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Mixotrophy, a physiological trait combining autotrophy and heterotrophy in one organism, significantly contributes to energy and matter transfer in aquatic ecosystems. However, understanding how environmental factors influence mixoplankton success across freshwater ecosystems has been uncertain. The grand écart hypothesis (GEH) posits that light and nutrient availability are key components of mixotrophs' niche, suggesting that ecosystem properties determine opposing gradients of light and nutrients, creating environmental filtering for mixotrophs. We hypothesized that ecosystem size, a property of lake ecosystems, mediates the prevalence of patterns and control mechanisms predicted by the GEH on mixoplankton relative biomass (MRB). Using data from 98 tropical lakes, we demonstrated that lake size mediates the inverse relationship between light and nutrient availability across ecosystems. Larger lakes have more light but low nutrients, while smaller lakes have more nutrients but greater shading. Light availability better explains MRB in small lakes, and nutrients better explain MRB in large lakes, with MRB values being higher in small lakes, with secondary influence from zooplankton herbivory. Our results validate the GEH as a significant framework for explaining patterns and control mechanisms of mixoplankton across tropical lakes. This study highlights the significance of lake size as an ecosystem property that generates opposing light and nutrient gradients, further emphasizing its importance for understanding mechanisms regulating freshwater phytoplankton community structure and functioning. Integrating lake size within the conceptual framework of the GEH could aid in explaining mixoplankton success over macroecological scales. [ABSTRACT FROM AUTHOR]

Mixotrophic protists combine photosynthesis and phagotrophy to obtain energy and nutrients. Because mixotrophs can act as either primary producers or consumers, they have a complex role in marine food webs and biogeochemical cycles. Many mixotrophs are also phenotypically plastic and can adjust their metabolic investments in response to resource availability. Thus, a single species's ecological role may vary with environmental conditions. Here, we quantified how light and food availability impacted the growth rates, energy acquisition rates, and metabolic investment strategies of eight strains of the mixotrophic chrysophyte, Ochromonas. All eight Ochromonas strains photoacclimated by decreasing chlorophyll content as light intensity increased. Some strains were obligate phototrophs that required light for growth, while other strains showed stronger metabolic responses to prey availability. When prey availability was high, all eight strains exhibited accelerated growth rates and decreased their investments in both photosynthesis and phagotrophy. Photosynthesis and phagotrophy generally produced additive benefits: In low‐prey environments, Ochromonas growth rates increased to maximum, light‐saturated rates with increasing light but increased further with the addition of abundant bacterial prey. The additive benefits observed between photosynthesis and phagotrophy in Ochromonas suggest that the two metabolic modes provide nonsubstitutable resources, which may explain why a tradeoff between phagotrophic and phototrophic investments emerged in some but not all strains. [ABSTRACT FROM AUTHOR]

Analysis of trait trade-offs, through which physiological traits requiring common resources are 'traded' to optimize competitive advantage, provides a route to simplify and more readily understand the complexities of ecology. The concept of trait trade-offs has found favour in plankton research, especially directed at phytoplankton, defined here as phototrophs incapable of phagotrophy. Mixoplankton, defined as protists that combine phototrophy and phagotrophy, are now recognized as being widespread and significant members of the protist plankton community; many photoflagellate 'phytoplankton' are actually mixoplankton, as are many '(microbial) zooplankton'. Mixoplankton might be expected to be dominant, being able to exploit different trophic strategies while simultaneously eliminating competitors. That mixoplankton are not dominant suggests that physiological trait trade-offs erode their apparent competitive edge. We present a systematic analysis of potential trait trade-offs in phototrophic protists focused on mixoplankton. We find no clear evidence to support trait trade-off arguments in plankton research, except perhaps for acquired phototrophy in mixoplanktonic ciliates versus zooplanktonic ciliates. Our findings suggest that the presence of various mixoplankton throughout the surface ocean waters is most likely explained by factors other than trait trade-offs. Diversities in mixoplankton form and function thus reflect that evolution of these organisms from very different lineages, provide them with advantages to function competitively in mature ecosystems with complex trophic interplay. Indeed, the complexity of those lineages is inconsistent with core trait trade-off definitions; there is no single ancestral mixoplankton nor a common environment supporting trait-trade-off-directed evolution. Trait trade-offs do not explain the breadth of mixoplankton ecophysiological capabilities. Diversity of mixoplankton form and function reflects phylogenetic diversity. Only one potential trait trade-off was identified which was for ciliates that steal chloroplasts. [ABSTRACT FROM AUTHOR]

Summary: Phytomyxea are intracellular biotrophic parasites infecting plants and stramenopiles, including the agriculturally impactful Plasmodiophora brassicae and the brown seaweed pathogen Maullinia ectocarpii. They belong to the clade Rhizaria, where phagotrophy is the main mode of nutrition. Phagocytosis is a complex trait of eukaryotes, well documented for free‐living unicellular eukaryotes and specific cellular types of animals. Data on phagocytosis in intracellular, biotrophic parasites are scant. Phagocytosis, where parts of the host cell are consumed at once, is seemingly at odds with intracellular biotrophy.Here we provide evidence that phagotrophy is part of the nutritional strategy of Phytomyxea, using morphological and genetic data (including a novel transcriptome of M. ectocarpii). We document intracellular phagocytosis in P. brassicae and M. ectocarpii by transmission electron microscopy and fluorescent in situ hybridization.Our investigations confirm molecular signatures of phagocytosis in Phytomyxea and hint at a small specialized subset of genes used for intracellular phagocytosis. Microscopic evidence confirms the existence of intracellular phagocytosis, which in Phytomyxea targets primarily host organelles.Phagocytosis seems to coexist with the manipulation of host physiology typical of biotrophic interactions. Our findings resolve long debated questions on the feeding behaviour of Phytomyxea, suggesting an unrecognized role for phagocytosis in biotrophic interactions. [ABSTRACT FROM AUTHOR]

Marine protists are eukaryotic trophic linkers that play a crucial role in iron recycling. Some marine protists have the ability of magnetotaxis, which they gain by consuming their ectosymbiotic bacteria. They graze and internalize the magnetotactic bacteria along with their magnetosome chains. Through egestion, marine protists avoid iron toxicity. Colloidal iron digestion by protists produces bioavailable iron for other marine organisms, passing to phytoplankton and mesozooplankton through the mesotrophic system. Indeed, ectosymbiotic bacteria and their protistan host form a microbial holobiont acting as an ecological unit. Some of the genetic mechanisms influencing the biosynthesis of magnetite in both prokaryotes and eukaryotes appear to be common. The recorded history of the magnetoreception ability of some marine protists goes back to the study by F.F. Torres de Araujo in 1986. After research over 35 years or more, it is safe to record that magnetotaxis in marine protists is yet to be fully understood, and might be similar to that of free-living magnetotactic bacteria. However, the attainment of magnetotaxis by protistan grazers through bacterivory and its role in iron recycling in the marine ecosystem is very interesting. The present article aims to provide an account of such interesting facts. [ABSTRACT FROM AUTHOR]

Light availability is the main regulator of primary production, shaping photosynthetic communities and their production of ecologically important biomolecules. In freshwater ecosystems, increasing dissolved organic carbon (DOC) concentrations, commonly known as browning, leads to lower light availability and the proliferation of mixotrophic phytoplankton. Here, a mixotrophic algal species (Cryptomonas sp.) was grown under five increasing DOC concentrations to uncover the plastic responses behind the success of mixotrophs in browning environments and their effect in the availability of nutritionally important biomolecules. In addition to the browning treatments, phototrophic, heterotrophic and mixotrophic growth conditions were used as controls. Despite reduced light availability, browning did not impair algal growth compared to phototrophic conditions. Comparative transcriptomics showed that genes related to photosynthesis were down‐regulated, whereas phagotrophy gene categories (phagosome, lysosome and endocytosis) were up‐regulated along the browning gradient. Stable isotope analysis of phospholipid fractions validated these results, highlighting that the studied mixotroph increases its reliance on heterotrophic processes with browning. Metabolic pathway reconstruction using transcriptomic data suggests that organic carbon is acquired through phagotrophy and used to provide energy in conjunction with photosynthesis. Although metabolic responses to browning were observed, essential fatty acid content was similar between treatments while sterol content was slightly higher upon browning. Together, our results provide a mechanistic model of how a mixotrophic alga responds to browning and how such responses affect the availability of nutritionally essential biomolecules for higher trophic levels. [ABSTRACT FROM AUTHOR]

Marine protists are eukaryotic trophic linkers that play a crucial role in iron recycling. Some marine protists have the ability of magnetotaxis, which they gain by consuming their ectosymbiotic bacteria. They graze and internalize the magnetotactic bacteria along with their magnetosome chains. Through egestion, marine protists avoid iron toxicity. Colloidal iron digestion by protists produces bioavailable iron for other marine organisms, passing to phytoplankton and mesozooplankton through the mesotrophic system. Indeed, ectosymbiotic bacteria and their protistan host form a microbial holobiont acting as an ecological unit. Some of the genetic mechanisms influencing the biosynthesis of magnetite in both prokaryotes and eukaryotes appear to be common. The recorded history of the magnetoreception ability of some marine protists goes back to the study by F.F. Torres de Araujo in 1986. After research over 35 years or more, it is safe to record that magnetotaxis in marine protists is yet to be fully understood, and might be similar to that of free-living magnetotactic bacteria. However, the attainment of magnetotaxis by protistan grazers through bacterivory and its role in iron recycling in the marine ecosystem is very interesting. The present article aims to provide an account of such interesting facts. [ABSTRACT FROM AUTHOR]

Two main theories have been put forward to explain the origin of mitochondria in eukaryotes: phagotrophic engulfment (undigested food) and microbial symbiosis (physiological interactions). The two theories generate mutually exclusive predictions about the order in which mitochondria and phagocytosis arose. To discriminate the alternatives, we have employed ancestral state reconstructions (ASR) for phagocytosis as a trait, phagotrophy as a feeding habit, the presence of mitochondria, the presence of plastids, and the multinucleated organization across major eukaryotic lineages. To mitigate the bias introduced by assuming a particular eukaryotic phylogeny, we reconstructed the appearance of these traits across 1789 different rooted gene trees, each having species from opisthokonts, mycetozoa, hacrobia, excavate, archeplastida, and Stramenopiles, Alveolates and Rhizaria. The trees reflect conflicting relationships and different positions of the root. We employed a novel phylogenomic test that summarizes ASR across trees which reconstructs a last eukaryotic common ancestor that possessed mitochondria, was multinucleated, lacked plastids, and was non-phagotrophic as well as non-phagocytic. This indicates that both phagocytosis and phagotrophy arose subsequent to the origin of mitochondria, consistent with findings from comparative physiology. Furthermore, our ASRs uncovered multiple origins of phagocytosis and of phagotrophy across eukaryotes, indicating that, like wings in animals, these traits are useful but neither ancestral nor homologous across groups. The data indicate that mitochondria preceded the origin of phagocytosis, such that phagocytosis cannot have been the mechanism by which mitochondria were acquired. [ABSTRACT FROM AUTHOR]

• Analysis of a seasonal plankton model with added mixotroph population. • Demonstrate potential for chaos in year-to year mixotroph density with enrichment. • Prediction of mixo-HABs may not be possible using environmental predictors alone. Mixotrophic nutrition, in which organisms combine aspects of heterotrophic and autotrophic nutrition, is a common characteristic amongst the phytoplankton, and especially amongst harmful algae bloom (HAB) species. To explore how the addition of a mixotrophic population influences a theoretical plankton system under different enrichment levels, we employ a previously published numerical plankton model that reproduces seasonal succession dynamics. Through this work we found potential for chaos to occur as the system was enriched. Focusing on occurrence of chaos in the mixotroph population, a novel finding, we conducted a bifurcation analysis which demonstrated a period doubling route to chaos. Further, we demonstrate that the occurrence of chaotic dynamics in the model system is not an artifact of the mixotroph life-history trait values used in our initial simulations, rather chaos was found to occur under a wide range of mixotroph trait value combinations. Through our trait analysis, we also found that regardless of whether or not the system was chaotic for a specific combination of values, variation in the mixotroph population peak population density could be high between consecutive years. These findings have implications that are important for the management of nutrients in aquatic systems and HABs. Results shed light on why it may be difficult to predict the occurrence of mixotrophic HABs based solely on environmental variables, when the year-to-year peak density of the mixotroph is chaotic or highly variable with enrichment. These results also demonstrate how initiation of a mixotrophic HAB might occur with enrichment, due to erratic population spikes, when broadcast allelopathic or toxic chemicals are at low concentrations, a novel bloom initiation mechanism. [ABSTRACT FROM AUTHOR]

The origin of the photosynthetic organelle in eukaryotes, the plastid, changed forever the evolutionary trajectory of life on our planet. Plastids are highly specialized compartments derived from a putative single cyanobacterial primary endosymbiosis that occurred in the common ancestor of the supergroup Archaeplastida that comprises the Viridiplantae (green algae and plants), red algae, and glaucophyte algae. These lineages include critical primary producers of freshwater and terrestrial ecosystems, progenitors of which provided plastids through secondary endosymbiosis to other algae such as diatoms and dinoflagellates that are critical to marine ecosystems. Despite its broad importance and the success of algal and plant lineages, the phagotrophic origin of the plastid imposed an interesting challenge on the predatory eukaryotic ancestor of the Archaeplastida. By engulfing an oxygenic photosynthetic cell, the host lineage imposed an oxidative stress upon itself in the presence of light. Adaptations to meet this challenge were thus likely to have occurred early on during the transition from a predatory phagotroph to an obligate phototroph (or mixotroph). Modern algae have recently been shown to employ linear tetrapyrroles (bilins) to respond to oxidative stress under high light. Here we explore the early events in plastid evolution and the possible ancient roles of bilins in responding to light and oxygen.

Constitutive mixoplankton-plastid-bearing microbial eukaryotes capable of both phototrophy and phagotrophy-are ubiquitous in marine ecosystems and facilitate carbon transfer to higher trophic levels within aquatic food webs, which supports enhanced sinking carbon flux. However, the regulation of the relative contribution of photosynthesis and prey consumption remains poorly characterized. We investigated the transcriptional dynamics behind this phenotypic plasticity in the prasinophyte green alga Pterosperma cristatum. Based on what is known of other mixoplankton species that cannot grow without photosynthesis (obligate phototrophs), we hypothesized that P. cristatum uses phagotrophy to circumvent the restrictions imposed on photosynthesis by nutrient depletion, to obtain nutrients from ingested prey, and to maintain photosynthetic carbon fixation. We observed an increase in feeding as a response to nutrient depletion, coinciding with an upregulation of expression for genes involved in essential steps of phagocytosis including prey recognition, adhesion and engulfment, transport and maturation of food vacuoles, and digestion. Unexpectedly, genes involved in the photosynthetic electron transfer chain, pigment biosynthesis, and carbon fixation were downregulated as feeding increased, implying an abatement of photosynthesis. Contrary to our original hypothesis, our results therefore suggest that depletion of inorganic nutrients triggered an alteration of trophic behavior from photosynthesis to phagotrophy in P. cristatum . While this behavior distinguishes P. cristatum from other groups of constitutive mixoplankton, its physiological response aligns with recent discoveries from natural microbial communities. These findings indicate that mixoplankton communities in nutrient-limited oceans can regulate photosynthesis against bacterivory based on nutrient availability., Competing Interests: None declared., (© The Author(s) 2024. Published by Oxford University Press on behalf of the International Society for Microbial Ecology.)

The stramenopiles are a large and diverse group of eukaryotes that possess various lifestyles required to thrive in a broad array of environments. The stramenopiles branch with the alveolates, rhizarians, and telonemids, forming the supergroup TSAR. Here, we present a new genus and species of aquatic nanoflagellated stramenopile: Mediocremonas mediterraneus, a free‐swimming heterotrophic predator. M. mediterraneus cell bodies measure between 2.0–4.0 μm in length and 1.2–3.7 μm in width, possessing two flagella and an oval body morphology. The growth and grazing rate of M. mediterraneus in batch cultures ranges from 0.68 to 1.83 d−1 and 1.99 to 5.38 bacteria/h, respectively. M. mediterraneus was found to be 93.9% phylogenetically similar with Developayella elegans and 94.7% with Develorapax marinus, two members within the class Developea. The phylogenetic position of the Developea and the ability of M. mediterraneus to remain in culture make it a good candidate for further genomic studies that could help us to better understand phagotrophy in marine systems as well as the transition from heterotrophy to phototrophy within the stramenopiles. [ABSTRACT FROM AUTHOR]

Many marine microbial eukaryotes combine photosynthetic with phagotrophic nutrition, but incomplete understanding of such mixotrophic protists, their functional diversity, and underlying physiological mechanisms limits the assessment and modeling of their roles in present and future ocean ecosystems. We developed an experimental system to study responses of mixotrophic protists to availability of living prey and light, and used it to characterize contrasting physiological strategies in two stramenopiles in the genus Ochromonas. We show that oceanic isolate CCMP1393 is an obligate mixotroph, requiring both light and prey as complementary resources. Interdependence of photosynthesis and heterotrophy in CCMP1393 comprises a significant role of mitochondrial respiration in photosynthetic electron transport. In contrast, coastal isolate CCMP2951 is a facultative mixotroph that can substitute photosynthesis by phagotrophy and hence grow purely heterotrophically in darkness. In contrast to CCMP1393, CCMP2951 also exhibits a marked photoprotection response that integrates non‐photochemical quenching and mitochondrial respiration as electron sink for photosynthetically produced reducing equivalents. Facultative mixotrophs similar to CCMP2951 might be well adapted to variable environments, while obligate mixotrophs similar to CCMP1393 appear capable of resource efficient growth in oligotrophic ocean environments. Thus, the responses of these phylogenetically close protists to the availability of different resources reveals niche differentiation that influences impacts in food webs and leads to opposing carbon cycle roles. [ABSTRACT FROM AUTHOR]

Mixotrophic protists combine phagotrophy and phototrophy within a single cell. Greater phagotrophic activity could reinforce the bypass of carbon (C) flux through the bacteria‐mixotroph link and thus lead to a more efficient transfer of C and other nutrients to the top of the trophic web. Determining how foreseeable changes in temperature and UVR affect mixotrophic trade‐offs in favor of one or the other nutritional strategy, along the mixotrophic gradient, is key to understanding the fate of carbon and mineral nutrients in the aquatic ecosystem. Our two main hypotheses were: (i) that increased warming and UVR will divert metabolism toward phagotrophy, and (ii) that the magnitude of this shift will vary according to the organism's position along the mixotrophic gradient. To test these hypotheses, we used two protists (Isochrysis galbana and Chromulina sp.) located in different positions on the mixotrophic gradient, subjecting them to the action of temperature and of UVR and their interaction. Our results showed that the joint action of these two factors increased the primary production:bacterivory ratio and stoichiometric values (N:P ratio) close to Redfield's ratio. Therefore, temperature and UVR shifted the metabolism of both organisms toward greater phototrophy regardless of the original position of the organism on the mixotrophic gradient. Weaker phagotrophic activity could cause a less efficient transfer of C to the top of trophic webs. [ABSTRACT FROM AUTHOR]

Mixotrophy is a combination of photosynthesis and direct access to organic carbon sources, mainly through osmotrophy or phagotrophy. This strategy is adopted by several, phylogenetically distinct, phytoplankton groups and is commonly occurring in marine, brackish and freshwater ecosystems. Traditionally, it has been put in relation to both scarcity of inorganic nutrients and poor light conditions. However, we observed blooms of the mixotrophic, toxic haptophyte Prymnesium parvum in different periods of the year and under variable resources availability. The analysis of a 6.5-year data set of phytoplankton weekly records from a Sicilian shallow lake (Biviere di Gela, south-eastern Sicily) allowed us to hypothesise that a depleted condition as regards inorganic nutrients is not the main fuel to the growth of P. parvum, neither this is due to light limitation. The results achieved show that an increased availability in suitable preys can stimulate the growth of this phagotrophic photoautotroph. Contemporarily, it was not found any clear environmental patterns to explain species dominance and growth patterns as related to inorganic nutrient availability. Moreover, it is shown that these organisms tend to monopolise resources when these become available irrespective of seasons, and under variable conditions as regards inorganic nutrient availability. [ABSTRACT FROM AUTHOR]

Growing attention to phytoplankton mixotrophy as a trophic strategy has led to significant revisions of traditional pelagic food web models and ecosystem functioning. Although some empirical estimates of mixotrophy do exist, a much broader set of in situ measurements are required to (i) identify which organisms are acting as mixotrophs in real time and to (ii) assess the contribution of their heterotrophy to biogeochemical cycling. Estimates are needed through time and across space to evaluate which environmental conditions or habitats favour mixotrophy: conditions still largely unknown. We review methodologies currently available to plankton ecologists to undertake estimates of plankton mixotrophy, in particular nanophytoplankton phago-mixotrophy. Methods are based largely on fluorescent or isotopic tracers, but also take advantage of genomics to identify phylotypes and function. We also suggest novel methods on the cusp of use for phago-mixotrophy assessment, including single-cell measurements improving our capacity to estimate mixotrophic activity and rates in wild plankton communities down to the single-cell level. Future methods will benefit from advances in nanotechnology, micromanipulation and microscopy combined with stable isotope and genomic methodologies. Improved estimates of mixotrophy will enable more reliable models to predict changes in food web structure and biogeochemical flows in a rapidly changing world. [ABSTRACT FROM AUTHOR]

Summary: Mixotrophy is a dominant metabolic strategy in ecosystems worldwide. Shifts in temperature (T) and light (i.e. the ultraviolet portion of spectrum (UVR)) are key abiotic factors that modulate the conditions under which an organism is able to live. However, whether the interaction between both drivers alters mixotrophy in a global‐change context remains unassessed.To determine the T × UVR effects on relative electron transport rates, nonphotochemical quenching, bacterivory, and bacterial production, we conducted an experiment with Isochrysis galbana populations grown mixotrophically, which were exposed to 5°C of cooling and warming with respect to the control (19°C) with (or without) UVR over light–dark cycles and different timescales.At the beginning of the experiment, cooling inhibited the relative electron transport and bacterivory rates, whereas warming depressed only bacterivory regardless of the radiation treatment. By the end of the experiment, warming and UVR conditions stimulated bacterivory. These reduced relative electron transport rates (c. 50% (warming) and > 70% (cooling)) were offset by increased (35%) cumulative bacterivory rates under warming and UVR conditions.We propose that mixotrophy constitutes an energy‐saving and a compensatory mechanism to gain carbon (C) when photosynthesis is impaired, and highlight the need to consider the natural environmental changes affecting the populations when we test the impacts of interacting global‐change drivers. [ABSTRACT FROM AUTHOR]

As the number of mixotrophic protists has been increasingly documented, “mixoplankton”, a third category separated from the traditional categorization of plankton into “phytoplankton” and “zooplankton”, has become a new paradigm and research hotspot in aquatic plankton ecology. While species of dinoflagellates are a dominant group among all recorded members of mixoplankton, the trophic modes of Karlodinium, a genus constituted of cosmopolitan toxic species, were reviewed due to their representative features as mixoplankton and harmful algal blooms (HABs)-causing dinoflagellates. Among at least 15 reported species in the genus, three have been intensively studied for their trophic modes, and all found to be phagotrophic. Their phagotrophy exhibits multiple characteristics: (1) omnivority, i.e., they can ingest a variety of preys in many forms; (2) flexibility in phagotrophic mechanisms, i.e., they can ingest small preys by direct engulfment and much bigger preys by myzocytosis using a peduncle; (3) cannibalism, i.e., species including at least K. veneficum can ingest the dead cells of their own species. However, for some recently described and barely studied species, their tropical modes still need to be investigated further regarding all of the above-mentioned aspects. Mixotrophy of Karlodinium plays a significant role in the population dynamics and the formation of HABs in many ways, which thus deserves further investigation in the aspects of physiological ecology, environmental triggers (e.g., levels of inorganic nutrients and/or presence of preys), energetics, molecular (genes and gene expression regulations) and biochemical (e.g., relevant enzymes and signal molecules) bases, origins, and evaluation of the advantages of being a phagotroph.

Mixotrophic flagellates can comprise significant proportions of plankton biomass in marine ecosystems. Despite the growing recognition of the importance of this ecological strategy, and the identification of major environmental factors controlling phagotrophic behavior (light and nutrients), the physiological and molecular mechanisms underlying mixotrophic behavior are still unclear. In this study, we performed RNA-Seq transcriptomic analysis for two mixotrophic prasinophytes, Micromonas polaris and Pyramimonas tychotreta, under dissolved nutrient regimes that altered their ingestion of bacteria prey. Though the strains examined were polar isolates, both belong to genera with widespread distribution. Our aim was to characterize the transcriptomes of these two non-model phytoflagellates, identify transcripts consistent with phagotrophic activity and assess their differential expression in response to nutrient stress. De novo assembly of the transcriptomes yielded large numbers of novel coding transcripts with no known match within public databases. A summary of the transcripts by Gene Ontology terms showed many expected expression patterns, including genes involved in photosynthetic pathways and enzymes implicated in nutrient uptake pathways. Searches of KEGG databases identified several genes associated with intra-cellular digestive pathways actively transcribed in both prasinophytes. Differential expression analysis showed a larger response in P. tychotreta, where 23,373 genes were up-regulated and 1,752 were down-regulated in the low nutrient treatment when phagotrophy was enhanced. In contrast, in M. polaris, low nutrient treatments resulted in up-regulation of 314 transcripts while down-regulating 371. With respect to phagotrophic-related expression, 37 genes were co-expressed in both P. tychotreta and M. polaris, and although the response was less pronounced in M. polaris, it is consistent with differences in observed ingestion behavior. This study presents the first genomic data for Pyramimonas tychotreta, and also contributes to the limited available data for Micromonas polaris. Furthermore, it provides insight into the presence of genes associated with phagocytosis within the Prasinophyceae and contributes to the understanding of potential target genes required for the construction of a complete model of gene regulation of phagocytic behavior in algae.

Phagotrophy of the harmful benthic dinoflagellates of the genus Ostreopsis has long been inferred based on observations of food particles present inside cells, but the prey has not yet been identified. This study aimed to investigate the seasonal dynamics of benthic dinoflagellates Ostreopsis spp. in temperate Korean coastal sites, with special emphasis on their phagotrophy. Further, prey species were identified by extracting the ingested food particles from single Ostreopsis cells and determining their rbcL and psaA gene sequences. High concentration of Ostreopsis cells was observed between June and October at all sites, when the water temperatures were higher than 19°C, exhibiting a marked temporal seasonality. The percentage of Ostreopsis cells containing ingested food particles exhibited large spatial and temporal variations among sampling sites, ranging from undetectable level to 29.5%, and was not always associated with Ostreopsis cell abundance. Phylogenetic analyses performed using both plastid-encoded rbcL and psaA genes revealed that all sequences obtained from the ingested food particles of Ostreopsis cells grouped within the class Florideophyceae, Rhodophyta. Our result clearly demonstrates that Ostreopsis species consume various macroalgae from Rhodophyta, but not protists, which have long been thought to be the potential prey. The results of this study provide a basis for better understanding the mixotrophic behavior and nutritional ecology of the harmful benthic dinoflagellate Ostreopsis species.

Abstract: Most research on microorganisms adapted to hypersaline habitats has focused on Archaea and Bacteria, with microbial eukaryotes receiving much less attention. Over the past 15 yr, our knowledge of phagotrophic microbial eukaryotes, i.e. protozoa, from hypersaline habitats has greatly improved through combinations of microscopy, molecular phylogenetics, environmental sequencing, transcriptomics and growth experiments. High salinity waters from salterns, other landlocked water masses and deep hypersaline anoxic basins contain unique and diverse halophilic protozoan assemblages. These have the potential to exert substantial grazing pressure on prokaryotes and other eukaryotes. They represent many separate evolutionary lineages; species of Heterolobosea, Bicosoecida, and Ciliophora have been most intensively characterized, with several proven to be extreme (or borderline extreme) halophiles. Transcriptomic examinations of the bicosoecid Halocafeteria (and the heteroloboseid Pharyngomonas) indicate that high‐salt adaptation is associated with a subtle shift in protein amino acid composition, and involves the differential expression of genes participating in ion homeostasis, signal transduction, stress management, and lipid remodeling. Instances of gene duplication and lateral transfer possibly conferring adaptation have been documented. Indirect evidence suggests that these protozoa use “salt‐out” osmoadaptive strategies. [ABSTRACT FROM AUTHOR]

• Use of flow cytometry to quantify ingestion rate of the toxic mixoplankton Prymnesium parvum. • Addition of algal prey significantly increased the growth rate of P. parvum both in inorganic phosphorus deficiency and sufficiency cultures. • Rates of ingestion and the lytic effects were stimulated by inorganic phosphorus deficiency in P. parvum. • P. parvum is able to use prey and/or debris derived from algae as phosphorus sources. Many harmful algae are mixoplanktonic, i.e. they combine phototrophy and phagotrophy, and this ability may explain their ecological success, especially when environmental conditions are not optimal for autotrophic growth. In this study, laboratory experiments were conducted with the mixotrophic and ichthyotoxic haptophyte Prymnesium parvum (strain CCAP 946/6) to test the effects of phosphorus (P) sufficiency and deficiency on its growth rate, phagotrophic and lytic activities. P-deficient P. parvum cultures were grown without or with addition of P in the form of inorganic phosphorus (nutrients) and/or living algal prey (i.e. the cryptophyte Teleaulax amphioxeia). The ingestion rate of P. parvum and prey mortality rate were calculated using flow cytometry measurements based on pigment-derived-fluorescence to distinguish between prey, predators and digesting predators. The first aim of the study was to develop a method taking into account the rate of digestion, allowing the calculation of ingestion rates over a two-week period. Growth rates of P. parvum were higher in the treatments with live prey, irrespective of the concentration of inorganic P, and maximum growth rates were found when both inorganic and organic P in form of prey were added (0.79 ± 0.07 d -1). In addition, the mortality rate of T. amphioxeia induced by lytic compounds was 0.2 ± 0.02 d -1 in the P-deficient treatment, while no mortality was observed under P-sufficiency in the present experiments. This study also revealed the mortality due to cell lysis exceeded that of prey ingestion. Therefore, additional experiments were conducted with lysed prey cells. When grown with debris from prey cells, the mean growth rate of P. parvum was similar to monocultures without additions of prey debris (0.30 ± 0.1 vs. 0.38 ± 0.03 d -1), suggesting that P. parvum is able to grow on prey debris, but not as fast as with live prey. These results provide the first quantitative evidence over two weeks of experiment that ingestion of organic P in the form of living prey and/or debris of prey plays an important role in P. parvum growth and may explain its ecological success in a nutrient-limited environments. [Display omitted] [ABSTRACT FROM AUTHOR]

Little is still known of the impacts of protist grazing on bacterioplankton communities in the dark ocean. Furthermore, the accuracy of assessments of in situ microbial activities, including protist grazing, can be affected by sampling artifacts introduced during sample retrieval and downstream manipulations. Potential artifacts may be increased when working with deep-sea samples or samples from chemically unique water columns such as oxygen minimum zones (OMZs). OMZs are oxygen-depleted regions in the ocean, where oxygen concentrations can drop to

Photosynthesis is a biochemical process essential for life, serving as the ultimate source of chemical energy for phototrophic and heterotrophic life forms. Since the machinery of the photosynthetic electron transport chain is quite complex and is unlikely to have evolved multiple independent times, it is believed that this machinery has been transferred to diverse eukaryotic organisms by endosymbiotic events involving a eukaryotic host and a phototrophic endosymbiont. Thus, photoautotrophy, as a benefit, is transmitted through the evolution of plastids. However, many eukaryotes became secondarily heterotrophic, reverting to hetero-osmotrophy, phagotrophy, or parasitism. Here, I briefly review the constructive evolution of plastid endosymbioses and the consequential switch to reductive evolution involving losses of photosynthesis and plastids and the evolution of parasitism from a photosynthetic ancestor.

Most biogeochemical/ecological models divide planktonic protists between phototrophs (phytoplankton) and heterotrophs (zooplankton). However, a large number of planktonic protists are able to combine several mechanisms of carbon and nutrient acquisition. Not representing these multiple mechanisms in biogeochemical/ecological models describing eutrophied coastal ecosystems can potentially lead to different conclusions regarding ecosystem functioning, especially regarding the success of harmful algae, which are often reported as mixotrophic. This modelling study investigates the implications for trophic dynamics of including 3 contrasting forms of mixotrophy, namely osmotrophy (using alkaline phosphatase activity, APA), non-constitutive mixotrophy (acquired phototrophy by microzooplankton) and also constitutive mixotrophy. The application is in the Southern North Sea, an ecosystem that faced, between 1985 and 2005, a significant increase in the nutrient supply N:P ratio (from 31 to 81 mol N:P). The comparison with a traditional model shows that, when the winter N:P ratio in the Southern North Sea is above 22 molN molP −1 (as occurred from mid-1990s), APA allows a 3–32% increase of annual gross primary production (GPP). In result of the higher GPP, the annual sedimentation increases as well as the bacterial production. By contrast, APA does not affect the export of matter to higher trophic levels because the increased GPP is mainly due to Phaeocystis colonies, which are not grazed by copepods. Under high irradiance, non-constitutive mixotrophy appreciably increases annual GPP, transfer to higher trophic levels, sedimentation, and nutrient remineralisation. In this ecosystem, non-constitutive mixotrophy is also observed to have an indirect stimulating effect on diatoms. Constitutive mixotrophy in nanoflagellates appears to have little influence on this ecosystem functioning. An important conclusion from this work is that contrasting forms of mixotrophy have different impacts on system dynamics and, due to the complex interactions in the ecosystem, their combined effect is not exactly the addition of the effects individually observed. It is thus important to describe such contrasting forms in an appropriate fashion. [ABSTRACT FROM AUTHOR]

Production of algae as renewable resources has been pursued actively. Attention has so far been focused on phototrophic algae. Explored here is a completely different approach that harnesses the growth ability of phagotrophic algae on microorganisms such as bacteria, which are abundantly present in waste, including activated sludge generated in wastewater treatment plants. Escherichia coli, a common bacterium, was used as an example of bacterial feed for the oleaginous phagotrophic alga Ochromonas danica. Growth characteristics and lipid contents of O. danica were determined under various conditions. Algal doubling time could be as short as 7 h, but increased to >20 h at low starting E. coli-to- O. danica ratios. The alga was also sensitive to step changes of pH and darkness. Without these factors, phagotrophic growth was similar in a wide range of conditions, even under total darkness and in deionized water with washed E. coli as the only food source. O. danica grew on E. coli with an average yield of about 25% (w/w) and a lipid content of 27% ± 8%. As a food source for phagotrophic algal growth, bacterial cells provided excess nitrogen, which was released as ammonia. Glucose supplementation was studied to avoid ammonia release and produce more algae with higher lipid content (33-34%). Results supported feasibility of this new approach and identified suitable process conditions for further development and application in real wastewater systems. Besides algal production, the approach can remove harmful bacteria from wastewater. [ABSTRACT FROM AUTHOR]

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Recently, a hitherto unknown feeding strategy, the toxic mucus trap, was discovered in the dinoflagellate Alexandrium pseudogonyaulax . In this study, over 40 strains of 8 different Alexandrium species ( A. ostenfeldii, A. tamarense, A. catenella, A. taylorii, A. margalefii, A. hiranoi, A. insuetum and A. pseudogonyaulax ) were screened for their ability to ingest prey and/or to form mucus traps. The mucus trap feeding strategy, where a mucus trap is towed by the longitudinal flagellum remains unique to A. pseudogonyaulax. In additional experiments, details of the trap were examined and quantified, such as speed and frequency of trap formation as well as what happens to the trap after the A. pseudogonyaulax cell detaches from it. The percentage of A. pseudogonyaulax cells producing a mucus trap and the number of prey cells caught increased with increasing prey concentration, whereas the physical size of the traps was independent of prey concentration. In one strain given an excess of prey, within 1 h over 90% of individual A. pseudogonyaulax cells had formed a trap, each containing an average of 45 prey cells. Individual A. pseudogonyaulax cells steadily produced traps and up to 5 traps were produced by a single A. pseudogonyaulax cell after only 24 h. The attachment of an A. pseudogonyaulax cell to the trap only ceased during, and just following, cell division. Prey cells were, to some extent, capable of escaping from the mucus trap, but the trap remained sticky and continued catching prey for up to 48 h after the trap had been abandoned by the A. pseudogonyaulax cell. These results reveal that the effects of the mucus trap extend far beyond the removal of prey through ingestion, and the potential impact of this strategy on surrounding cells is high. [ABSTRACT FROM AUTHOR]

Background: Ochromonas is a genus of mixotrophic chrysophytes that is found ubiquitously in many aquatic environments. Species in this genus can be important consumers of bacteria but vary in their ability to perform photosynthesis. We studied the effect of light and bacteria on growth and gene expression of a predominantly phagotrophic Ochromonas species. Axenic cultures of Ochromonas sp. were fed with heat-killed bacteria (HKB) and grown in constant light or darkness. RNA was extracted from cultures in the light or in the dark with HKB present (Light + HKB; Dark + HKB), and in the light after HKB were depleted (Light + depleted HKB). Results: There were no significant differences in the growth or bacterial ingestion rates between algae grown in light or dark conditions. The availability of light led to a differential expression of only 8% of genes in the transcriptome. A number of genes associated with photosynthesis, phagotrophy, and tetrapyrrole synthesis was upregulated in the Light + HKB treatment compared to Dark + HKB. Conversely, the comparison between the Light + HKB and Light + depleted HKB treatments revealed that the presence of HKB led to differential expression of 59% of genes, including the majority of genes involved in major carbon and nitrogen metabolic pathways. Genes coding for unidirectional enzymes for the utilization of glucose were upregulated in the presence of HKB, implying increased glycolytic activities during phagotrophy. Algae without HKB upregulated their expression of genes coding for ammonium transporters, implying uptake of inorganic nitrogen from the culture medium when prey were unavailable. Conclusions: Transcriptomic results agreed with previous observations that light had minimal effect on the population growth of Ochromonas sp. However, light led to the upregulation of a number of phototrophy- and phagotrophyrelated genes, while the availability of bacterial prey led to prominent changes in major carbon and nitrogen metabolic pathways. Our study demonstrated the potential of transcriptomic approaches to improve our understanding of the trophic physiologies of complex mixotrophs, and revealed responses in Ochromonas sp. not apparent from traditional culture studies. [ABSTRACT FROM AUTHOR]

There is increasing awareness that many terrestrial and aquatic organisms are not strictly heterotrophic or autotrophic but rather mixotrophic. Mixotrophy is an intermediate nutritional strategy, merging autotrophy and heterotrophy to acquire organic carbon and/or other elements, mainly N, P or Fe. We show that both terrestrial and aquatic mixotrophs fall into three categories, namely necrotrophic (where autotrophs prey on other organisms), biotrophic (where heterotrophs gain autotrophy by symbiosis) and absorbotrophic (where autotrophs take up environmental organic molecules). Here we discuss their physiological and ecological relevance since mixotrophy is found in virtually every ecosystem and occurs across the whole eukaryotic phylogeny, suggesting an evolutionary pressure towards mixotrophy. Ecosystem dynamics tend to separate light from non-carbon nutrients (N and P resources): the biological pump and water stratification in aquatic ecosystems deplete non-carbon nutrients from the photic zone, while terrestrial plant successions create a canopy layer with light but devoid of non-carbon soil nutrients. In both aquatic and terrestrial environments organisms face a grand écart (dancer's splits, i.e., the need to reconcile two opposing needs) between optimal conditions for photosynthesis vs. gain of non-carbon elements. We suggest that mixotrophy allows adaptation of organisms to such ubiquist environmental gradients, ultimately explaining why mixotrophic strategies are widespread. [ABSTRACT FROM AUTHOR]

Predation by grazing protists in aquatic habitats can influence prokaryotic community structure and provides a source of new, labile organic matter. Due to methodological difficulties associated with studies of deep-sea (below photic zone) microbiota, trophic interactions between eukaryotes and prokaryotes in mesopelagic and bathypelagic realms are largely obscured. Further complicating matters, examinations of trophic interactions using water samples that have been exposed to upwards of hundreds of atmospheres of pressure change prior to initiating experiments can potentially introduce significant artifacts. Here we present results of the first study of protistan grazing in water layers ranging from the euphotic zone to the bathypelagic, utilizing the Microbial Sampler-Submersible Incubation Device (MS-SID) that makes possible in situ studies of microbial activities. Protistan grazing in the mesopelagic and bathypelagic realm of the East Mediterranean Sea was quantified using fluorescently labeled prokaryotes (FLP) prepared from the naturally-occurring prokaryotic assemblages. These studies reveal daily prokaryotic removal due to grazing ranging from 31.3±5.9% at 40 m depth to 0.5±0.3% at 950 m. At 3540 m depth, where a chemocline habitat exists with abundant and active prokaryotes above Urania basin, the daily consumption of prokaryotes by protists was 19.9±6.6% of the in situ abundance. [ABSTRACT FROM AUTHOR]

The origin of the photosynthetic organelle in eukaryotes, the plastid, changed forever the evolutionary trajectory of life on our planet. Plastids are highly specialized compartments derived from a putative single cyanobacterial primary endosymbiosis that occurred in the common ancestor of the supergroup Archaeplastida that comprises the Viridiplantae (green algae and plants), red algae, and glaucophyte algae. These lineages include critical primary producers of freshwater and terrestrial ecosystems, progenitors of which provided plastids through secondary endosymbiosis to other algae such as diatoms and dinoflagellates that are critical to marine ecosystems. Despite its broad importance and the success of algal and plant lineages, the phagotrophic origin of the plastid imposed an interesting challenge on the predatory eukaryotic ancestor of the Archaeplastida. By engulfing an oxygenic photosynthetic cell, the host lineage imposed an oxidative stress upon itself in the presence of light. Adaptations to meet this challenge were thus likely to have occurred early on during the transition from a predatory phagotroph to an obligate phototroph (or mixotroph). Modern algae have recently been shown to employ linear tetrapyrroles (bilins) to respond to oxidative stress under high light. Here we explore the early events in plastid evolution and the possible ancient roles of bilins in responding to light and oxygen.

The dinoflagellate Prorocentrum donghaiense is a dominant harmful algal bloom (HAB) species on the East China Sea (ECS) coast. The co-occurrence of Karlodinium veneficum with P. donghaiense is often observed and can later develop into dense blooms. However, the role of K. veneficum in P. donghaiense population dynamics is unknown. In the current study, three K. veneficum (GM1, GM2, and GM3) strains were isolated from the ECS with one (GM1) from a mixed, dense bloom of P. donghaiense and other HAB species. All three isolates had identical ITS sequences that were concordant with the species designation. Unique karlotoxin congeners were isolated from one strain (GM2). The sterol compositions of P. donghaiense and K. veneficum were consistent with sensitivity to karlotoxin in the former and insensitivity in the latter. Additional experimentation showed that: (1) in monocultures, higher growth rate of P. donghaiense than K. veneficum is observed in nutrient-enriched and nutrient-depleted media. In co-cultures, the growth of P. donghaiense is inhibited; (2) feeding on P. donghaiense by K. veneficum is clearly demonstrated by fluorescent dye tracking; and (3) the isolated karlotoxin is lethal to P. donghaiense in a concentration-dependent manner. From these studies we propose that K. veneficum may play a negative role in P. donghaiense bloom maintenance and that P. donghaiense may in turn be a bloom initiator as a prey item for K. veneficum . [ABSTRACT FROM AUTHOR]

Comparative studies of the mitochondrial proteome have identified a conserved core of proteins descended from the α-proteobacterial endosymbiont that gave rise to the mitochondrion and was the source of the mitochondrial genome in contemporary eukaryotes. A surprising result of phylogenetic analyses is the relatively small proportion (10-20%) of the mitochondrial proteome displaying a clear α-proteobacterial ancestry. A large fraction of mitochondrial proteins typically has detectable homologs only in other eukaryotes and is presumed to represent proteins that emerged specifically within eukaryotes. A further significant fraction of the mitochondrial proteome consists of proteins with homologs in prokaryotes, but without a robust phylogenetic signal affiliating them with specific prokaryotic lineages. The presumptive evolutionary source of these proteins is quite different in contending models of mitochondrial origin. [ABSTRACT FROM AUTHOR]

The genus Esoptrodinium Javornický consists of freshwater, athecate dinoflagellates with an incomplete cingulum. Strains isolated thus far feed on microalgae and most possess obvious pigmented chloroplasts, suggesting mixotrophy. However, some geographic isolates lack obvious pigmented chloroplasts. The purpose of this study was to comparatively examine this difference and the associated potential for mixotrophy among different isolates of Esoptrodinium. All isolates phagocytized prey cells through an unusual hatch-like peduncle located on the ventral episome, and were capable of ingesting various protist taxa. All Esoptrodinium isolates required both food and light to grow. However, only the tested strain with visible pigmented chloroplasts benefited from light in terms of increased biomass (phototrophy). Isolates lacking obvious chloroplasts received no biomass benefit from light, but nevertheless required light for sustained growth (i.e., photoobligate, but not phototrophic). Isolates with visible chloroplasts exhibited chlorophyll autofluorescence and formed a monophyletic psbA gene clade that suggested Esoptrodinium possesses inherited, peridinoid-type plastids. One isolate with cryptic, barely visible plastids lacked detectable chlorophyll and exhibited an apparent loss-of-function mutation in psbA, indicating the presence of nonphotosynthetic plastids. The other isolate that lacked visible chloroplasts lacked both detectable chlorophyll and an amplifiable psbA sequence. The results demonstrate mixotrophy quantitatively for the first time in a freshwater dinoflagellate, as well as apparent within-clade loss of phototrophy along with a correlated mutation sufficient to explain that phenotype. Phototrophy is a variable trait in Esoptrodinium; further study is required to determine if this represents an inter- or intraspecific (allelic) characteristic in this taxon. [ABSTRACT FROM AUTHOR]

In freshwater ecosystems a vast diversity of elusive protists exists that specifically feed on microalgae. Due to difficulties in isolation and long-term maintenance, most of these are still poorly known. In this study stable, bacteria-free cultures of several limnetic, algivorous amoeboflagellates were investigated by light microscopy and molecular phylogenetic analyses. All strains represent naked, biflagellate cells, either occurring as rigid flagellates or as surface-attached amoebae. They perforate cell walls of certain Zygnematophyceae and Chlorophyceae (Viridiplantae) and phagocytose algal cell contents. Time-lapse microscopy revealed the feeding behaviour, locomotional processes and life histories of the amoeboflagellates. Clear differences in cell morphology and food range specificity led to the description of two new, monotypic genera Orciraptor and Viridiraptor, which occupy similar, but distinct ecological niches in aquatic ecosystems as ‘necrophytophagous’ and ‘parasitoid’ protists, respectively. Molecular phylogenetic analyses based on 18S rDNA sequence data demonstrated that Orciraptor and Viridiraptor belonged to ‘clade X’ within the order Glissomonadida (Cercozoa, Rhizaria). In conclusion, we established the phenotypic identity of a clade, which until now was exclusively known from environmental sequences, and erect the new family Viridiraptoridae for ‘clade X’. Its algivorous members are compared with other glissomonads and nomenclatural, methodological and ecological aspects of these novel ‘raptorial’ amoeboflagellates are discussed. [Copyright &y& Elsevier]

Abstract: The genus Pfiesteria includes two toxigenic species, Pfiesteria piscicida and Pfiesteria shumwayae, that are thinly thecate dinoflagellates with apparently cosmopolitan distribution, especially in shallow, poorly flushed, eutrophic estuaries. They are heterotrophic prey generalists that typically feed via phagotrophy and prefer live fish or their fresh tissues as food. They can also engage in limited mixotrophy through temporary retention of kleptochloroplasts from algal prey. Toxicity is highly variable among strains, ranging from apparently nontoxic to highly toxic. Some strains produce a group of hydrophilic toxins with metal-mediated free radical production. Various metals can be involved in the toxin congeners, and the purified toxins are highly labile. These toxins can adversely affect mammalian cells as well as fish. Toxic strains are capable of killing fish by both toxins and physical attack from feeding upon epidermis and other tissues. Non-inducible strains do not produce sufficient toxin to kill fish, but some are capable of causing larval fish death by physical attack. From 1991 to 1998, Pfiesteria spp. were linked to major kills of juvenile Atlantic menhaden (Brevoortia tyrannus), mostly at densities of ≥4(3)×102 to 103 (rarely, 104)flagellatecellsmL−1. These kills mainly occurred in the second largest and largest estuaries on the U.S. mainland, especially two main tributaries of the Albemarle-Pamlico Estuarine System, following decades of hurricane-free conditions. Between kills, Pfiesteria abundance was low in surface waters (<10cellsmL−1), and the available evidence suggests that the populations were mostly in the lower water column and within surficial sediments. Apparently highly sensitive to scouring effects from major storms, Pfiesteria populations have been sparse in the affected estuaries since several hurricanes struck the Albemarle-Pamlico in the late 1990s. Recent research highlights include characterization of a novel group of Pfiesteria toxins, culture of a toxigenic strain on a sterile fish cell line, axenic culture on a semi-defined medium, the discovery of a new mode of heterotrophic feeding in dinoflagellates as manifested by Pfiesteria, and other advances in understanding the nutritional ecology and prey acquisition of these harmful dinoflagellates. [Copyright &y& Elsevier]

We studied the phagotrophic ability of dinoflagellate strain Karenia mikimotoi KM-Lü (isolated from the South China Sea), using fluorescent microspheres, bacteria isolated from the culture of K. mikimotoi and a marine microalgae Isochrysis galbana. We found that K. mikimotoi cultured under conditions of high light intensity could ingest fluorescent microspheres (diameters 0.5 and 2.0 μm) and fluorescence-labeled bacteria and microalgae. Under a low light intensity, however, only fluorescent microspheres (diameter 0.5 μm) and fluorescence-labeled microalgae were ingested. K. mikimotoi showed better growth by ingesting living marine bacteria or microalgae I. galbana than the controls, either in nutrient-depleted or nutrient-replete conditions. In nutrient-depleted conditions, the growth of K. mikimotoi was more significant with I. galbana as the prey item. In conclusion, the harmful dinoflagellate K. mikimotoi from the South China Sea has apparent phagotrophic ability, and some marine bacteria and microalgae may promote the growth of K. mikimotoi. [ABSTRACT FROM AUTHOR]

Abstract: A growing body of data suggests that fungi have gained genes by horizontal gene transfer (HGT). This is an exciting result because fungi at first glance represent the most recalcitrant of all organisms to gene transfer, possessing robust cell walls and having lost phagotrophic capacities because they feed exclusively by osmotrophy. Nonetheless, a number of mechanisms have been implicated in gene transfer including: anastomosis of cellular structures, conjugation-like transfer between bacteria and yeasts, and exchange of supernumerary chromosomes. Despite absence of clearly identified mechanisms driving gene transfer in fungi, genome analysis has provided evidence for a number of fungal genes derived from foreign genomes by HGT. We briefly summarise current approaches to identifying HGT using genome data and make the case that phylogenetic analysis is the best approach to find and test potential examples of HGT. By applying this approach we have collected as many datasets as we could find for which phylogenetic analyses have been used as evidence of HGT and re-tested all 340 examples using updated taxon sampling. This approach enabled us to provide further supporting evidence for 323 examples of HGT, representing a significant pattern of transfer from both prokaryotes (mainly bacteria) and fungi into fungal genomes. Annotation of the HGTs suggests that these transfers have added to the core nutrient-processing metabolic network of many fungi, expanding the sugar, nitrogen, amino acid, nucleobase, and macromolecule metabolism of fungal microbes. Furthermore, these transfers appear to have added a significant number of new genes to the secretome and transporter repertoire of fungi, implying that gene transfer has added to the osmotrophic capacity of many fungal species. [Copyright &y& Elsevier]

Abstract: Heterotrophic bacteria, cyanobacteria, and picoeukaryotic algae dominate the plankton community of high nutrient-low chlorophyll (HNLC) areas of the eastern equatorial Pacific (EEP). While grazing on these picoplankton is often attributed to aplastidic zooflagellates, mixotrophic nanoflagellates (phagotrophic phototrophs) may also exert a large grazing pressure. We assessed the relative contributions of mixotrophic nanoplankton and obligate heterotrophs to picoplankton phagotrophy in mixed-layer water of the EEP using 0.8-μm Fluorescently-Labeled Bacteria (FLB). Obligate heterotrophs and phototrophs were distinguished from their ratios of microscopically measured red (chlorophyll a) to green (proflavin-stained protein) fluorescence. Sampling sites were located along a nutrient gradient formed by a tropical instability wave at 0.5°N between 123.5°W and 128°W and at 1.75°N, 125°W. The majority of ingested particles were found within 3-5μm flagellates, with 54% of the demonstrated phagotrophs belonging to the high-pigment putatively phototrophic population and obligate heterotrophs responsible for 51% of the demonstrated phagotrophy due to their greater propensity to ingest multiple prey. The importance of mixotrophy as a means of alleviating nutrient stress is indicated by a strong inverse relationship between the proportion of community FLB uptake by mixotrophs and ambient nutrient concentration. Low ambient Fe concentration and a demonstrated community response to Fe-addition in shipboard grow-out experiments suggest that mixotrophs were primarily engaging in phagotrophy to offset Fe-deficiencies. [Copyright &y& Elsevier]

Lindehoff, Elin, Edna Granéli, and Patricia M. Glibert, 2010. Influence of Prey and Nutritional Status on the Rate of Nitrogen Uptake by Prymnesium parvum (Haptophyte). Journal of the American Water Resources Association (JAWRA) 46(1):121-132. DOI: 10.1111/j.1752-1688.2009.00396.x We studied how the specific nitrogen (N) uptake rates of nitrate (NO3−), urea, and the amino acids, glutamic acid and glycine, by Prymnesium parvum were affected by (1) the change from N-deficient status to N-sufficient status of the P. parvum cells, (2) presence of prey from a natural Baltic Sea plankton community, and (3) the composition of prey as affected by additions of terrestrial originated dissolved organic matter (DOM) or inorganic nutrients. Nitrogen-deficient P. parvum (16 μM NO3− and 4 μM PO4−, molar N:P ratio of 4:1) were mixed with a natural Baltic plankton community and given PO43− and (1) NO3− (control) or (2) high molecular weight DOM, >1 kDa concentrated from sewage effluent (+DOM), in a molar N:P ratio of 9-10:1. With additions of 15N-enriched substrates, rates of N uptake from NO3−, urea, and the amino acids glycine and glutamic acid were measured every 24 h for 72 h. Initial N-deficient P. parvum were highly toxic (3.7 ± 0.9 × 10−4 mg Sap equiv/cell) and toxic allelochemicals were released into the medium causing the natural plankton community to lyse. Rates of N uptake differed between the “control” and the “+DOM” treatments over time; total (sum of the N substrates measured) absolute uptake rates (ρcell, fmol N/cell/h) at ambient culture conditions were significantly higher (ANOVA, p < 0.05) in the more toxic “control” treatments compared with the “+DOM” treatments after 48 h. In the “control” treatment, the total ρcell increased significantly (ANOVA, p < 0.01) from time 0 to 48 h, while in the “+DOM” treatment there was no significant increase. Released organic nutrients from the lysed plankton cells may have increased uptake rates of amino acids and urea by P. parvum. All uptake rates declined in all treatments by 72 h. Total dissolved N uptake rates at ambient culture conditions were estimated to make up about 10% of the N P. parvum are potentially capable of ingesting from particulate prey. [ABSTRACT FROM AUTHOR]