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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]

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]

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]

Pfiesteria shumwayae Glasgow et J. M. Burkh. [= Pseudopfiesteria shumwayae (Glasgow et J. M. Burkh.) Litaker, Steid., P. L. Mason, Shields et P. A. Tester] is a heterotrophic dinoflagellate commonly found in temperate, estuarine waters. P. shumwayae can feed on other protists, fish, and invertebrates, but research on the biochemical requirements of this species has been restricted by the lack of axenic cultures. An undefined, biphasic culture medium was formulated that supported the axenic growth of two of three strains of P. shumwayae. The medium contained chicken egg yolk as a major component. Successful growth depended on the method used to sterilize the medium, and maximum cell yields (104 · mL−1) were similar to those attained in previous research when P. shumwayae was cultured with living fish or microalgae. Additionally, P. shumwayae flagellate cells ingested particles present in the biphasic medium, allowing detailed observations of feeding behavior. This research is an initial step toward a chemically defined axenic culture medium and determination of P. shumwayae metabolic requirements. [ABSTRACT FROM AUTHOR]

A small, freshwater dinoflagellate with an incomplete cingulum, identified as Esoptrodinium gemma Javornický (= Bernardinium bernardinense sensu auctt. non sensu Chodat), was maintained in mixed culture and examined using light and serial section TEM. Vegetative flagellate cells, large cells with two longitudinal flagella (planozygotes), and cysts were examined. The cells displayed a red eyespot near the base of the longitudinal flagellum, made of two or three layers of pigment globules not bounded by a membrane. Yellow-green, band-shaped chloroplasts, bounded by three membranes and containing lamella with three thylakoids, were present in both flagellate cells and cysts. Most cells had food vacuoles, containing phagotrophically ingested chlamydomonads or chlorelloid green algae; ingestion occurred through the ventral area, involving a thin pseudopod apparently driven by the peduncle. The pusule was tubular, with numerous diverticula in its distal portion, and opened into the longitudinal flagellar canal. Three roots were associated with each pair of flagellar bases, both in vegetative cells and in a planozygote. The longitudinal microtubular root bifurcated around the longitudinal basal body. The planozygote contained a single peduncle and associated structures, and a single transverse flagellar canal with the two converging transverse flagella. Using two ciliates as outgroup species, phylogenetic analyses based on maximum parsimony, neighbor-joining and posterior probability (Bayesian analysis) supported a clade comprising Esoptrodinium, Tovellia, and Jadwigia. [ABSTRACT FROM AUTHOR]

Cryptoperidiniopsoids are an unclassified group of delicately thecate heterotrophic dinoflagellates known to be common in eastern U.S. estuarine waters. Over the past 10 years cryptoperidiniopsoids were isolated from different geographical regions and cultured with cryptophyte algal prey. In the seven clonal isolates examined, reproduction was strongly linked to the availability of prey cells. The dinoflagellates phagocytized the contents of prey cells through a tube-like peduncle, similarly as close relatives of Pfiesteria spp. and several other heterotrophic species. Cell division occurred while encysted, most commonly yielding two biflagellated offspring. Abundant fusing gametes, phagotrophic planozygotes, and cysts with a pronounced nuclear cyclosis characterized persistent sexuality. Cysts with nuclear cyclosis produced two flagellated offspring cells. The resistance of reproductive cysts to antimicrobial treatments was examined, and a simple high-yield technique was developed for population synchronization while ridding the dinoflagellates of most contaminating vacuolar prey DNA and external contaminants. The DNA content and population DNA profiles of synchronously excysted cryptoperidiniopsoids from different isolates were measured using flow cytometry and were related to the life history of these and other dinoflagellates. Cryptophyte-fed cultures with versus without extracellular bacteria were compared, and bacteria apparently promoted cryptoperidiniopsoid feeding and growth. Externally bacteria-free dinoflagellates were cultured in media enriched with dissolved organic nutrients, and nutritional benefit may have occurred in some treatments. The potential for mixotrophic nutrition from maintenance of cryptophyte chloroplasts was examined using flow cytometrically sorted cells, but evidence of kleptoplastidy was not found in these isolates under the conditions imposed. [ABSTRACT FROM AUTHOR]

Pfiesteria shumwayae is a heterotrophic dinoflagellate with a widespread distribution in temperate-subtropical estuarine waters. In this study, five clonal isolates from the eastern coast of North America, one from New Zealand, and a mixed composite of clones were cultured in aquaria and fed live fish. Division, sexuality, and phagotrophic feeding on fish were studied by LM, SEM, and flow cytometry. The development of reproductive cysts isolated from aquaria was followed. Synchronously excysted flagellate populations were examined for sexuality and then for feeding behavior and reproduction when given larval fish. Reproductive cysts varied in size and underwent protoplast division(s), most commonly producing two to eight biflagellated offspring. Fusing gametes, resulting planozygotes, and nuclear cyclosis were documented as evidence of sexuality. Gametes emerged from cysts, and fusions were approximately isogamous. Resulting planozygotes had two longitudinal flagella and one transverse flagellum and apparently fed before encysting. Distinct and lengthy chromosome movements (nuclear cyclosis) occurred in presumed zygotic cysts before nuclear division(s). These cysts did not exhibit dormancy in growing cultures and produced two or four biflagellated offspring. Flagellated cells fed on surficial fish tissues and then encysted for reproduction. Stages indicating a completed sexual cycle (fusion, planozygotes, and nuclear cyclosis) were uncommon or absent in clonal cultures but were relatively abundant in the mixed clone culture. Self-sterility factors apparently influenced sexuality. Starved populations formed quiescent cysts that released swimming cells when food was provided. Pfiesteria shumwayae was similar in reproduction and sexuality to closely related species. [ABSTRACT FROM AUTHOR]

Cryptoperidiniopsoids are an unclassified group of delicately thecate heterotrophic dinoflagellates known to be common in eastern U.S. estuarine waters. Over the past 10 years cryptoperidiniopsoids were isolated from different geographical regions and cultured with cryptophyte algal prey. In the seven clonal isolates examined, reproduction was strongly linked to the availability of prey cells. The dinoflagellates phagocytized the contents of prey cells through a tube-like peduncle, similarly as close relatives of Pfiesteria spp. and several other heterotrophic species. Cell division occurred while encysted, most commonly yielding two biflagellated offspring. Abundant fusing gametes, phagotrophic planozygotes, and cysts with a pronounced nuclear cyclosis characterized persistent sexuality. Cysts with nuclear cyclosis produced two flagellated offspring cells. The resistance of reproductive cysts to antimicrobial treatments was examined, and a simple high-yield technique was developed for population synchronization while ridding the dinoflagellates of most contaminating vacuolar prey DNA and external contaminants. The DNA content and population DNA profiles of synchronously excysted cryptoperidiniopsoids from different isolates were measured using flow cytometry and were related to the life history of these and other dinoflagellates. Cryptophyte-fed cultures with versus without extracellular bacteria were compared, and bacteria apparently promoted cryptoperidiniopsoid feeding and growth. Externally bacteria-free dinoflagellates were cultured in media enriched with dissolved organic nutrients, and nutritional benefit may have occurred in some treatments. The potential for mixotrophic nutrition from maintenance of cryptophyte chloroplasts was examined using flow cytometrically sorted cells, but evidence of kleptoplastidy was not found in these isolates under the conditions imposed. [ABSTRACT FROM AUTHOR]

Pfiesteria shumwayae is a heterotrophic dinoflagellate with a widespread distribution in temperate-subtropical estuarine waters. In this study, five clonal isolates from the eastern coast of North America, one from New Zealand, and a mixed composite of clones were cultured in aquaria and fed live fish. Division, sexuality, and phagotrophic feeding on fish were studied by LM, SEM, and flow cytometry. The development of reproductive cysts isolated from aquaria was followed. Synchronously excysted flagellate populations were examined for sexuality and then for feeding behavior and reproduction when given larval fish. Reproductive cysts varied in size and underwent protoplast division(s), most commonly producing two to eight biflagellated offspring. Fusing gametes, resulting planozygotes, and nuclear cyclosis were documented as evidence of sexuality. Gametes emerged from cysts, and fusions were approximately isogamous. Resulting planozygotes had two longitudinal flagella and one transverse flagellum and apparently fed before encysting. Distinct and lengthy chromosome movements (nuclear cyclosis) occurred in presumed zygotic cysts before nuclear division(s). These cysts did not exhibit dormancy in growing cultures and produced two or four biflagellated offspring. Flagellated cells fed on surficial fish tissues and then encysted for reproduction. Stages indicating a completed sexual cycle (fusion, planozygotes, and nuclear cyclosis) were uncommon or absent in clonal cultures but were relatively abundant in the mixed clone culture. Self-sterility factors apparently influenced sexuality. Starved populations formed quiescent cysts that released swimming cells when food was provided. Pfiesteria shumwayae was similar in reproduction and sexuality to closely related species. [ABSTRACT FROM AUTHOR]

The growth and grazing characteristics of Poterioochromonas malhamensis (Pringsheim) Peterfi (= Ochromonas malhamensis Pringsheim) (ca. 8 μm) feeding on phytoplankton, including the cyanobacteria Synechococcus sp. (ca. 2 μm) and Microcystis viridis (A. Brown) Lemmermann (ca. 6 μm) and the green alga Chlorella pyrenoidosa Chick (ca. 13 μm), were investigated in laboratory experiments involving the following treatments: (1) light without added algal prey (autotrophy), (2) light with added algal prey (mixotrophy), and (3) dark with added algal prey (phagotrophy). There were significantly higher cell numbers under mixotrophic and phagotrophic growth than under autotrophic growth. With phytoplankton as food, growth rates under both mixotrophy and phagotrophy were about two or three times higher than those under autotrophy, indicating that the algal diets were readily able to support the population growth of P. malhamensis. There were no significant differences in growth rate between mixotrophic and phagotrophic cultures during exponential growth. The ingestion rate of P. malhamensis with algal prey was also similar under both continuous light and dark. Poterioochromonas malhamensis ingested on average 0.27 M. viridis cells·flagellate-1·h-1 and 0.18 C. pyrenoidosa cells·flagellate-1·h-1 in continuous light and 0.25 M. viridis cells·flagellate-1·h-1 and 0.18 C. pyrenoidosa cells·flagellate-1·h-1 in continuous dark during exponential growth. The results showed that light had no effect on the growth and ingestion rates of P. malhamensis for phagotrophy during exponential growth. However, phagotrophic populations of P. malhamensis were incapable of growth in continuous darkness for longer than 5 days. Populations of P. malhamensis showed no increase when prey was added again after 4 days in continuous darkness, indicating that... [ABSTRACT FROM AUTHOR]

The marine dinoflagellate Gyrodinium resplendens Hulburt is a mixotroph. It possesses chloroplasts and is photosynthetic, and it also feeds phagotrophically on another dinoflagellate, Prorocentrum minimum (Pavillard) Schiller. The species could be cultivated only in food-replete cultures. When kept in cultures without food, cellular chl a content and photosynthetic activity of G. resplendens decreased and growth ceased after a few days. In food-replete cultures, G. resplendens could grow strictly heterotrophically in darkness, but growth rate was then three times lower than in food-replete cultures kept in light. It is suggested that the main importance of phagotrophy is to acquire a growth factor essential to photosynthetic growth. The addition of soil extract or amino acids to the growth medium induced enhanced photosynthetic growth of the species even without the presence of particulate food, but only for approximately 2 weeks. Long-term starvation of G. resplendens led to loss of the ability to feed, and therefore starved cells eventually reached a point of no return where neither photosynthesis nor phagotrophy could sustain further growth. Light microscopical observations on G. resplendens revealed new morphological and behavioral details of the species. [ABSTRACT FROM AUTHOR]

Feeding in Peranema trichophorum (Ehrenberg 1838) Stein 1878 was first observed over a century ago, yet there is still contention over how the feeding apparatus is used in feeding. Using video microscopy and scanning microscopy, this study documents two types of feedings. Peranema may engulf prey cells whole. Immotile cells are preferred, but moving cells occasionally are engulfed. Details of the early stages of engulfment are presented using scanning electron microscopy. A second method of feeding begins with the attachment of Peranema to a prey cell. The rods of the feeding apparatus then are repeatedly scaped over the surface of the prey until a tear occurs in the cell. The feeding apparatus is inserted into the opening and the internal cell contents are sucked out. The anterior flagellum is inserted into the prey to help remove the cell's contents. [ABSTRACT FROM AUTHOR]

Gymnodinium acidotum Nygaard, a blue-green dinoflagellate previously shown to contain cryptophycean chloroplasts and other organelles, was observed from water and soil samples and in culture. The dinoflagellate excysts from soil samples as a mononucleated colorless cell that is positively phototactic. Colorless cell in unialgal culture remain colorless and can only be maintained less than one week whereas pigmented cells cultured unialgally grow for 10 days but then decline rapidly. Colorless cells cultured with Chroomonas spp. regain chloroplasts and have been maintained in mixed culture for nine months. Fifty-seven percent of the dinoflagellates from mixed cultures are binucleated, and three cells have been observed possibly ingesting cryptophytes. We suggest that cryptophycean chloroplasts are retained and possibly utilized by G. acidotum for at least ten days and the digested. [ABSTRACT FROM AUTHOR]

Two morphologically distinct species of free-swinming dinoflagellates belonging to the genus Gyrodinium utilize the spine and rhizopodial environments of planktonic foraminifera and colonial radiolaria as microhabitats. Up to 84% of the sarcodines examined in a given population were associated with these dinoflagellate species possess choloroplasts, indicating they are capable of autotrophy. [SUP14]C-labelling experiments with the radiolarian-associated dinoflagellate demonstrate that it can take up inorganic carbon under both light and dark conditions. Ultrastructural evidence suggests the foraminiferal dinoflagellate may be capable of phagotrophy. Hence, these algae should be considered mixotrophs. An possibly feeding occurs in the foraminiferal-associated Gyrodinium and is documented with electron microscopy. Ultrastructural examination suggests this organelle may be hydrostatically controlled and may be an extension of the sac pusule. [ABSTRACT FROM AUTHOR]