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Corals have complex symbiotic associations that can be influenced by the environment. We compare symbiotic dinoflagellate (family: Symbiodiniaceae) associations and the microbiome of five scleractinian coral species from three different reef habitats in Palau, Micronesia. Although pH and temperature corresponded with specific host‐Symbiodiniaceae associations common to the nearshore and offshore habitats, bacterial community dissimilarity analyses indicated minimal influence of these factors on microbial community membership for the corals Coelastrea aspera, Psammocora digitata, and Pachyseris rugosa. However, coral colonies sampled close to human development exhibited greater differences in microbial community diversity compared to the nearshore habitat for the coral species Coelastrea aspera, Montipora foliosa, and Pocillopora acuta, and the offshore habitat for Coelastrea aspera, while also showing less consistency in Symbiodiniaceae associations. These findings indicate the influence that habitat location has on the bacterial and Symbiodiniaceae communities comprising the coral holobiont and provide important considerations for the conservation of coral reef communities, especially for island nations with increasing human populations and development. [ABSTRACT FROM AUTHOR]

The existence of widespread species with the capacity to endure diverse, or variable, environments are of importance to ecological and genetic research, and conservation. Such "ecological generalists" are more likely to have key adaptations that allow them to better tolerate the physiological challenges of rapid climate change. Reef‐building corals are dependent on endosymbiotic dinoflagellates (Family: Symbiodiniaceae) for their survival and growth. While these symbionts are biologically diverse, certain genetic types appear to have broad geographic distributions and are mutualistic with various host species from multiple genera and families in the order Scleractinia that must acquire their symbionts through horizontal transmission. Despite the considerable ecological importance of putative host‐generalist symbionts, they lack formal species descriptions. In this study, we used molecular, ecological, and morphological evidence to verify the existence of five new host‐generalist species in the symbiodiniacean genus Cladocopium. Their geographic distribution and prevalence among host communities corresponds to prevailing environmental conditions at both regional and local scales. The influence that each species has on host physiology may partially explain regional differences in thermal sensitivities among coral communities. The potential increased prevalence of a generalist species that endures environmental instability is a consequential ecological response to warming oceans. Large‐scale shifts in symbiont dominance could ensure reef coral persistence and productivity in the near term. Ultimately, these formal designations should advance scientific communication and generate informed research questions on the physiology and ecology of coral‐dinoflagellate mutualisms. [ABSTRACT FROM AUTHOR]

Ocean warming is increasingly affecting marine ecosystems across the globe. Reef‐building corals are particularly affected by warming, with mass bleaching events increasing in frequency and leading to widespread coral mortality. Yet, some corals can resist or recover from bleaching better than others. Such variability in thermal resilience could be critical to reef persistence; however, the scientific community lacks standardized diagnostic approaches to rapidly and comparatively assess coral thermal vulnerability prior to bleaching events. We present the Coral Bleaching Automated Stress System (CBASS) as a low‐cost, open‐source, field‐portable experimental system for rapid empirical assessment of coral thermal thresholds using standardized temperature stress profiles and diagnostics. The CBASS consists of four or eight flow‐through experimental aquaria with independent water masses, lighting, and individual automated temperature controls capable of delivering custom modulating thermal profiles. The CBASS is used to conduct daily thermal stress exposures that typically include 3‐h temperature ramps to multiple target temperatures, a 3‐h hold period at the target temperatures, and a 1‐h ramp back down to ambient temperature, followed by an overnight recovery period. This mimics shallow water temperature profiles observed in coral reefs and prompts a rapid acute heat stress response that can serve as a diagnostic tool to identify putative thermotolerant corals for in‐depth assessments of adaptation mechanisms, targeted conservation, and possible use in restoration efforts. The CBASS is deployable within hours and can assay up to 40 coral fragments/aquaria/day, enabling high‐throughput, rapid determination of thermal thresholds for individual genotypes, populations, species, and sites using a standardized experimental framework. [ABSTRACT FROM AUTHOR]

The global impacts of climate change are evident in every marine ecosystem. On coral reefs, mass coral bleaching and mortality have emerged as ubiquitous responses to ocean warming, yet one of the greatest challenges of this epiphenomenon is linking information across scientific disciplines and spatial and temporal scales. Here we review some of the seminal and recent coral-bleaching discoveries from an ecological, physiological, and molecular perspective. We also evaluate which data and processes can improve predictive models and provide a conceptual framework that integrates measurements across biological scales. Taking an integrative approach across biological and spatial scales, using for example hierarchical models to estimate major coral-reef processes, will not only rapidly advance coral-reef science but will also provide necessary information to guide decision-making and conservation efforts. To conserve reefs, we encourage implementing mesoscale sanctuaries (thousands of km2) that transcend national boundaries. Such networks of protected reefs will provide reef connectivity, through larval dispersal that transverse thermal environments, and genotypic repositories that may become essential units of selection for environmentally diverse locations. Together, multinational networks may be the best chance corals have to persist through climate change, while humanity struggles to reduce emissions of greenhouse gases to net zero. [ABSTRACT FROM AUTHOR]

Active chlorophyll a fluorometry is a well‐established tool for noninvasively diagnosing coral functional state, but has not yet been developed as a rapid phenotyping (functional screening) platform as for agriculture and forestry. Here, we present a proof‐of‐concept using Light‐Induced Fluorescence Transient‐Fast Repetition Rate fluorometry (LIFT‐FRRf) to identify coral photobiological‐based phenotypes in the context of rapidly scaling coral propagation practices on the northern Great Barrier Reef. For example, resolving light niche plasticity to inform transplantation, and identifying functionally diverse colonies to maximize stock selection. We first used optically diverse laboratory‐reared corals and coral endosymbiont (Symbiodiniaceae) isolates to develop a phenotyping approach integrating FRRf instantaneous kinetic parameters (light harvesting, electron turnover rates) and light‐dependent parameters (dynamic "quenching" terms, saturating light intensity [EK]). Subsequent field‐based LIFT‐FRRf phenotyping of coral from a selective (2‐4 m depth) reef habitat revealed that widely topographically dispersed plating Acropora taxa exhibited broad light niche plasticity (EK variance) underpinned by multiple phenotypes that were predominantly differentiated by minimum electron turnover capacity; fluorometer configurations that cannot resolve kinetic parameters will thus likely have more limited capacity to resolve phenotypes. As such, plating Acropora have broad propagation potential in terms of multiple functional variants for stock and across diverse light environments (growth, transplantation). In contrast, coral taxa (Pocillopora verrucosa, Echinopora lamellosa) with relatively restricted topographic dispersion exhibited less light niche plasticity and only single phenotypes, thereby imposing more constraints for propagation. We discuss the core technical, operational, and conceptual steps required to develop more sophisticated coral phenotyping platforms. [ABSTRACT FROM AUTHOR]

Reef‐building corals in the genus Porites are one of the most important constituents of Indo‐Pacific reefs. Many species within this genus tolerate abnormally warm water and exhibit high specificity for particular kinds of endosymbiotic dinoflagellates that cope with thermal stress better than those living in other corals. Still, during extreme ocean heating, some Porites exhibit differences in their stress tolerance. While corals have different physiological qualities, it remains unknown whether the stability and performance of these mutualisms is influenced by the physiology and genetic relatedness of their symbionts. We investigated two ubiquitous Pacific reef corals, Porites rus and Porites cylindrica, from warmer inshore and cooler offshore reef systems in Palau. While these corals harbored a similar kind of symbiont in the genus Cladocopium (within the ITS2 C15 subclade), rapidly evolving genetic markers revealed evolutionarily diverged lineages corresponding to each Porites species living in each reef habitat. Furthermore, these closely related Cladocopium lineages were differentiated by their densities in host tissues, cell volume, chlorophyll concentration, gross photosynthesis, and photoprotective pathways. When assessed using several physiological proxies, these previously undifferentiated symbionts contrasted in their tolerance to thermal stress. Symbionts within P. cylindrica were relatively unaffected by exposure to 32℃ for 14 days, whereas P. rus colonies lost substantial numbers of photochemically compromised symbionts. Heating reduced the ability of the offshore symbiont associated with P. rus to translocate carbon to the coral. By contrast, high temperatures enhanced symbiont carbon assimilation and delivery to the coral skeleton of inshore P. cylindrica. This study indicates that large physiological differences exist even among closely related symbionts, with significant implications for thermal susceptibility among reef‐building Porites. [ABSTRACT FROM AUTHOR]

Dinoflagellates within the family Symbiodiniaceae are widespread and fuel metabolism of reef‐forming corals through photosynthesis. Adaptation in capacity to harvest and utilize light, and "safely" process photosynthetically generated energy is a key factor regulating their broad ecological success. However, whether such adaptive capacity similarly extends to how Symbiodiniaceae species and genotypes assimilate inorganic carbon (Ci) remains unexplored. We build on recent approaches exploring functional diversity of fitness traits to identify whether Ci uptake and incorporation could be reconciled with evolutionary adaptation among Symbiodiniaceae. We examined phylogenetically diverse Symbiodiniaceae cultures (23 isolates, 6 genera) to track how carbon was invested into cellular uptake, excretion, and growth (cell size, division, storage). Gross carbon uptake rates (GPC) over 1 h varied among isolates grown at 26°C (0.63–3.08 pg C [cell h]−1) with no evident pattern with algal phylogeny. Intriguingly, net carbon uptake rates (24 h) were often higher (1.01–5.54 pg C [cell h]−1) than corresponding values of GPC—we discuss how such GPC measurements may reflect highly conserved biological characteristics for cultured cells linked to high metabolic dependency on photorespiration and heterotrophy. Three isolates from different genera (Cladocopium goreaui, Durusdinium trenchii, and Effrenium voratum) were additionally grown at 20°C and 30°C. Here, Ci uptake consistently decreased with temperature‐driven declines in growth rate, suggesting environmental regulation outweighs phylogenetic organization of carbon assimilation capacity among Symbiodiniaceae. Together, these data demonstrate environmental regulation and ecological success among Symbiodiniaceae likely rests on plasticity of upstream photosynthetic processes (light harvesting, energy quenching, etc.) to overcome evolutionary‐conserved limitations in Ci functioning. [ABSTRACT FROM AUTHOR]

Symbioses between cnidarians and symbiotic dinoflagellates ( Symbiodinium) are ecologically important and physiologically diverse. This diversity contributes to the spatial distribution of specific cnidarian- Symbiodinium associations. Physiological variability also exists within Symbiodinium species, yet we know little regarding its relevance for the establishment of symbiosis under different environmental conditions. Two putatively conspecific Symbiodinium strains (both ITS2-type A4) were isolated from the sea anemone Exaiptasia pallida and placed into unialgal culture. Thermal tolerance of these cultures was compared following heating from 26°C to 33.5°C over 18 days. Photosystem II function was negatively impacted by heating in one strain while PSII function in the other showed little response to elevated temperature. Additionally, infection of Symbiodinium cells into aposymbiotic anemones was assessed for both strains at 26°C and 30.5°C. The heat-sensitive strain had greater infection success at 26°C, while there was no difference in infection between the two strains at the higher temperature. Results from this work suggest that variability in thermal optima or -tolerance within Symbiodinium spp. has relevance for early stages of host- Symbiodinium interactions. Thus, varying infectiousness among differentially heat-sensitive Symbiodinium strains could provide a mechanism for the emergence of novel and potentially resilient cnidarian- Symbiodinium associations in a rapidly warming environment. [ABSTRACT FROM AUTHOR]

Here we report the effects of elevated pCO2 on the model symbiotic anemone Exaiptasia pallida and how its association with three different strains of the endosymbiotic dinoflagellate Symbiodinium minutum (ITS2-type B1) affects its response. Exposure to elevated pCO2 (70.9 Pa) for 28 d led to an increased effective quantum yield of PSII in actinic light within two of the alga-anemone combinations. Autotrophic carbon fixation, along with the rate of carbon translocated to the animal, were significantly elevated with high pCO2. Elevated pCO2 exposure also coincided with significantly greater asexual budding rates in all tested anemones. Further, differences in photochemistry and carbon translocation rates suggest subtle differences in the response to pCO2 among the three strains of S. minutum and their host anemones. This illustrates the potential for physiological diversity at the subspecies level for this ecologically important dinoflagellate. Positive alterations in photosynthesis, carbon utilization, and fitness within this model symbiosis suggest a potential benefit from ocean acidification (OA) not yet observed within corals, which may enable these anthozoans to gain a greater ecological presence under future OA conditions. [ABSTRACT FROM AUTHOR]

Dinoflagellates of the genus Symbiodinium express broad diversity in both genetic identity (phylogeny) and photosynthetic function to presumably optimize ecological success across extreme light environments; however, whether differences in the primary photobiological characteristics that govern photosynthetic optimization are ultimately a function of phylogeny is entirely unresolved., We applied a novel fast repetition rate fluorometry approach to screen genetically distinct Symbiodinium types ( n = 18) spanning five clades (A-D, F) for potential phylogenetic trends in factors modulating light absorption (effective cross-section, reaction center content) and utilization (photochemical vs dynamic nonphotochemical quenching; [1 - C] vs [1 - Q]) by photosystem II ( PSII)., The variability of PSII light absorption was independent of phylogenetic designation, but closely correlated with cell size across types, whereas PSII light utilization intriguingly followed one of three characteristic patterns: (1) similar reliance on [1 - C] and [1 - Q] or (2) preferential reliance on [1 - C] (mostly A, B types) vs (3) preferential reliance on [1 - Q] (mostly C, D, F types), and thus generally consistent with cladal designation., Our functional trait-based approach shows, for the first time, how Symbiodinium photosynthetic function is governed by the interplay between phylogenetically dependent and independent traits, and is potentially a means to reconcile complex biogeographic patterns of Symbiodinium phylogenetic diversity in nature. [ABSTRACT FROM AUTHOR]

Mass coral bleaching events caused by elevated seawater temperatures result in extensive coral loss throughout the tropics, and are projected to increase in frequency and severity. If bleaching becomes an annual event later in this century, more than 90% of coral reefs worldwide may be at risk of long-term degradation. While corals can recover from single isolated bleaching and can acclimate to recurring bleaching events that are separated by multiple years, it is currently unknown if and how they will survive and possibly acclimatize to annual coral bleaching. Here, we demonstrate for the first time that annual coral bleaching can dramatically alter thermal tolerance in Caribbean corals. We found that high coral energy reserves and changes in the dominant algal endosymbiont type ( Symbiodinium spp.) facilitated rapid acclimation in Porites divaricata, whereas low energy reserves and a lack of algal phenotypic plasticity significantly increased susceptibility in Porites astreoides to bleaching the following year. Phenotypic plasticity in the dominant endosymbiont type of Orbicella faveolata did not prevent repeat bleaching, but may have facilitated rapid recovery. Thus, coral holobiont response to an isolated single bleaching event is not an accurate predictor of its response to bleaching the following year. Rather, the cumulative impact of annual coral bleaching can turn some coral species 'winners' into 'losers', and can also facilitate acclimation and turn some coral species 'losers' into 'winners'. Overall, these findings indicate that cumulative impact of annual coral bleaching could result in some species becoming increasingly susceptible to bleaching and face a long-term decline, while phenotypically plastic coral species will acclimatize and persist. Thus, annual coral bleaching and recovery could contribute to the selective loss of coral diversity as well as the overall decline of coral reefs in the Caribbean. [ABSTRACT FROM AUTHOR]

Growing concerns over ocean acidification have highlighted the need to critically understand inorganic carbon acquisition and utilization in marine microalgae. Here, we contrast these characteristics for the first time between two genetically distinct dinoflagellate species of the genus Symbiodinium (phylotypes A13 and A20) that live in symbiosis with reef-forming corals., Both phylotypes were grown in continuous cultures under identical environmental conditions. Rubisco was measured using quantitative Western blots, and radioisotopic 14C uptake was used to characterize light- and total carbon dioxide (TCO2)-dependent carbon fixation, as well as inorganic carbon species preference and external carbonic anhydrase activity., A13 and A20 exhibited similar rates of carbon fixation despite cellular concentrations of Rubisco being approximately four-fold greater in A13. The uptake of CO2 over [ABSTRACT FROM AUTHOR]

Microalgae possess numerous cellular mechanisms specifically employed for acclimating the photosynthetic pathways to changes in the physical environment. Despite the importance of coral-dinoflagellate symbioses, little focus has been given as to how the symbiotic algae ( Symbiodinium spp.) regulate the expression of their photosynthetic genes. This study used real-time PCR to investigate the transcript abundance of the plastid-encoded genes, psbA (encoding the D1 protein of photosystem II) and psaA (encoding the P700 protein in photosystem I), within the cultured Symbiodinium ITS-2 (internal transcribed spacer region) types A20 and A13. Transcript abundance was monitored during a low to high-light shift, as well as over a full diel light cycle. In addition, psaA was characterized in three isolates (A20, A13, and D4-5) and noted as another example of a dinoflagellate plastid gene encoded on a minicircle. In general, the overall incongruence of transcript patterns for both psbA and psaA between the Symbiodinium isolates and other models of transcriptionally controlled chloroplast gene expression (e.g., Pisum sativum [pea], Sinapis alba [mustard seedling], and Synechocystis sp. PCC 6803 [cyanobacteria]) suggests that Symbiodinium is reliant on posttranscriptional mechanisms for homeostatic regulation of its photosynthetic proteins. [ABSTRACT FROM AUTHOR]

Periodic and seasonal exposure to high light is a common occurrence for many near-shore and estuarine phytoplankton. Rapid acclimatization to shifts in light may provide an axis by which some species of phytoplankton can outcompete other microalgae. Patterns of photoacclimation and photosynthetic capacity in the raphidophyte Heterosigma akashiwo (Hada) Hada ex Hara et Chihara isolated from the mid-Atlantic of the United States were followed in continuous cultures at low- and high-light intensities, followed by reciprocal shifts to the opposite light level. The maximum quantum yield ( F v /F m) as well as the photosynthetic cross-section (σ PSII) of photosystem II was higher in high-light cultures compared to low-light cultures. Significant diurnal variability in photochemistry and photoprotection was noted at both light levels, and high-light-acclimated cultures displayed greater variability in photoprotective pathways. When shifted from low to high light, there was only a slight and temporary decline in maximum quantum yield, while cell specific growth more than doubled within 24 h. Rapid acclimation to high light was facilitated by short-term photoprotection (nonphotochemical quenching), reduced PSII reaction center connectivity, and electron transport. Short-term increases in de-epoxidated xanthophyll pigments contributed to nonphotochemical protection, but lagged behind initial increases in nonphotochemical quenching and were not the primary pathway of photoprotection in this alga. By 48 h, photochemistry of cultures shifted from low to high light resembled long-term high-light-acclimated cultures. This isolate of H. akashiwo appears well poised to exploit rapid shifts in light by using unique cellular adjustments in light harvesting and photochemistry. [ABSTRACT FROM AUTHOR]

Increased seawater pCO2, and in turn 'ocean acidification' (OA), is predicted to profoundly impact marine ecosystem diversity and function this century. Much research has already focussed on calcifying reef-forming corals (Class: Anthozoa) that appear particularly susceptible to OA via reduced net calcification. However, here we show that OA-like conditions can simultaneously enhance the ecological success of non-calcifying anthozoans, which not only play key ecological and biogeochemical roles in present day benthic ecosystems but also represent a model organism should calcifying anthozoans exist as less calcified (soft-bodied) forms in future oceans. Increased growth (abundance and size) of the sea anemone ( Anemonia viridis) population was observed along a natural CO2 gradient at Vulcano, Italy. Both gross photosynthesis ( PG) and respiration (R) increased with pCO2 indicating that the increased growth was, at least in part, fuelled by bottom up (CO2 stimulation) of metabolism. The increase of PG outweighed that of R and the genetic identity of the symbiotic microalgae ( Symbiodinium spp.) remained unchanged (type A19) suggesting proximity to the vent site relieved CO2 limitation of the anemones' symbiotic microalgal population. Our observations of enhanced productivity with pCO2, which are consistent with previous reports for some calcifying corals, convey an increase in fitness that may enable non-calcifying anthozoans to thrive in future environments, i.e. higher seawater pCO2. Understanding how CO2-enhanced productivity of non- (and less-) calcifying anthozoans applies more widely to tropical ecosystems is a priority where such organisms can dominate benthic ecosystems, in particular following localized anthropogenic stress. [ABSTRACT FROM AUTHOR]

Dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP) are sulfur compounds that may function as antioxidants in algae. Symbiotic dinoflagellates of the genus Symbiodinium show strain-specific differences in their susceptibility to temperature-induced oxidative stress and have been shown to contain high concentrations of DMSP. We investigated continuous cultures of four strains from distinct phylotypes (A1, A13, A2, and B1) that can be characterized by differential thermal tolerances. We hypothesized that strains with high thermal tolerance have higher concentrations of DMSP and DMS in comparison to strains with low thermal tolerance. DMSP concentrations were strain-specific with highest concentrations occurring in A1 (225 ± 3.5 mmol · L−1 cell volume [CV]) and lowest in A2 (158 ± 3.8 mmol · L−1 CV). Both strains have high thermal tolerance. Strains with low thermal tolerance (A13 and B1) showed DMSP concentrations in between these extremes (194 ± 19.0 and 160 ± 6.1 mmol · L−1 CV, respectively). DMS data further confirmed this general pattern with high DMS concentrations in A1 and A13 (4.1 ± 1.22 and 2.1 ± 0.37 mmol · L−1 CV, respectively) and low DMS concentrations in A2 and B1 (0.3 ± 0.06 and 0.5 ± 0.22 mmol · L−1 CV, respectively). Hence, the strain-specific differences in DMSP and DMS concentrations did not match the different abilities of the four phylotypes to withstand thermal stress. Future work should quantify the possible dynamics in DMSP and DMS concentrations during periods of high oxidative stress in Symbiodinium sp. and address the role of these antioxidants in zooxanthellate cnidarians. [ABSTRACT FROM AUTHOR]

Raphidophytes (class Raphidophyceae) produce high levels of reactive oxygen species (ROS), yet little is known regarding cellular scavenging mechanisms needed for protection against these radicals. Enzymatic activities of the antioxidants superoxide dismutase (SOD) and catalase (CAT) were measured in conjunction with the production of superoxide (O) and hydrogen peroxide (HO) in batch cultures of five different raphidophytes species during early exponential, late-exponential, and stationary growth phases. The greatest concentrations of O per cell were detected during exponential growth with reduced levels in stationary phases in raphidophytes Heterosigma akashiwo (Hada) Hada ex Y. Hara et Chihara, Chattonella marina (Subrahman.) Y. Hara et Chihara, and Chattonella antiqua (Hada) Ono (strain 18). Decreasing trends from exponential to stationary phases for SOD activity and HO per cell were observed in all species tested. Significant correlations between O per cell and SOD activity per cell over growth phase were only observed in three raphidophytes ( Heterosigma akashiwo, Chattonella marina, and Chattonella antiqua strain 18), likely due to different cellular locations of externally released O radicals and intracellular SOD enzymes measured in this study. CAT activity was greatest at early exponential phase for several raphidophytes, but correlations between HO per cell and CAT activity per cell were only observed for Fibrocapsa japonica Toriumi et Takano, Chattonella antiqua (strain 18), and Chattonella subsalsa Biecheler. Our results suggest that SOD and CAT play important protective roles against ROS during exponential growth of several raphidophytes, while other antioxidant pathways may play a larger role for scavenging ROS during later growth. [ABSTRACT FROM AUTHOR]

Occurrences whereby cnidaria lose their symbiotic dinoflagellate microalgae ( Symbiodinium spp.) are increasing in frequency and intensity. These so-called bleaching events are most often related to an increase in water temperature, which is thought to limit certain Symbiodinium phylotypes from effectively dissipating absorbed excitation energy that is otherwise used for photochemistry. Here, we examined photosynthetic characteristics and hydrogen peroxide (H2O2) production, a possible signal involved in bleaching, from two Symbiodinium types (a thermally “tolerant” A1 and “sensitive” B1) representative of cnidaria– Symbiodinium symbioses of reef-building Caribbean corals. Under steady-state growth at 26°C, a higher efficiency of PSII photochemistry, rate of electron turnover, and rate of O2 production were observed for type A1 than for B1. The two types responded very differently to a period of elevated temperature (32°C): type A1 increased light-driven O2 consumption but not the amount of H2O2 produced; in contrast, type B1 increased the amount of H2O2 produced without an increase in light-driven O2 consumption. Therefore, our results are consistent with previous suggestions that the thermal tolerance of Symbiodinium is related to adaptive constraints associated with photosynthesis and that sensitive phylotypes are more prone to H2O2 production. Understanding these adaptive differences in the genus Symbiodinium will be crucial if we are to interpret the response of symbiotic associations, including reef-building corals, to environmental change. [ABSTRACT FROM AUTHOR]

Little is known about the combined impacts of future CO2 and temperature increases on the growth and physiology of marine picocyanobacteria. We incubated Synechococcus and Prochlorococcus under present-day (380 ppm) or predicted year-2100 CO2 levels (750 ppm), and under normal versus elevated temperatures (+4°C) in semicontinuous cultures. Increased temperature stimulated the cell division rates of Synechococcus but not Prochlorococcus. Doubled CO2 combined with elevated temperature increased maximum chl a–normalized photosynthetic rates of Synechococcus four times relative to controls. Temperature also altered other photosynthetic parameters (α, Φmax, Ek, and ) in Synechococcus, but these changes were not observed for Prochlorococcus. Both increased CO2 and temperature raised the phycobilin and chl a content of Synechococcus, while only elevated temperature increased divinyl chl a in Prochlorococcus. Cellular carbon (C) and nitrogen (N) quotas, but not phosphorus (P) quotas, increased with elevated CO2 in Synechococcus, leading to ∼20% higher C:P and N:P ratios. In contrast, Prochlorococcus elemental composition remained unaffected by CO2, but cell volume and elemental quotas doubled with increasing temperature while maintaining constant stoichiometry. Synechococcus showed a much greater response to CO2 and temperature increases for most parameters measured, compared with Prochlorococcus. Our results suggest that global change could influence the dominance of Synechococcus and Prochlorococcus ecotypes, with likely effects on oligotrophic food-web structure. However, individual picocyanobacteria strains may respond quite differently to future CO2 and temperature increases, and caution is needed when generalizing their responses to global change in the ocean. [ABSTRACT FROM AUTHOR]

The harmful bloom alga Aureococcus anophagefferens Hargraves et Sieburth can survive prolonged darkness, which could facilitate overwinter survival and dispersal by anthropogenic vectors such as ballast water. Experiments were conducted to examine the biochemical and photosynthetic changes in cells during dark storage. Cells were stored in the dark for periods from 2 to 14 days and were sampled at days 0, 1, 2, 4, 7, 10, and 14. Samples from days 2, 7, and 14 were monitored during a recovery period of 4–5 days. Physiological and photosynthetic parameters were measured during the dark storage and recovery periods. Cultures resumed growth quickly when returned to light, and bacterial counts remained constant during the dark storage period but increased rapidly during recovery periods. Cellular protein, carbohydrate, and lipid concentrations declined slightly during the dark period. There were no changes in chl a per cell or in RUBISCO per cell during 14 days of darkness. The data therefore suggest that A. anophagefferens is able to maintain its photosynthetic apparatus during dark storage periods of at least 2 weeks and relies on cellular reserves until it is returned to light to resume photosynthesis. During the recovery period in the light, the cells are able to acclimate rapidly to current light levels and resume growth. [ABSTRACT FROM AUTHOR]

The capacity for photoacclimation to light at 100 or 600 μmol photons·m−2·s−1 and the subsequent response to thermal stress was examined in four genetically distinct cultures of symbiotic dinoflagellates in the genus Symbiodinium with the ITS2 designations A1, A1.1, B1, and F2. While all algal types showed typical signs of photoacclimation to high light via a reduction in chl a, there was a differential response in cellular growth, photosystem II (PSII) activity, and the chl a-specific absorption coefficient between cultures. When maintained at 32°C for up to 10 days, significant variation in the susceptibility to thermal stress was observed in the rate of loss in PSII activity and electron transport, PSII reaction center degradation, and cellular growth. The order of thermal tolerance did not change between the two light levels. However, as expected, loss in photosynthetic function was exacerbated in the thermally sensitive phylotypes (B1 and A1.1) when acclimated to the higher light intensity. There was no consistent relationship between thermal tolerance and changes in light energy dissipation via non-photochemical pathways. Phylotypes F2 and A1 showed a high degree of thermal tolerance, yet the cellular responses to light and temperature were markedly different between these algae. The F2 isolate showed the greatest capacity for photoacclimation and growth at high light and temperature, while the A1 isolate appeared to adjust to thermal stress by a slight decline in PSII activity and a significant decline in growth, possibly at the expense of increased photosystem and cellular repair rates. [ABSTRACT FROM AUTHOR]

We examined the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera at glacial (19 Pa; 190 ppm), current (39 Pa; 380 ppm), and proj ected year 2100 (76 Pa; 750 ppm) CO2 levels. Rates of N2 and CO2 fixation and growth increased in step with increasing partial pressure of CO2 (pCO2), but only under Fe-replete conditions. N2 and carbon fixation rates at 75 Pa CO2 were 1.4-1.8-fold and 1.2-2.0-fold higher, respectively, relative to those at present day and glacial pCO2 levels. In Fe-replete cultures, cellular Fe and molybdenum quotas varied threefold and were linearly related to N2 fixation rates and to external pCO2. However, N2 fixation and trace metal quotas were decoupled from pCO2 in Fe-limited Crocosphaera. Higher CO2 and Fe concentrations both resulted in increased cellular pigment contents and affected photosynthesis vs. irradiance parameters. If these results also apply to natural Crocosphaera populations, anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation, but this effect may be tempered by Fe availability. Possible biogeochemical consequences may include elevated inputs of new nitrogen to the ocean and increased potential for Fe and/or phosphorus limitation in the future high-CO2 ocean, and feedbacks to atmospheric pCO2 in both the near future and over glacial to interglacial timescales. [ABSTRACT FROM AUTHOR]

The photobiology and distribution of dinoflagellates in the genus Symbiodinium was investigated for eight common reef coral species over a depth range of 1-25 m on a coral reef in Belize. The genetic identification of symbionts using polymerase chain reaction-denaturing gradient electrophoresis of the internal transcribed spacer 2 region revealed marked differences in host specificity and depth zonation for certain symbiont types. Each host taxon was found to associate with a limited subset of symbionts that exist in the region. Intraspecific variation was greatest at the shallower sites (1-8 m), where as many as five distinctive symbionts were distributed among a single host population (e.g., Montastraea faveolata). At depth (15-25 m), variation from colony to colony was minimal, where one algal type associated with most or all the colonies of a species. The maximal photochemical efficiency and light-acclimated efficiency of photosystem II (PSII) were determined by active chlorophyll fluorescence and used to assess potential differences in photosynthetic potential. Under normal ambient conditions, little or no physiological differences were noted among different symbionts occurring in the same species of coral at a particular depth, yet interspecific differences in PSII efficiency were noted between coral species at the same depth. Short-term bleaching experiments showed that symbionts B1 and C7 within M. faveolata experienced a higher degree of thermally induced photoinhibition relative to A4a symbionts in Porites astreoides. The differential patterns of PSII inactivation observed within M. faveolata could be explained by the presence of different symbiont populations within this coral. Differences in in situ maximum excitation pressure on PSII between symbionts within some corals may provide a predictive measure of how different species of coral or individual colonies with different symbionts would respond to natural thermal stress events. [ABSTRACT FROM AUTHOR]