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1. Selective Uptake of Pelagic Microbial Community Members by Caribbean Reef Corals

   https://doi.org/10.1128/AEM.03175-20  

   Hoadley, Kenneth D. ; Hamilton, Maria ; Poirier, Camille L. ; Choi, Chang Jae ; Yung, Cheuk-Man ; Worden, Alexandra Z. ( April 2021 , Applied and Environmental Microbiology)

   Johnson, Karyn N. (Ed.)

   ABSTRACT Coral reefs are possible sinks for microbes; however, the removal mechanisms at play are not well understood. Here, we characterize pelagic microbial groups at the CARMABI reef (Curaçao) and examine microbial consumption by three coral species: Madracis mirabilis , Porites astreoides , and Stephanocoenia intersepta . Flow cytometry analyses of water samples collected from a depth of 10 m identified 6 microbial groups: Prochlorococcus , three groups of Synechococcus , photosynthetic eukaryotes, and heterotrophic bacteria. Minimum growth rates (μ) for Prochlorococcus , all Synechococcus groups, and photosynthetic eukaryotes were 0.55, 0.29, and 0.45 μ day −1 , respectively, and suggest relatively high rates of productivity despite low nutrient conditions on the reef. During a series of 5-h incubations with reef corals performed just after sunset or prior to sunrise, reductions in the abundance of photosynthetic picoeukaryotes, Prochlorococcus and Synechococcus cells, were observed. Of the three Synechococcus groups, one decreased significantly during incubations with each coral and the other two only with M. mirabilis. Removal of carbon from the water column is based on coral consumption rates of phytoplankton and averaged between 138 ng h −1 and 387 ng h −1 , depending on the coral species. A lack of coral-dependent reduction in heterotrophic bacteria, differences in Synechococcus reductions, and diurnal variation in reductions of Synechococcus and Prochlorococcus , coinciding with peak cell division, point to selective feeding by corals. Our study indicates that bentho-pelagic coupling via selective grazing of microbial groups influences carbon flow and supports heterogeneity of microbial communities overlying coral reefs. IMPORTANCE We identify interactions between coral grazing behavior and the growth rates and cell abundances of pelagic microbial groups found surrounding a Caribbean reef. During incubation experiments with three reef corals, reductions in microbial cell abundance differed according to coral species and suggest specific coral or microbial mechanisms are at play. Peaks in removal rates of Prochlorococcus and Synechococcus cyanobacteria appear highest during postsunset incubations and coincide with microbial cell division. Grazing rates and effort vary across coral species and picoplankton groups, possibly influencing overall microbial composition and abundance over coral reefs. For reef corals, use of such a numerically abundant source of nutrition may be advantageous, especially under environmentally stressful conditions when symbioses with dinoflagellate algae break down. 

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2. Microbial food web dynamics in the oceanic Gulf of Mexico

   https://doi.org/10.1093/plankt/fbab021  

   Landry, Michael R ; Selph, Karen E ; Stukel, Michael R ; Swalethorp, Rasmus ; Kelly, Thomas B ; Beatty, Jennifer L ; Quackenbush, Cameron R ( April 2021 , Journal of Plankton Research)

   null (Ed.)

   Abstract Phytoplankton growth and microzooplankton grazing rates were measured in repeated profiles of dilution experiments incubated in situ on a drift array in order to assess microbial production and food web characteristics in the oligotrophic bluefin tuna spawning habitat of the Gulf of Mexico (May peak spawning seasons, 2017–2018). Grazing often exceeded growth with the processes more balanced overall in the surface mixed layer, but biomass accumulated in the mid-euphotic zone. Community production estimates (260–500 mg C m−2 day−1) were low compared to similar open-ocean studies in the Pacific Ocean. Prochlorococcus was a consistent major contributor (113–204 mg C m−2 day−1) to productivity, while diatoms and dinoflagellates (2–10 and 4–13 mg C m−2 day−1, respectively) were consistently low. Prymnesiophytes, the most dynamic component (34–134 mg C m−2 day−1), co-dominated in 2017 experiments. Unexpected imbalances in grazing relative to production were observed for all picoplankton populations (Prochlorococcus, Synechococcus and heterotrophic bacteria), suggesting a trophic cascade in the absence of mesozooplankton predation on large microzooplankton. Study sites with abundant larval tuna had the shallowest deep chlorophyll maxima and significant net positive phytoplankton growth below the mixed layer. 

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3. Effects of Temperature and Nutrient Supply on Resource Allocation, Photosynthetic Strategy, and Metabolic Rates of Synechococcus sp.

   https://doi.org/10.1111/jpy.12983  

   Fernández‐González, Cristina ; Pérez‐Lorenzo, María ; Pratt, Nicola ; Moore, C. Mark ; Bibby, Thomas S. ; Marañón, Emilio ; Raven, ed., J. ( March 2020 , Journal of Phycology)

   Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but their role is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (SynechococcusPCC7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO,rbcL) and light (PhotosystemII, psbA) photosynthetic reactions, the content of chlorophylla, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found thatrbcL and psbA abundance increased with nutrient supply, whereas a temperature‐induced increase in psbA occurred only in nutrient‐replete treatments. Low temperature and abundant nutrients caused increased RuBisCOabundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient‐sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates inSynechococcussp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark‐reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments.

    

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4. Data and code from an examination of growth rates of cyanobacteria co-cultured with a heterotrophic bacterium, Alteromonas, under either present-day or predicted future pCO2 conditions

   https://doi.org/10.26008/1912/bco-dmo.925841.1  

   Morris, James J ; Entwistle, Elizabeth ; Lu, Zhiying ; Kuhl, Matthew ; Durrant, Alexander ( January 2024 , Biological and Chemical Oceanography Data Management Office (BCO-DMO))

   The CO2 content of Earth's atmosphere is rapidly increasing due to human consumption of fossil fuels. Models based on short-term culture experiments predict that major changes will occur in marine phytoplankton communities in the future ocean, but these models rarely consider how the evolutionary potential of phytoplankton or interactions within marine microbial communities may influence these changes. Here we experimentally evolved representatives of four phytoplankton functional types (silicifiers, calcifiers, coastal cyanobacteria, and oligotrophic cyanobacteria) in co-culture with a heterotrophic bacterium, Alteromonas, under either present-day or predicted future pCO2 conditions. The data and analysis code in this dataset show that the growth rates of cyanobacteria generally increased under both conditions, and the growth defects observed in ancestral Prochlorococcus cultures at elevated pCO2 and in axenic culture were diminished after evolution. Evolved Alteromonas were also poorer "helpers" for Prochlorococcus, supporting the assertion that the interaction between Prochlorococcus and heterotrophic bacteria is not a true mutualism but rather a competitive interaction stabilized by Black Queen processes. This work provides new insights on how phytoplankton will respond to anthropogenic change and on the evolutionary mechanisms governing the structure and function of marine microbial communities. 

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5. Sharp transitions in phytoplankton communities across estuarine to open ocean waters of the tropical Pacific

   Tucker, Sarah ; Rii, Yoshimi ; Freel, Kelle ; Kotubetey, Keliʻiahonui ; Kawelo, Hiʻilei ; Winter, Kawika ; Rappé, Michael ( March 2024 , BioXriv)

   Islands in the tropical Pacific supply elevated nutrients to nearshore waters that enhance phytoplankton biomass and create hotspots of productivity in otherwise nutrient-poor oceans. Despite the importance of these hotspots in supporting nearshore food webs, the fine-scale spatial and temporal variability of phytoplankton enhancement and changes in the underlying phytoplankton communities across nearshore to open ocean systems remain poorly understood. In this study, a combination of flow cytometry, pigment analyses, 16S rRNA gene amplicons, and metagenomic sequencing provide a synoptic view of phytoplankton dynamics over a four-year, near-monthly time-series across coastal Kāneʻohe Bay, Hawaiʻi, spanning from an estuarine Indigenous aquaculture system to the adjacent offshore environment. Through comparisons with measurements taken at Station ALOHA located in the oligotrophic North Pacific Subtropical Gyre, we elucidated a sharp and persistent transition between picocyanobacterial communities, from Synechococcus abundant in the nearshore to Prochlorococcus proliferating in offshore and open ocean waters. In comparison to immediately adjacent offshore waters and the surrounding open ocean, phytoplankton biomass within Kānʻeohe Bay was dramatically elevated. While phytoplankton community composition revealed strong seasonal patterns, phytoplankton biomass positively correlated with wind speeds, rainfall, and wind direction, and not water temperatures. These findings reveal sharp transitions in ocean biogeochemistry and phytoplankton dynamics across estuarine to open ocean waters in the tropical Pacific and provide a foundation for quantifying deviations from baseline conditions due to ongoing climate change. 

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