- Martiny, Jennifer B.
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- National Science Foundation
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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 inmore »
Abstract. Oligotrophic regions play a central role in global biogeochemical cycles, with microbial communities in these areas representing an important term in global carbon budgets. While the general structure of microbial communities has been well documented in the global ocean, some remote regions such as the western tropical South Pacific (WTSP) remain fundamentally unexplored. Moreover, the biotic and abiotic factors constraining microbial abundances and distribution remain not well resolved. In this study, we quantified the spatial (vertical and horizontal) distribution of major microbial plankton groups along a transect through the WTSP during the austral summer of 2015, capturing important autotrophic and heterotrophic assemblages including cytometrically determined abundances of non-pigmented protists (also called flagellates). Using environmental parameters (e.g., nutrients and light availability) as well as statistical analyses, we estimated the role of bottom–up and top–down controls in constraining the structure of the WTSP microbial communities in biogeochemically distinct regions. At the most general level, we found a
typical tropical structure, characterized by a shallow mixed layer, a clear deep chlorophyll maximum at all sampling sites, and a deep nitracline. Prochlorococcus was especially abundant along the transect, accounting for 68±10.6% of depth-integrated phytoplankton biomass. Despite their relatively low abundances,more »
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.
Glass, Jennifer B. (Ed.)ABSTRACT Standard methods for calculating microbial growth rates (μ) through the use of proxies, such as in situ fluorescence, cell cycle, or cell counts, are critical for determining the magnitude of the role bacteria play in marine carbon (C) and nitrogen (N) cycles. Taxon-specific growth rates in mixed assemblages would be useful for attributing biogeochemical processes to individual species and understanding niche differentiation among related clades, such as found in Synechococcus and Prochlorococcus . We tested three novel DNA sequencing-based methods (iRep, bPTR, and GRiD) for evaluating the growth of light-synchronized Synechococcus cultures under different light intensities and temperatures. In vivo fluorescence and cell cycle analysis were used to obtain standard estimates of growth rate for comparison with those of the sequence-based methods (SBM). None of the SBM values were correlated with growth rates calculated by standard techniques despite the fact that all three SBM were correlated with the percentage of cells in S phase (DNA replication) over the diel cycle. Inaccuracy in determining the time of maximum DNA replication is unlikely to account entirely for the absence of a relationship between SBM and growth rate, but the fact that most microbes in the surface ocean exhibit some degree ofmore »
Plasticity in the grazing ecophysiology of Florenciella (Dichtyochophyceae), a mixotrophic nanoflagellate that consumes Prochlorococcus and other bacteriaMixotrophic nanoflagellates can account for more than half of the bacterivory in the sunlit ocean, yet very little is known about their ecophysiology. Here, we characterize the grazing ecology of an open‐ocean mixotroph in the genus Florenciella (class Dictyochophyceae). Members of this class were indirectly implicated as major consumers of Prochlorococcus and Synechococcus in the oligotrophic North Pacific Subtropical Gyre, but their phagotrophic capabilities have never been investigated. Our studies showed that Florenciella readily consumed Prochlorococcus, Synechococcus, and heterotrophic bacteria, and that the ingested prey relieved nutrient limitations on growth. Florenciella grew faster (3 d−1) in nitrogen‐deplete medium given sufficient live Synechococcus, than in nitrogen‐replete K medium (2 d−1), but it did not grow in continuous darkness. Grazing rates were substantially higher under nutrient limitation and showed a hint of diel variability, with rates tending to be highest near the end of the light period. An apparent trade‐off between the maximum clearance rate (5 nL Florenciella−1 h−1) and the maximum ingestion rate (up to ∼ 10 prey cells Florenciella−1 h−1) across experiments suggests that grazing behavior may also vary in response to prey concentration. If the observed grazing rates are representative of other open‐ocean mixotrophs, their collective activity could account for amore »