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1. Plasticity in the grazing ecophysiology of Florenciella (Dichtyochophyceae), a mixotrophic nanoflagellate that consumes Prochlorococcus and other bacteria

   https://doi.org/10.1002/lno.11585  

   Li, Qian ; Edwards, Kyle F. ; Schvarcz, Christopher R. ; Selph, Karen E. ; Steward, Grieg F. ( August 2020 , Limnology and Oceanography)

   Mixotrophic 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 genusFlorenciella(class Dictyochophyceae). Members of this class were indirectly implicated as major consumers ofProchlorococcusandSynechococcusin the oligotrophic North Pacific Subtropical Gyre, but their phagotrophic capabilities have never been investigated. Our studies showed thatFlorenciellareadily consumedProchlorococcus,Synechococcus, and heterotrophic bacteria, and that the ingested prey relieved nutrient limitations on growth.Florenciellagrew faster (3 d⁻¹) in nitrogen‐deplete medium given sufficient liveSynechococcus, 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 nLFlorenciella⁻¹h⁻¹) and the maximum ingestion rate (up to ∼ 10 prey cellsFlorenciella⁻¹h⁻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 a significant fraction of the daily cyanobacterial mortality. This study provides essential parameters for understanding the grazing ecology of a common marine mixotroph and the first characterization of mixotrophic nanoflagellate functional responses when feeding on unicellular cyanobacteria, the dominant marine primary producers in the oligotrophic ocean.

    

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2. Microbial and nutrient dynamics in mangrove, reef, and seagrass waters over tidal and diurnal time scales

   https://doi.org/10.3354/ame01944  

   Becker, CC ; Weber, L ; Suca, JJ ; Llopiz, JK ; Mooney, TA ; Apprill, A ( October 2020 , Aquatic Microbial Ecology)

   In coral reefs and adjacent seagrass meadow and mangrove environments, short temporal scales (i.e. tidal, diurnal) may have important influences on ecosystem processes and community structure, but these scales are rarely investigated. This study examines how tidal and diurnal forcings influence pelagic microorganisms and nutrient dynamics in 3 important and adjacent coastal biomes: mangroves, coral reefs, and seagrass meadows. We sampled for microbial ( Bacteria and Archaea ) community composition, cell abundances and environmental parameters at 9 coastal sites on St. John, US Virgin Islands that spanned 4 km in distance (4 coral reefs, 2 seagrass meadows and 3 mangrove locations within 2 larger bays). Eight samplings occurred over a 48 h period, capturing day and night microbial dynamics over 2 tidal cycles. The seagrass and reef biomes exhibited relatively consistent environmental conditions and microbial community structure but were dominated by shifts in picocyanobacterial abundances that were most likely attributed to diel dynamics. In contrast, mangrove ecosystems exhibited substantial daily shifts in environmental parameters, heterotrophic cell abundances and microbial community structure that were consistent with the tidal cycle. Differential abundance analysis of mangrove-associated microorganisms revealed enrichment of pelagic oligotrophic taxa during high tide and enrichment of putative sediment-associated microbes during low tide. Our study underpins the importance of tidal and diurnal time scales in structuring coastal microbial and nutrient dynamics, with diel and tidal cycles contributing to a highly dynamic microbial environment in mangroves, and time of day likely contributing to microbial dynamics in seagrass and reef biomes. 

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3. Pico‐ and nanoplankton communities on a near to offshore transect along the continental shelf of the northwestern Gulf of Mexico in the aftermath of Hurricane Harvey

   https://doi.org/10.1002/lno.11788  

   Kurtay, Gulce ; Prevost, Hans J. ; Stauffer, Beth A. ( May 2021 , Limnology and Oceanography)

   Hurricane Harvey delivered over 124 trillion liters of freshwater to the Texas–Louisiana coast and the northwestern Gulf of Mexico (GOM) in late August‐early September 2017. Environmental conditions, size‐fractionated phytoplankton biomass, and pico‐ and nanoplankton abundances (picocyanobacteria, picoeukaryotes, autotrophic, and heterotrophic nanoplankton) were characterized along nearshore‐offshore transects prior to Hurricane Harvey (late July 2017) and in the 3 weeks to 6 months following the storm (September 2017 to March 2018). To understand the extent to which observed changes in the aquatic environment and plankton communities could be attributed to Hurricane Harvey (vs. seasonal or interannual variability), salinity, temperature, and phytoplankton biomass from historical data (2006–2018) were also analyzed. Nearshore stations from September and October 2017 showed significantly lower salinities and overall phytoplankton biomass compared to historical data. Inorganic nitrogen concentrations were minimal in October. Pico‐ and nanoplankton abundances were lower in September and October than prior to the storm, with the exception of picocyanobacteria. In contrast, post‐storm biomass at mid‐shelf stations was within the historical average, while pico‐ and nanoplankton abundances were higher. Offshore stations showed little change in biomass or abundances following the storm. Pre‐storm assemblages of pico‐ and nanoplankton in July 2017 were distinct from those in post‐storm months, and variance in these assemblages and specific group abundances was tied to inorganic nutrients, salinity, and temperature. These results point to significant changes in important members of the plankton that occurred in GOM continental shelf waters following a major hurricane, with important implications for oceanic food webs and biogeochemical cycles.

    

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4. Broad phylogenetic and functional diversity among mixotrophic consumers of Prochlorococcus

   https://doi.org/10.1038/s41396-022-01204-z  

   Li, Qian ; Edwards, Kyle F. ; Schvarcz, Christopher R. ; Steward, Grieg F. ( February 2022 , The ISME Journal)

   Small eukaryotic phytoplankton are major contributors to global primary production and marine biogeochemical cycles. Many taxa are thought to be mixotrophic, but quantitative studies of phagotrophy exist for very few. In addition, little is known about consumers of Prochlorococcus, the abundant cyanobacterium at the base of oligotrophic ocean food webs. Here we describe thirty-nine new phytoplankton isolates from the North Pacific Subtropical Gyre (Station ALOHA), all flagellates ~2–5 µm diameter, and we quantify their ability to graze Prochlorococcus. The mixotrophs are from diverse classes (dictyochophytes, haptophytes, chrysophytes, bolidophytes, a dinoflagellate, and a chlorarachniophyte), many from previously uncultured clades. Grazing ability varied substantially, with specific clearance rate (volume cleared per body volume) varying over ten-fold across isolates and six-fold across genera. Slower grazers tended to create more biovolume per prey biovolume consumed. Using qPCR we found that the haptophyte Chrysochromulina was most abundant among the isolated mixotrophs at Station ALOHA, with 76–250 cells mL−1 across depths in the upper euphotic zone (5–100 m). Our results show that within a single ecosystem the phototrophs that ingest bacteria come from many branches of the eukaryotic tree, and are functionally diverse, indicating a broad range of strategies along the spectrum from phototrophy to phagotrophy.

    

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5. Protistan Communities Within the Galápagos Archipelago With an Emphasis on Micrograzers

   https://doi.org/10.3389/fmars.2022.811979  

   Jang, Se Hyeon ; Lim, Prisca ; Torano, Olivia ; Neave, Erika F. ; Seim, Harvey ; Marchetti, Adrian ( May 2022 , Frontiers in Marine Science)

   The Galápagos Archipelago is a globally significant biodiversity hotspot. However, compared to the relatively well-known megafauna, the distribution and ecological significance of marine protists in this system are poorly understood. To gain an understanding of the protistan assemblages across trophic modes, an intensive oceanographic survey was conducted in the Galápagos Marine Reserve (GMR) in October of 2018. The Equatorial Undercurrent (EUC)-influenced region had higher chlorophyll- a (Chl- a ) concentrations than those of the eastern regions of the archipelago, along with higher abundances of protistan grazers. Specifically, proportions of autotrophic and potentially mixotrophic dinoflagellates were higher in the EUC, whereas in the eastern regions, heterotrophic dinoflagellates and chlorophytes dominated. Taxonomic composition and biochemical indicators suggested proportions of micrograzers and their associated heterotrophic biomass was higher in the oligotrophic, low Chl- a regions in the east. We also report observations from a dinoflagellate bloom in the western archipelago, which was heavily influenced by upwelling of the EUC. The red tide-forming dinoflagellate Scrippsiella lachrymosa was highly detected through light microscopy and DNA amplicon sequencing. In addition, the heterotrophic dinoflagellate Polykrikos kofoidii was detected and, based on cell densities observed in this study and grazing rates obtained from the literature, estimated to potentially graze up to 62% of S. lachrymosa bloom population. Our findings thus provide new insights into the composition of micrograzers and their potential roles in structuring protistan communities in the Galápagos Archipelago. 

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