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1. Heterotrophic Bacteria Enhance the Aggregation of the Marine Picocyanobacteria Prochlorococcus and Synechococcus

   https://doi.org/10.3389/fmicb.2019.01864  

   Cruz, Bianca N ; Neuer, Susanne ( August 2019 , Frontiers in microbiology)

   null (Ed.)

   Marine picocyanobacteria are ubiquitous primary producers across the world’s oceans, and play a key role in the global carbon cycle. Recent evidence stemming from in situ investigations have shown that picocyanobacteria are able to sink out of the euphotic zone to depth, which has traditionally been associated with larger, mineral ballasted cells. The mechanisms behind the sinking of picocyanobacteria remain a point of contention, given that they are too small to sink on their own. To gain a mechanistic understanding of the potential role of picocyanobacteria in carbon export, we tested their ability to form “suspended” (5–60 μm) and “visible” (ca. > 0.1 mm) aggregates, as well as their production of transparent exopolymer particles (TEP)—which are a key component in the formation of marine aggregates. Additionally, we investigated if interactions with heterotrophic bacteria play a role in TEP production and aggregation in Prochlorococcus and Synechococcus by comparing xenic and axenic cultures. We observed TEP production and aggregation in batch cultures of axenic Synechococcus, but not in axenic Prochlorococcus. Heterotrophic bacteria enhanced TEP production as well as suspended and visible aggregate formation in Prochlorococcus, while in Synechococcus, aggregation was enhanced with no changes in TEP. Aggregation experiments using a natural plankton community dominated by picocyanobacteria resulted in aggregation only in the presence of the ballasting mineral kaolinite, and only when Synechococcus were in their highest seasonal abundance. Our results point to a different export potential between the two picocyanobacteria, which may be mediated by interactions with heterotrophic bacteria and presence of ballasting minerals. Further studies are needed to clarify the mechanistic role of bacteria in TEP production and aggregation of these picocyanobacteria. 

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2. 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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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. Siderophores as an iron source for picocyanobacteria in deep chlorophyll maximum layers of the oligotrophic ocean

   https://doi.org/10.1038/s41396-022-01215-w  

   Hogle, Shane L. ; Hackl, Thomas ; Bundy, Randelle M. ; Park, Jiwoon ; Satinsky, Brandon ; Hiltunen, Teppo ; Biller, Steven ; Berube, Paul M. ; Chisholm, Sallie W. ( March 2022 , The ISME Journal)

   Prochlorococcus and Synechococcus are the most abundant photosynthesizing organisms in the oceans. Gene content variation among picocyanobacterial populations in separate ocean basins often mirrors the selective pressures imposed by the region’s distinct biogeochemistry. By pairing genomic datasets with trace metal concentrations from across the global ocean, we show that the genomic capacity for siderophore-mediated iron uptake is widespread in Synechococcus and low-light adapted Prochlorococcus populations from deep chlorophyll maximum layers of iron-depleted regions of the oligotrophic Pacific and S. Atlantic oceans: Prochlorococcus siderophore consumers were absent in the N. Atlantic ocean (higher new iron flux) but constituted up to half of all Prochlorococcus genomes from metagenomes in the N. Pacific (lower new iron flux). Picocyanobacterial siderophore consumers, like many other bacteria with this trait, also lack siderophore biosynthesis genes indicating that they scavenge exogenous siderophores from seawater. Statistical modeling suggests that the capacity for siderophore uptake is endemic to remote ocean regions where atmospheric iron fluxes are the smallest, especially at deep chlorophyll maximum and primary nitrite maximum layers. We argue that abundant siderophore consumers at these two common oceanographic features could be a symptom of wider community iron stress, consistent with prior hypotheses. Our results provide a clear example of iron as a selective force driving the evolution of marine picocyanobacteria.

    

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5. 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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