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ABSTRACT Phytoplankton community composition during austral summer 2022 in the Argo Abyssal Plain (Argo Basin), a 5000-m deep area northwest of the Australian continent in the eastern Indian Ocean, is described in detail, including phytoplankton abundance, biomass, size structure, taxonomic identifications through DNA and pigment analyses, as well as the percent of functional mixotrophs. The region was characterized by warm (up to 30.5°C), stratified, oligotrophic (nitrogen-limited) waters, with integrated euphotic zone (EZ) chlorophyll a (CHLa) of 13 mg m^-2. The EZ mean CHLawas low in the upper layer (0.085 µg L^-1) and 0.32 µg L^-1at the pronounced deep CHLamaxima. EZ-integrated phytoplankton carbon averaged 1229 mg C m^-2.Prochlorococcuswas the dominant taxon throughout the EZ, but the lower EZ had ∼4-times more eukaryotic carbon biomass than the upper EZ, along with a distinct community. In the upper EZ, prymnesiophytes, dinoflagellates and prasinophyte taxa without prasinoxanthin had the highest contributions to monovinyl chlorophyll a (MV-CHLa). In the lower EZ the community was more diverse, with prymnesiophytes, dinoflagellates, prasinophyte taxa with prasinoxanthin, pelagophytes, and cryptophytes all comprising significant contributions to MV-CHLa. Diatoms were a minor part of the community. In the upper EZ, a higher percent of the community showed mixotrophy (35-84%) relative to the lower EZ (30-51%). Although a low abundance, nitrogen-fixing organisms (symbionts of diatoms and cyanobacteria taxa) were ubiquitous. Overall, the community was similar to that found at the Hawaii Ocean Time-series site and the central Gulf of Mexico. 

Abstract In the ocean, dissolved organic phosphorus (DOP) supports the health and productivity of marine phytoplankton, a phenomenon most often investigated under inorganic phosphate (Pi) scarcity. However, microbial DOP acquisition in Pi replete environments remains poorly understood. Here, we conducted a combination of nutrient addition experiments, alkaline phosphatase (AP) rate measurements, and metatranscriptomics along an onshore-to-offshore gradient in the California Current Ecosystem (CCE), an upwelling region relatively replete in Pi. We found that AP activity (APA) and eukaryotic gene transcripts for DOP utilization were present throughout the CCE. In bottle incubations, APA was upregulated in response to iron (Fe) and nitrogen (N) additions. Major contributors to these trends included atypical alkaline phosphatases (AP_aty) of diatoms in upwelling areas, and unclassified phosphodiesterases (other PDE) of multiple eukaryotic taxa in offshore regimes. APA and gene expression dynamics were not coupled to phytoplankton growth, suggesting that phytoplankton experience underlying P stress, or a state of cellular metabolism caused by Pi scarcity, even in regions primarily growth-limited by other elements. AP_atyand PDE (other) genes were highly abundant among the microbial community phosphatase pool, highlighting the importance of detecting these atypical and unclassified proteins via manual curation of metatranscriptomics data. Altogether, these results emphasize the functional diversity of phosphatases sustaining microbial community health in diverse and productive marine habitats. 

The micronutrient iron is essential for phytoplankton growth due to its central role in a wide variety of key metabolic processes including photosynthesis and nitrate assimilation. As a result of scarce bioavailable iron in seawater, marine primary productivity is often iron-limited with future iron supplies remaining uncertain. Although evolutionary constraints resulted in high cellular iron requirements, phytoplankton evolved diverse mechanisms that enable uptake of multiple forms of iron, storage of iron over short and long timescales, and modulation of their iron requirement under stress. Genomics continues to increase our understanding of iron-related proteins that are homologous to those characterized in other model organisms, while recently, molecular and cell biology have been revealing unique genes and processes with connections to iron acquisition or use. Moreover, there are an increasing number of examples showing the interplay between iron uptake and extracellular processes such as boundary layer chemistry and microbial interactions. 

Coastal Antarctic marine ecosystems are significant in carbon cycling because of their intense seasonal phytoplankton blooms. Southern Ocean algae are primarily limited by light and iron (Fe) and can be co-limited by cobalamin (vitamin B₁₂). Micronutrient limitation controls productivity and shapes the composition of blooms which are typically dominated by either diatoms or the haptophytePhaeocystis antarctica. However, the vitamin requirements and ecophysiology of the keystone speciesP. antarcticaremain poorly characterized. Using cultures, physiological analysis, and comparative omics, we examined the response ofP. antarcticato a matrix of Fe-B₁₂conditions. We show thatP. antarcticais not auxotrophic for B₁₂, as previously suggested, and identify mechanisms underlying its B₁₂response in cultures of predominantly solitary and colonial cells. A combination of proteomics and proteogenomics reveals a B₁₂-independent methionine synthase fusion protein (MetE-fusion) that is expressed under vitamin limitation and interreplaced with the B₁₂-dependent isoform under replete conditions. Database searches return homologues of the MetE-fusion protein in multiplePhaeocystisspecies and in a wide range of marine microbes, including other photosynthetic eukaryotes with polymorphic life cycles as well as bacterioplankton. Furthermore, we find MetE-fusion homologues expressed in metaproteomic and metatranscriptomic field samples in polar and more geographically widespread regions. As climate change impacts micronutrient availability in the coastal Southern Ocean, our finding thatP. antarcticahas a flexible B₁₂metabolism has implications for its relative fitness compared to B₁₂-auxotrophic diatoms and for the detection of B₁₂-stress in a more diverse set of marine microbes. 

Oysters play an important role in coastal ecology and are a globally popular seafood source. However, their filter-feeding lifestyle enables coastal pathogens, toxins, and pollutants to accumulate in their tissues, potentially endangering human health. While pathogen concentrations in coastal waters are often linked to environmental conditions and runoff events, these do not always correlate with pathogen concentrations in oysters. Additional factors related to the microbial ecology of pathogenic bacteria and their relationship with oyster hosts likely play a role in accumulation but are poorly understood. In this study, we investigated whether microbial communities in water and oysters were linked to accumulation of Vibrio parahaemolyticus, Vibrio vulnificus, or fecal indicator bacteria. Site-specific environmental conditions significantly influenced microbial communities and potential pathogen concentrations in water. Oyster microbial communities, however, exhibited less variability in microbial community diversity and accumulation of target bacteria overall and were less impacted by environmental differences between sites. Instead, changes in specific microbial taxa in oyster and water samples, particularly in oyster digestive glands, were linked to elevated levels of potential pathogens. For example, increased levels of V. parahaemolyticus were associated with higher relative abundances of cyanobacteria, which could represent an environmental vector for Vibrio spp. transport, and with decreased relative abundance of Mycoplasma and other key members of the oyster digestive gland microbiota. These findings suggest that host and microbial factors, in addition to environmental variables, may influence pathogen accumulation in oysters.