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To investigate how pico-and nano-plankton respond to oceanographic conditions in the Southwestern Atlantic Ocean, we assessed the influence of a summer intrusion of the South Atlantic Central Water (SACW) on the spatial and vertical dynamics of planktonic abundance and carbon biomass across environmental gradients. Seawater samples were collected from six depths within the euphotic zone at nine oceanographic stations in a transect on the Brazilian continental shelf in January 2013. The abundance of pico-and nano-plankton populations was determined by flow cytometry, and carbon biomass was calculated based on conversion factors from the literature. The autotrophic Synechococcus spp., picoeukaryotes, and nanoeukaryotes were more abundant in the surface layers of the innermost stations influenced by Coastal Water (maximum of 1.19 × 10 5 , 1.5 × 10 4 , and 8.61 × 10 3 cell·mL −1 , respectively), whereas Prochlorococcus spp. dominated (max. of 6.57 × 10 4 cell·mL −1 ) at the outermost stations influenced by Tropical Water and in the uplifting layers of the SACW around a depth of 100 m. Numerically, heterotrophic bacterial populations were predominant, with maximum concentrations (2.11 × 10 6 cell·mL −1 ) recorded in the surface layers of the inner and mid shelves in Coastal Water and the upper limits of the SACW. Nutrient-rich (high silicate and phosphate) and relatively less saline waters enhanced the picoeukaryotic biomass, while Synechococcus and heterotrophic bacteria were linked to higher temperatures, lower salinities, and higher inputs of ammonia and dissolved organic carbon. The relative importance of each group to carbon biomass partitioning under upwelling conditions is led by heterotrophic bacteria, followed by picoeukaryotes, Synechococcus and Prochlorococcus, and when the SACW is not as influential, the relative contribution of each phytoplanktonic group is more evenly distributed. In addition to habitat preferences, the physical structure of oligotrophic waters has a large impact on the vertical and spatial distribution patterns of picoplankton, reflecting the strong effect of the SACW intrusion.

13Seamounts are often covered with Fe and Mn oxides, known as ferromanganese (Fe-Mn) crusts. Future mining of 14 these crusts is predicted to have significant effects on biodiversity in mined areas. Although microorganisms have 15 been reported on Fe-Mn crusts, little is known about the role of crusts in shaping microbial communities. Here, 16we investigated microbial community based on 16S rRNA gene sequences retrieved from Fe-Mn crusts, coral 17 skeleton, calcarenite and biofilm at crusts of the Rio Grande Rise (RGR). RGR is a prominent topographic feature 18 in the deep southwestern Atlantic Ocean with Fe-Mn crusts. Our results revealed that crust field of the RGR harbors 19 a usual deep-sea microbiome. We observed differences of microbial structure according to the sampling location 20 and depth, suggesting an influence of water circulation and availability of particulate organic matter. Bacterial and 21 archaeal groups related to oxidation of nitrogen compounds, such as Nitrospirae, Nitrospinae phyla, 22Nitrosopumilus within Thaumarchaeota group were present on those substrates. Additionally, we detected 23 abundant assemblages belonging to methane oxidation, i. e. Ca. Methylomirabilales (NC10) and SAR324 24 (Deltaproteobacteria). The chemolithoautotrophs associated with ammonia-oxidizing archaea and nitrite-oxidizing 25 bacteria potentially play an important role as primary producers in the Fe-Mn substrates from RGR. These results 26 provide the first insights into the microbial diversity and potential ecological processes in Fe-Mn substrates from 27 the Atlantic Ocean. This may also support draft regulations for deep-sea mining in the region.28 29

Marine microbes control the flux of matter and energy essential for life in the oceans. Until now, the distribution and diversity of planktonic microorganisms above Fe-Mn crusts has received relatively little attention. Future mining\dredging of these minerals is predicted to affect microbial diversity and functioning in the deep sea. Here, we studied the ecology of planktonic microbes among pelagic environments of an Fe-Mn deposit region, at Rio Grande Rise, Southwestern Atlantic Ocean. We investigated microbial community composition using high-throughput sequencing of 16S rRNA genes and their abundance estimated by flow cytometry. Our results showed that the majority of picoplanktonic was found in epi- and mesopelagic waters, corresponding to the Tropical Water and South Atlantic Central Water. Bacterial and archaeal groups related to phototrophy, heterotrophy and chemosynthesis, such as Synechococcales, Sar11 (Proteobacteria) and Nitrosopumilales (Thaumarchaeota) were the main representatives of the pelagic microbial community. Additionally, we detected abundant assemblages involved in biodegradation of marine organic matter and iron oxidation at deep waters, i.e., Pseudoalteromonas and Alteromonas. No differences were observed in microbial community alpha diversity. However, we detected differences in community structure between water masses, suggesting that changes in an environmental setting (i.e. nutrient availability or circulation) play a significant role in structuring the pelagic zones, also affecting the meso- and bathypelagic microbiome.HighlightsRio Grande Rise pelagic microbiomePicoplankton carbon biomass partitioning through pelagic zonesUnique SAR11 Clade I oligotype in the shallowest Tropical WaterHigher number of shared oligotypes between deepest water massesNitrogen, carbon and sulfur may be important contributors for the pelagic microbiome

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