ABSTRACT Dark carbon fixation (DCF) is a source of new and labile carbon in the deep ocean, while heterotrophic microbial production (HMP) promotes organic matter transfer through the microbial loop. Despite their ecological relevance, there is a scientific gap regarding the estimates of DCF and HMP in the Southwestern Atlantic Ocean. Thus, the aim of this study was to investigate the spatial distribution of DCF and HMP; their relevance to the ocean carbon cycle; their relationship with environmental parameters and amongst themselves on the upper slope of Santos Basin. The samples were collected at three different water depths and sediment layers aboard the R/V Alpha Crucis in November 2019. DCF and HMP rates were measured by 14C-bicarbonate and 3H-leucine incorporation, respectively, and incubated in the dark. In the water column, DCF rates varied from 1.51 x 101 to 3.24 x 102 µg C m-3 h-1, which were one to two orders of magnitude lower than the HMP rates, from 1.26 x 102 to 1.48 x 104 µg C m-3 h-1. In the sediments, the DCF ranged from 1.15 x 104 to 1.83 x 105 µg C m-3 h-1, while HMP was one to four orders of magnitude lower, 3.22 x 101 to 1.56 x 103 µ gC m-3 h-1. DCF rates were significantly higher in the sediments, due to a higher availability of energy sources than in the oligotrophic water above. The HMP had higher rates in the water column as it is deeply dependent on organic matter derived from photosynthesis. This is the first study to investigate DCF and HMP considering the water column and sediments of the Southwestern Atlantic Ocean, thus contributing to a better understanding of the microbial role in the marine carbon cycle and ecosystem functioning. (DCF (HMP loop Ocean Thus Basin RV R V 2019 14Cbicarbonate Cbicarbonate 14C bicarbonate 3Hleucine Hleucine 3H leucine H incorporation respectively dark 151 1 51 1.5 10 324 3 24 3.2 m3 m m- h1, h1 h 1, h-1 126 26 1.2 148 48 1.4 h1. 1. 115 15 1.1 183 83 1.8 322 22 156 56 above photosynthesis functioning 201 5 32 2 3. h- 12 14 4 11 18 8 20

Abstract Deep-sea sediments comprise one of the largest habitats on Earth. The archaeal groups contribute to a large fraction of the deep-sea benthic biomass, playing a key role in biogeochemical cycles. However, their diversity in deep-sea benthic ecosystems remains poorly understood, mostly because only recently novel taxa have been proposed, thus remodeling the phylogenetic tree. Despite the dificulty in obtaining cultivated representatives, the metabolic capabilities of archaea have lately been described through metagenomic data, indicating that archaeal taxa are highly versatile. Here, we aimed to reveal the diversity of archaeal communities in surface (0 to 15 cm depth) and subsurface (200 cm depth) sediments from a carbonate-rich region in the Southwestern Atlantic upper slope. We performed 16S rRNA gene sequencing, and found that the archaeal composition in surface sediments was mainly dominated by ammonia-oxidizing archaea within Nitrososphaeria class. The distribution of Nitrososphaeria ASVs (amplicon sequence variants) indicates the presence of several species or ecotypes. Contrastingly, the subsurface sediment was dominated by uncultivated anaerobic and poorly known archaea, including representatives of all supergroups (Asgard, TACK, DPANN and Euryarchaeota). These archaea have been described as having potentially diverse metabolic capabilities, including autotrophic and heterotrophic pathways, such as acetogenesis, methylotrophy, and degradation of labile and recalcitrant organic compounds. This indicates an important role in the remineralization of organic matter in the SW (Southwest) Atlantic slope. They are likely enriched due to the transport and mixing of sediments by the IWBC (Intermediate Western Boundary Current) along the continental slope. However, further studies are needed to reveal the geochemical and oceanographic drivers of the archaeal distribution. This study provides the first description of the archaeal communities in carbonate-rich sediments in the SW Atlantic slope, and adds new biodiversity insights to this geological feature, which is considered a vulnerable marine ecosystem, thus helping for future conservation strategies.

Abstract Pockmarks are geological features that often sustain hydrocarbon-related communities of microorganisms when active, seeping oil or methane. Microbial communities of inactive pockmarks have not been well studied until the present. Plenty of pockmarks in the Southwestern Atlantic Ocean have been discovered. However, no information is available about the organic characteristics in association with the microbial diversity related to methane and non-methane hydrocarbon consumption. This study examined the identity and potential ecology of the methane-related microbial community in an inactive SW Atlantic Carbonate-enriched Pockmark (SWACP). Undisturbed sediment cores were enriched with CH4 99.5% at 5°C, with samples harvested at 16h, 120h, 240h, 720h, and 960h, followed by metataxonomics functional prediction, and the correlation of microbial groups with incubation and enrichment types. The SWACP is depleted in organic compounds, and chemosynthetic production is dominant. Incubated mini-cores of sediment were affected by incubation time and enrichment type, which influenced the microbial composition. Although several taxa were shared among all sediment samples, specific groups per enrichment type and incubation time were observed. These communities comprised taxa previously reported in marine bottom waters, carbonate crusts, active cold-seeps, and inactive pockmarks. The methane-enriched taxa were predominantly related to aerobic methanotrophy, methylotrophy, aerobic and anaerobic non-methane hydrocarbon degradation, and fermentation. This study brings the first survey of the key microbial groups in methane fluxes of a Brazilian deep-sea pockmark, providing data for understanding the ecology surrounding the SW Atlantic gas field areas.