Abstract The bearded fireworm Hermodice carunculata is widely distributed across tropical and subtropical Atlantic and Mediterranean Oceans and was previously considered to be mostly associated with shallow reefs. We provide here data on the distribution, abundance and habitat use of H. carunculata across a euphotic-mesophotic gradient (0-90 m) in the Saint Peter and Saint Paul’s Archipelago (SPSPA, Mid Atlantic Ridge, Brazil). Samples were obtained using SCUBA and a Remote Operated Vehicle (ROV). A total of 189 individuals were observed and a sharp increase in abundance with depth was recorded, particularly from 50 m depth onwards. In the mesophotic zone (50-90 m) individuals were closely associated with branching black-corals (Tanacetipathes spp.) and predation over black-corals and the scleractinian Madracis decactis was commonly sighted. A Boosted Regression Tree model indicated black-coral abundance as the main driver of H. carunculata abundance, suggesting that preference for optimal habitats to hide/forage is more important than depth per se on the bathymetric distribution of the fireworm. The high abundances of H. carunculata in the mesophotic zone, and its predation on keystone benthic cnidarians, suggests that this species play important roles in the dynamics of deep reefs.

Abstract Carbonate mounds and pockmarks are geologically and ecologically important features distributed worldwide in the world’s oceans. In the present study, we present a chemical characterization of deep-sea scleractinian coral skeletons collected in these geomorphological areas at the southeastern continental margin of Brazil. Coral samples were collected from ten sampling stations during cruises aboard the R/V Alpha Crucis, in 2019. Three species of scleractinian corals were studied: Enallopsammia rostrata, Solenosmilia variabilis, and Desmophyllum pertusum. Stable isotopes of carbon and oxygen (δ13C and δ18O), metals, and phosphorus present in the coral carbonate skeletons were analyzed. Corals are recognized as archives of physical-chemical variations in the marine environment, and the Element/Ca ratios, δ13C, and δ18O allowed for the characterization of the studied areas. Chemical composition found in pockmark areas indicated affinity to terrigenous and particulate materials input (Ba/Ca, Fe/Ca, Mn/Ca, Li/Ca, and Mg/Ca). Greater availability of nutrients and anthropogenic materials (Pb/Ca, Cd/Ca, Zn/Ca, and P/Ca) is also likely to occur in this region, with some elemental ratios higher than those measured in other oceans. These mounds can act as barriers for metals from land flows. Also, corals benefit from a higher food supply due to stronger currents. The corals at the top of the Alpha Crucis Carbonate Ridge receive significant marine influence. Most coral samples have carbonate of aragonitic origin, except for a specimen of D. pertusum, which presented carbonate of biogenic calcite and aragonite. The results demonstrate the potential of scleractinian corals in the chemical characterization of the deep ocean and the need for further investigation of carbonate mound areas from the SW Atlantic.

Abstract Pockmarks are circular or elliptical structures formed at the seabed by the expulsion of gas from the subsurface. They are widely distributed along the continental margin off southeastern Brazil and can be over a kilometer wide and 100 meters deep. However, studies concerning the organic characteristics of these pockmark areas are scarce. This study sought to evaluate the organic composition of the sedimentary matter in pockmark areas located in the continental slope region of the southern Brazilian coast. Hydrocarbons, sterols, long-chain alcohols, stable isotopes of C and N, total organic carbon, and total nitrogen were assessed to provide an organic molecular characterization of the pockmarks located in the Southwestern Atlantic Ocean. These compounds did not reflect the organic characteristics of the scape of fluids that generate pockmark structures.

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.

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.

Resumo Dentre as teorias mais aceitas de como surgiu a fauna de oceano profundo está a de que organismos de águas rasas migraram para águas mais profundas e lá especiaram até originarem as comunidades de mar profundo atuais. No entanto, muitos organismos são impedidos de fazer longas migrações verticais por conta do aumento da pressão hidrostática. Larvas da ascídia Polyandrocarpa zorritensis foram submetidas a pressões de 1, 50, 100 e 200 atm. Taxas de sobrevivência, assentamento e metamorfose foram avaliadas após incubação de 24 horas em uma câmara de pressão. A maioria das larvas assentou (84%, 62%, 83% e 77%, respectivamente) e completou a metamorfose (93%, 59%, 85% e 60%) em todas as pressões. A mortalidade ficou abaixo de 15% para todas as pressões, com exceção do tratamento de 50 atm, em que a mortalidade ficou em 38%. Quase 100% das larvas sobreviventes completaram a metamorfose nos tratamentos de 1, 50 e 100 atm. No entanto, para o tratamento de 200 atm, 1/3 dos indivíduos ainda estavam no desenvolvimento larval e apresentaram um desenvolvimento atrasado. Esses resultados sugerem que larvas de ascídia podem suportar valores de pressão hidrostática que são encontrados no oceano profundo. Portanto, é provável que as atuais espécies de ascídias de oceano profundo podem tê-lo colonizado por meio de migrações verticais em algumas gerações.

Abstract How deep-sea fauna evolved is a question still being investigated. One of the most accepted theories is that shallow water organisms migrated to deeper waters and gave origin to the deep-sea communities. However, many organisms are prevented from performing long vertical migrations by the increasing hydrostatic pressure. Tadpole larvae of the ascidian Polyandrocarpa zorritensis were submitted to pressure treatments of 1, 50, 100 and 200 atm. Survival, settlement and metamorphosis rates were verified after 24 hour incubation in a pressure chamber. The majority of larvae settled (84%, 62%, 83% and 77% respectively) and successfully underwent metamorphosis (93%, 59%, 85% and 60%) in all pressure treatments. Larval mortality was of less than 15% in all treatments, except for the 50 atm treatment, which presented 38% mortality. Nearly 100% of the surviving larvae underwent metamorphosis in the treatments of 1, 50 and 100 atm. However, 1/3 of the individuals were still in their larval stages in the 200 atm treatment and presented delayed development. These data suggest that ascidian larvae can withstand the hydrostatic pressure levels found in the deep-sea. It is therefore feasible that the current abyssal ascidian species may have colonized the deep-sea through vertical migration and in only a few generations.

Bivalves tridacnídeos são habitantes conspícuos dos recifes da região do Indo-Pacífico e são cultivados e comercializados para os mercados alimentício e aquarista. No estudo apresentado foram determinadas as taxas de crescimento diário durante a primeira semana de vida da larva do bivalve ornamental Tridacna crocea. As matrizes foram induzidas à desova por meio de uma injeção intragonadal de serotonina realizada através do orifício bissal. Após desova, ovócitos foram coletados, fertilizados e mantidos em béqueres de vidro e tanques de fluxo contínuo tratados com antibióticos para evitar contaminação. Larvas foram alimentadas em duas ocasiões com a microalga Isochrysis galbana e zooxantelas foram oferecidas também por duas vezes. O comprimento larval foi medido através de mesa digitalizadora conectada em um microcomputador. A mortalidade larval foi exponencial durante as primeiras 48 horas de vida, diminuindo em seguida. A taxa média de crescimento foi de 11,3 μm dia-1, aumentando para 18,0 μm dia-1 após a adição de simbiontes. A sobrevivência aumentou para 75% após a adição de zooxantelas. Os resultados apresentam a curva de crescimento para a larva de T. crocea e sugerem que a aquisição de simbiontes pela larva pode ser importante para o crescimento larval e sobrevivência mesmo antes delas completarem a metamorfose.

Tridacnid clams are conspicuous inhabitants of Indo-Pacific coral reefs and are traded and cultivated for the aquarium and food industries. In the present study, daily growth rates of larvae of the giant clam Tridacna crocea were determined in the laboratory during the first week of life. Adults were induced to spawn via intra-gonadal serotonin injection through the byssal orifice. After spawning oocytes were collected, fertilized and kept in 3 L glass beakers and raceways treated with antibiotics to avoid culture contamination. Larvae were fed twice with the microalga Isochrysis galbana and zooxanthellae were also offered twice during the veliger stage (days 4 and 6). Larval length was measured using a digitizing tablet coupled to a microcomputer. Larval mortality was exponential during the first 48 hours of life declining significantly afterwards. Mean growth rate was 11.3 μm day-1, increasing after addition of symbionts to 18.0 μm day-1. Survival increased to ca. 75% after the addition of zooxanthellae. The results describe the growth curve for T. crocea larvae and suggest that the acquisition of symbionts by larvae may be useful for larval growth and survival even before larvae have attained metamorphosis.