Contribution of chemoautotrophy and heterotrophy to the microbial carbon cycle in the Southwestern Atlantic Ocean

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 C-14-bicarbonate and H-3-leucine incorporation, respectively, and incubated in the dark. In the water column, DCF rates varied from 1.51 x 10(1) to 3.24 x 10(2) mu g C m(-3) h(-1), which were one to two orders of magnitude lower than the HMP rates, from 1.26 x 10(2) to 1.48 x 10(4) mu g C m(-3) h(-1). In the sediments, the DCF ranged from 1.15 x 10(4) to 1.83 x 10(5) mu g C m(-3) h(-1), while HMP was one to four orders of magnitude lower, 3.22 x 10(1) to 1.56 x 10(3) mu 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.