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

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 14 C-bicarbonate and 3 H-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 µg 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 µg C m -3 h -1 . In the sediments, the DCF ranged from 1.15 x 10 4 to 1.83 x 10 5 µg 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 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.

Inorganic carbon assimilation by planktonic community in Santos Basin, Southwestern Atlantic Ocean

“…Chemosynthesis has also been reported to be higher and more variable in mesopelagic compared to bathypelagic zones (Reinthaler et al, 2010); however, the rates in this study were homogeneous between pelagic zones. Our results corroborate those of Li et al (1993) for the North Atlantic and Sargasso Sea and of Passos et al (2022) for the Santos Basin upper slope, where oceanic chemoautotrophy did not vary with depth. Farías et al (2009) found that, on average, chemosynthetic production was divided equally in the euphotic and aphotic layers, suggesting that dark carbon fixation can be crucial in euphotic zones, the usual established realm of photosynthesis.…”

“…Although the chemosynthetic rates in this study are the first that we are aware of in the spatial width of the Santos Basin, they are in the same order of Planktonic photo-and chemoautotrophy in Santos Basin Ocean and Coastal Research 2023, v71(suppl 3):e23006 13 Kutner et al magnitude as those found in the South Atlantic by Prakash et al (1991), which ranged from 0.17 to 3.81 mg C m -3 h -1 . Signori et al (2018) measured dark carbon fixation from 0.001 to 2.72 mg C m -3 h -1 in GB, and Passos et al (2022) reported rates between 0.02 and 0.3 mg C m -3 h -1 on the upper slope region of the basin. Farías et al (2009) measured high rates in the upwelling off the Chilean coast, varying between 0.2 and 145 mg C m -3 d -1 (0.008 -6.04 mg C m -3 h -1 ) in the euphotic zone and between 0.16 and 117 mg C m -3 d -1 (0.007 -4.87 mg C m -3 h -1 ) in the aphotic zone.…”

“…Based on other studies (Reinthaler et al, 2010;Zhou et al, 2017;Passos et al, 2022), the lack of association between dark carbon fixation and environmental parameters is common. Reinthaler et al (2010) described that these relationships were weak but indicated that the large variability in the rates suggests the occurrence of "hotspots" of higher activity in the water column as opposed to a homogeneous ocean.…”