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

Abstract

Primary production is essential in shaping biogeochemical cycles and microbial and ecosystem dynamics. The distribution of chemosynthetic rates in pelagic zones and their participation in the carbon cycle, especially when compared to photosynthetic rates in the Southwestern Atlantic Ocean, are poorly constrained. This study aimed to measure pelagic photo- and chemosynthetic productivity and to analyze their spatial distribution and abiotic drivers. Samples for photosynthesis experiments collected at the surface and deep chlorophyll maximum (DCM) were incubated with ¹⁴C-bicarbonate at eight light levels, simulating in situ conditions. Samples for chemosynthesis experiments were collected throughout the water column, from the surface, DCM, 250 m, 900 m, 1,200 m, and 2,300 m, and were incubated in the dark. Rates were analyzed using statistical tests to verify spatial differences between groups of samples and generalized linear models to identify correlations with environmental variables (temperature, salinity, density, mixed layer depth, dissolved oxygen, nitrite, nitrate, silicate, phosphate, turbidity, CDOM, and phycoerythrin and chlorophyll-a concentrations). Moreover, both processes were integrated from the surface to the DCM and compared at the same stations to determine the relative contribution in the epipelagic zone. The photosynthetic and chemosynthetic rates were, on average, 3.00 ± 3.26 mg C m^-3 h^-1 and 0.97 ± 1.22 mg C m^-3 h^-1, respectively. In most stations, chemosynthesis represented an average of 10.2% of total primary productivity, but surpassed photosynthesis in three experiments (reaching 63.4 – 78.8%). Photosynthesis displayed a clear offshore-onshore gradient, along with correlated CDOM concentrations, indicating an autochthonous production of the latter. Chemosynthesis, on the other hand, exhibited high variability and lack of prediction by studied environmental variables, with isolated points of substantially higher activity.

Keywords:
Primary production; Photoautotrophy; Chemoautotrophy; Carbon Cycle; Microbial dynamics