Bacterial production prevails over photo- and chemosynthesis in a eutrophic tropical lagoon

Dark carbon fixation is a combination of processes performed by microorganisms to fix inorganic carbon driven the energy generated by chemical reactions. Our study quantified microbial primary and secondary production rates to evaluate the relative importance of dark carbon fixation in relation to photosynthetic rates and heterotrophic bacterial production and their contribution to the carbon cycle in a tropical eutrophic and human-impacted lagoon (Rodrigo de Freitas Lagoon, Brazil). Samples were collected daily at a fixed point to assess short-term temporal variations and at six different stations to assess spatial variability. Water samples were incubated using C-14-bicarbonate at 100% of light and in the dark to quantify photosynthesis and dark carbon fixation, respectively; and H-3-leucine incubations were performed to measure bacterial production. Environmental parameters (temperature, salinity, dissolved oxygen, dissolved organic carbon and inorganic nutrients) were measured to characterize the hydrology of the studied area, and to evaluate their influence on the microbial production rates. There were no significant changes in production rates over time or within different depths. Bacterial production rates varied significantly over space, and were correlated with nitrogen compounds, temperature, salinity and pH. However, a short rainfall event, although not strong or extreme, promoted rapid changes in the shallow water column, such as a decrease in temperature and nitrogen compounds, inducing responses to microbial production rates, such as higher rates of dark carbon fixation and bacterial production at the bottom and lower rates of photosynthesis. Heterotrophic bacterial production was significantly higher than dark and light carbon fixation, being the most important carbon-cycling process in Rodrigo de Freitas Lagoon, possibly favored by the high allochthonous organic matter and nutrient inputs. This study showed that, on average, dark carbon fixation accounted for 30% at the surface and 43% near the bottom of the total primary production and up to 10% of heterotrophic production in the water column, which may be related to the availability of ammonia and methane. We concluded that, even for this short-term temporal and spatial assessment, dark carbon fixation is relevant, although usually disregarded, and this process should be taken into account when evaluating carbon budgets, metabolic balance and ecosystem functioning in tropical coastal lagoons.Funding Agencies|CNPqNational Council for Scientific and Technological Development (CNPq) [CNPq 155776/2014-2]