Bacterial community composition and colored dissolved organic matter in a coastal upwelling ecosystem

Abstract

The aim of the present study was to correlate changes in dissolved organic matter (DOM) composition, as characterized through humic- and protein-like fluorescence, with changes in the abundance of major bacterial groups (Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria and Bacteroidetes) and bacterial activity in an upwelling system during the season of low productivity. Sampling was conducted under 2 contrasting periods characterized by low (February) and high (October) precipitation. In October, the mean humic-like DOM fluorescence in surface waters (3.1 ppb equivalents of quinine sulphate [ppb QS]) was higher than the annual average for this coastal zone (2.2 ppb QS), which was attributed to enhanced continental runoff. Alphaproteobacteria and Bacteroidetes were the most abundant groups, accounting for about 13 and 16% of total bacterial abundance, respectively. Betaproteobacteria were detectable only during the rainy period, accounting for 2 to 9% of total bacterial abundance. The bulk dissolved organic carbon concentration similarly explained the relative abundance of Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria (ca. 50% of total variability). By contrast, a strong correlation was found between the humic-like DOM fluorescence and the relative abundance of the Betaproteobacteria group, explaining 68% of the total variability. Multivariate linear regression analyses revealed that the relative abundance of Betaproteobacteria has the greatest influence on bacterial carbon fluxes, explaining 61 and 65% of bulk bacterial activity and biomass variability, respectively. Despite their relatively low abundance, Betaproteobacteria might play a relevant biogeochemical role in this coastal transition ecosystem during the low productivity period as allochthonous DOM consumers