MICROBIAL RESPONSES TO ORGANIC MATTER ADDITIONS IN CONTRASTING AREAS IN MARTINEZ AND BAKER FJORDS

Patagonian fjords in southern Chile are characterized by high spatial variability in physical, biological and chemical characteristics influencing organic matter degradation by aquatic microbial communities. These communities are naturally subjected to organic matter originated from terrestrial (drainage, rivers, precipitation and ice melting) or from seawater at deep layers. To study this, we experimentally subjected estuarine and marine picoplankton communities of 0.7 µm and 1.6 µm size classes to a “marine”? organic matter (OM) addition and determine their response through time (0, 5 and 15 days). To study this, bacteria composition changes, picoplankton abundance, dissolved organic carbon (DOC) and d13C were determined. Our results showed that natural (no OM additions) estuarine and marine bacteria was dominated by Alphaproteobacteria (mainly SAR11 clade) and Gammaproteobacteria, respectively. OM addition resulted in an enrichment through the incubation time associated with Rhodobacterales and Alteromonadales independently of their estuarine or margin origin, and with Epsilonproteobacteria (Campilobacteriales), at the Steffen Fjord. Picoplankton abundance increased with time, but higher cell numbers were found in OM treatments in both size classes (>2x105 cell mL-1). DOC concentration decreased after eight days of incubation in both treatments and sample origin. In total, our results indicate that the estuarine and marine portions of the fjord hold a similar potential to degrade the new organic matter offer thanks to a versatile picoplanktonic community mainly from seawater origin.