Abstract
Patagonian fjords and channels in southern Chile are heterogeneous ecosystems characterized by the interaction of estuarine and marine waters influencing physical-chemical conditions and biological assemblages. Besides salinity, microbial communities from estuarine and marine origin are naturally subjected to changing organic matter quality and variable nutrient concentrations. In this study, we tackle the response of the bacterial community from estuarine and marine origins associated with two size classes (<0.7 µm and <1.6 µm) to the addition of sterile phytoplankton-derived exudates (PDE) compared to control conditions (no addition). Picoplanktonic cell abundance, active bacterial composition analyzed through 16S rRNA sequencing, changes in dissolved organic carbon (DOC) and δ13C were determined over 5 and 15 days after PDE addition. Our results showed that the active marine bacteria were richer and more diverse than their estuarine counterparts, and were dominated by Alphaproteobacteria and Gammaproteobacteria, respectively. PDE addition in both the fractions and the sample origin resulted in an enrichment throughout the incubation of Rhodobacteracea and Cryimorphaceae families, whereas Epsilonproteobacteria (Arcobacteraceae) were mainly favored in the estuarine experiments. Picoplankton abundance increased with time, but higher cell numbers were found in PDE treatments in both size classes (>2 × 105 cell mL−1). In all the experiments, DOC concentration decreased after eight days of incubation, but shifts in δ13C organic matter composition were greater in the estuarine experiments. Overall, our results indicate that despite their different origins (estuarine versus marine), microbial communities inhabiting the fjord responded to PDE with a faster effect on marine active bacteria.
Highlights
The ecosystem englobing Patagonian fjords is one of the world’s largest estuarine environments, which supports high biological production and vertical carbon export fluxes [1–4]
We study the differential responses of estuarine and marine bacterial communities to the introduction of fresh DOM derived from phytoplankton and hypothesize that the expected “priming effect” could favor the activity of specific heterotrophic bacteria inhabiting estuarine areas usually exposed to more recalcitrant organic matter
The DOM experiments consisted of six glass bottles per size fraction that were enriched with 0.5 mg L−1 of dissolved organic carbon (DOC) from phytoplanktonderived exudates
Summary
The ecosystem englobing Patagonian fjords is one of the world’s largest estuarine environments, which supports high biological production and vertical carbon export fluxes [1–4]. An exacerbated spatial and temporal variability in particulate and dissolved organic matter (POM and DOM) is an inherent characteristic of the fjords and channels of Chilean Patagonia [1–3] This area is characterized by high primary production due to the occurrence of phytoplankton blooms during spring [4,9]. The interplay between microbial consumers and DOM release by primary consumers via extracellular release, cell breakdown (predation) or viral lysis is a key component of marine carbon cycling The release of this DOM, which is usually considered highly labile and can be degraded by the heterotrophic bacterial community, can enhance the remineralization of semi-labile and recalcitrant organic matter through the “priming effect” [17–20]. We evaluated changes in the abundance, composition, and diversity of bacterioplankton communities as a response to phytoplankton-derived exudates (PDE; labile DOM)
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