Adaptations of microbial communities and dissolved organics to seasonal pressures in a mesotrophic coastal Mediterranean lake

Abstract

In lake ecosystems, changes in eukaryotic and prokaryotic microbes and the concentration and availability of dissolved organic matter (DOM) produced within or supplied to the system by allochthonous sources are components that characterize complex processes in the microbial loop. We address seasonal changes of microbial communities and DOM in the largest Croatian lake, Vrana. This shallow lake is connected to the Adriatic Sea and is impacted by agricultural activity. Microbial community and DOM structure were driven by several environmental stressors, including drought, seawater intrusion and heavy precipitation events. Bacterial composition of different lifestyles (free-living and particle-associated) differed and only a part of the particle-associated bacteria correlated with microbial eukaryotes. Oscillations of cyanobacterial relative abundance along with chlorophyll a revealed a high primary production season characterized by increased levels of autochthonous DOM that promoted bacterial processes of organic matter degradation. From our results, we infer that in coastal freshwater lakes dependent on precipitationevaporation balance, prolonged dry season coupled with heavy irrigation impact microbial communities at different trophic levels even if salinity increases only slightly and allochthonous DOM inputs decrease. These pressures, if applied more frequently or at higher concentrations, could have the potential to overturn the trophic state of the lake.

Salinity as a Determinant Structuring Microbial Communities in Coastal Lakes

“…Quantitative changes in the communities of microorganisms are related to the seasonal supply of organic carbon [60]. Therefore, seasonal changes in the structure of the prokaryotic community in the coastal lakes may be the result of the availability of organic matter, the concentration of which is regulated not only by hydrological connectivity with the sea but also input of allochthonous matter from lake catchment [31]. Based on the short generation times of many bacteria, together with their rapid evolution and remarkable trophic versatility, environmental boundaries can be crossed more frequently than is the case for plants or animals [53].…”

“…The bacterial community was analysed using an EUB338I-III oligonucleotide probe, targeting total bacteria [30]. Within the bacteria, the main taxonomic groups were determined using: ALF968, BET42a, GAM42a, and DELTA 495abc probes for Alpha-, Beta-, Gamma-, and Deltaproteobacteria, respectively [36,37]; a CF319a probe for Cytophaga-Flavobacteria [31]; and a HGC69a probe for Actinobacteria [38]. The characteristic oligonucleotide probes used in this study are presented in Table 2.…”

“…This may result from stress caused by both anthropogenic pollution and the pressure of environmental parameters [25,26]. Despite the growing interest in the influence of abiotic factors on the structure of microbial communities in coastal ecosystems [6,17,[27][28][29] biotic changes in the low-salinity ecosystems still require more attention of researchers [30][31][32]. Observations of changes in the structure of bacterioplankton within coastal lakes would also allow the creation of a model for the early detection of changes in freshwater-to-brackish ecotone zones within a global range.…”