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Bacteria are major drivers of biogeochemical nutrient cycles and energy fluxes in marine environments, yet how bacterial communities respond to environmental change is not well known. Metagenomes allow examination of genetic responses of the entire microbial community to environmental change. However, it is challenging to link metagenomes directly to biogeochemical process rates. Here, we investigate metagenomic responses in natural bacterioplankton communities to simulated environmental stressors in the Baltic Sea, including increased river water input, increased nutrient concentration, and reduced oxygen level. This allowed us to identify informative prokaryotic gene markers, responding to environmental perturbation. Our results demonstrate that metagenomic and metabolic changes in bacterial communities in response to environmental stressors are influenced both by the initial community composition and by the biogeochemical factors shaping the functional response. Furthermore, the different sources of dissolved organic matter (DOM) had the largest impact on metagenomic blueprint. Most prominently, changes in DOM loads influenced specific transporter types reflecting the substrate availability and DOC assimilation and consumption pathways. The results provide new knowledge for developing models of ecosystem structure and biogeochemical cycling in future climate change scenarios and advance our exploration of the potential use of marine microorganisms as markers for environmental conditions., QC 20181012

Climate change is predicted to induce substantial changes in precipitation patterns across the globe. In Northern Europe, precipitation is expected to increase more than the global average (particularly in northern Scandinavia), causing increased river runoff. The Baltic Sea is one of the largest brackish environments on earth with a catchment area that spans 14 countries, encompassing primarily forested areas and agricultural landscapes. Despite the acknowledged role of marine bacteria in nutrient cycling, there is a lack of knowledge in their metabolic responses to inorganic and organic nutrient loading from riverine runoff. We investigated the bacterial growth and gene expression responses in a mesocosm experiment in which river water from boreal forest- (enriched in humic substances) or agriculture- influenced catchment areas were added to Baltic Sea Proper water. The riverine nutrient input triggered extensive phytoplankton blooms and bacterial growth, most notably in the agriculture river treatment. Interestingly, bacterial gene expression analysis (metatranscriptomics) showed similar responses to agriculture and humic river inputs at the start of the experiment (before the phytoplankton bloom), but expression patterns diverged significantly upon bloom senescence.Notably, transcripts associated with phosphate metabolism were significantly enriched , whereas transcripts related to nitrogen metabolism were significantly lower in the agriculture river treatment compared to the boreal forest river treatment. The opposite pattern was observed in the boreal forest river water treatment. Overall, our results showed that interactions between river nutrient loading and phytoplankton organic matter are important in regulating bacterial activities and responses at the molecular level. This suggests that bacterial transformations of organic matter and nutrient cycling in coastal waters and estuarine environments are sensitive to changes in precipitation patterns in a catc

Climate change is projected to induce substantial changes in precipitation patterns across the globe. In Northern Europe, precipitation is expected to increase more than the global average (particularly in northern Scandinavia), causing increased river runoff. The Baltic Sea is one of the largest brackish environments on earth with a catchment area that spans 14 countries, encompassing primarily forested areas and agricultural landscapes. Despite the acknowledged role of marine bacteria in nutrient cycling, there is a lack of knowledge in their metabolic responses to inorganic and organic nutrient loading from riverine runoff. We investigated the bacterial growth and gene expression responses in a mesocosm experiment in which river water from boreal forest- (enriched in humic substances) or agriculture-influenced catchment areas (enriched in nitrogen) were added to Baltic Proper water. The riverine nutrient input triggered extensive phytoplankton blooms and bacterial growth, most notably in the agriculture river treatment. Interestingly, bacterial gene expression analysis (metatranscriptomics) showed similar responses to agriculture and humic river inputs at the start of the experiment during phytoplankton development, but expression patterns diverged upon bloom senescence. This indicated that interactions between river nutrient loading and phytoplankton organic matter are important in regulating bacterial activities and responses at the molecular level. Notably, transcripts associated with phosphate metabolism were significantly enriched in the agriculture river treatment compared to the boreal forest river treatment, whereas transcripts related to nitrogen metabolism were significantly lower. The opposite pattern was observed in the boreal forest river water treatment. This suggests that bacterial transformations of organic matter and nutrient processing in coastal environments are sensitive to alterations in the precipitation-induced riverine runoff in a catchmen