Abstract
Frequencies and biomass of Baltic Sea cyanobacterial blooms are expected to be higher in future climate conditions, but also of longer duration as a result of increased sea surface temperature. Concurrently, climate predictions indicate a reduced salinity in the Baltic Sea. These climate-driven changes are expected to alter not solely the phytoplankton community but also the role of microbial communities for nutrient remineralization. Here, we present the response of summer plankton communities (filamentous cyanobacteria, picocyanobacteria, and heterotrophic bacteria) to the interplay of increasing temperature (from 16 to 18°C and 20°C) and reduced salinity (from salinity 6.9 to 5.9) in the Baltic Proper (NW Gotland Sea) using a microcosm approach. Warmer temperatures led to an earlier peak of cyanobacterial biomass, while yields were reduced. These conditions caused a decrease of nitrogen-fixers (Dolichospermum sp.) biomass, while non nitrogen-fixers (Pseudanabaena sp.) increased. Salinity reduction did not affect cyanobacterial growth nor community composition. Among heterotrophic bacteria, Actinobacteria showed preference for high temperature, while Gammaproteobacteria thrived at in situ temperature. Heterotrophic bacteria community changed drastically at lower salinity and resembled communities at high temperature. Picocyanobacteria and heterotrophic bacterial biomass had a pronounced increase associated with the decay of filamentous cyanobacteria. This suggests that shifts in community composition of heterotrophic bacteria are influenced both directly by abiotic factors (temperature and salinity) and potentially indirectly by cyanobacteria. Our findings suggest that at warmer temperature, lower yield of photosynthetic cyanobacteria combined with lower proportion of nitrogen-fixers in the community could result in lower carbon export to the marine food web with consequences for the decomposer community of heterotrophic bacteria.
Highlights
Interactions between photosynthetic phytoplankton and heterotrophic bacterioplankton largely affect the fate of primary production in most aquatic systems (Azam and Malfatti, 2007)
Chlorophyll a was a good proxy for cyanobacterial biomass during the experiment as they were highly correlated (R∼0.9; p < 0.001), we can ensure that phytoplankton community was mainly composed of cyanobacteria
A less pronounced development of heterotrophic bacteria biomass was detected at 16◦C (Figure 1D), but no significant differences were detected between the treatments
Summary
Interactions between photosynthetic phytoplankton and heterotrophic bacterioplankton (decomposers) largely affect the fate of primary production in most aquatic systems (Azam and Malfatti, 2007). Microbial Response to Future Climate Conditions ecosystem with particulate and dissolved organic matter (Biddanda and Benner, 1997) This organic matter is partly transferred to higher trophic levels through grazing, while 50% is decomposed and remineralized by heterotrophic bacteria (Cole, 1982). Among the most important threats, the constant increase of human activities in open and coastal areas, especially industrialized agriculture, has enhanced eutrophication in the Baltic Sea (Wulff et al, 2007) and led to the expansion of hypoxic zones in both deep and coastal areas (Zillén and Conley, 2010) with occasional release of phosphate to the pelagic zone (Vahtera et al, 2007) In these nutrient conditions, blooms of cyanobacteria are possible due to the presence of nitrogen-fixing taxa such as the filamentous Aphanizomenon sp., Dolichospermum sp. During summer in the Baltic Sea, filamentous and single-celled cyanobacteria represent the major contributors to primary and export production in the upper layer, above thermocline
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