Effects of climate changes on the microbial activities and prokaryotic abundances in the euphotic layer of the Central Mediterranean Sea

Prokaryotic microorganisms are at the base of marine food web and are sensitive to climate changes. In the Mediterranean Sea, an important link between surface water warming and microbial activities is hypothesized to exist on a seasonal and inter-annual scale, with high amounts of organic matter utilized by prokaryotes and recycled in warm periods. To test this hypothesis, from 1998 to 2013, the prokaryotic abundance and microbial activities (enzymatic activities, prokaryotic heterotrophic production, and respiration), together with trophic parameters (chlorophyll, particulate organic carbon, and nitrogen), were measured in the euphotic layer of the Ionian Sea (Central Mediterranean). A statistical simultaneous analysis of the environmental and prokaryotic data was performed with the aim of depicting the evolution of microbial activities and their implications in the biogeochemical cycles. Results showed that despite the large spatial and temporal variability, temperature was the main factor affecting the variability of microbial abundance and metabolism with significant differences on both seasonal and inter-annual scales. Particularly interesting is the increase of production in summer and of the respiration in winter. In spite of low levels of chlorophyll and organic carbon, an increase of prokaryotic heterotrophic production, coupled with that of alkaline phosphatase, was found. As a consequence, the increase of prokaryotic biomass was bottom-up sustained. Most of prokaryotic activities and respiration rates were significantly correlated to organic carbon pools; the process of organic matter decomposition was frequently enhanced at higher temperature, and both prokaryotic production and growth efficiency increased in summer. In view of climate changes, the rise in temperature was associated to the increase of heterotrophic activities and this trend can have a predictive value to describe the microbial processes in the Central Mediterranean Sea.