Annual dynamics of DOM and microbial community interactions in a Mediterranean coastal area: mutual influence and environmental drivers

Marine dissolved organic matter (DOM) and microbes are strictly linked by complex and dynamic processes. DOM represents the main source of energy for microbial communities and its concentration and chemical properties influence the microbe’s structure and activity. Microbes on the other hand can affect the DOM pool by changing its concentration and quality. Composition and properties of the DOM and the microbial community are therefore interdependent. On top of this, the variability of the environment drives changes on both, affecting each one’s properties and dynamics, and consequently their interaction. The main goal of this study is to investigate the microbes-DOM interplay in a coastal area of the Mediterranean Sea and to assess its variability in relation to environmental conditions. Our hypothesis is that the environmental variability (i.e. seasonal changes in salinity, temperature, biological activity…) affects the concentration and properties of potentially bioavailable DOM and its interaction with the microbial community. To achieve this goal, incubation experiments were carried out monthly for 1 year. Surface water was filtered through 1.2 µm filters to remove potential microbes’ grazers and most of phototrophic organisms and incubated in the dark at the in-situ temperature. DOC concentration, DOM optical properties (absorption and fluorescence), microbial prokaryotic abundance, secondary heterotrophic C production, and enzyme activity were followed for 30 days. Samples for the same parameters were collected from the surface water at the same time as the experimental sampling to gain information on the annual dynamics of both the DOM and the microbial community. The study was carried out in the framework of the National Biodiversity Future Center (NBFC), at the Long-Term Ecological Research (LTER) “C1” coastal station, located in the Gulf of Trieste (northern Adriatic Sea). This station has been studied since 1986 and provides a solid background to contextualize seasonal environmental changes. To the best of our knowledge, this is the first study investigating the DOM-microbial community relationship with such resolution over an entire year and allowing to estimate the potentially bioavailable DOM and its variability with changing environmental conditions. The North Adriatic Sea, as well as all the Mediterranean Sea is already experiencing the effects of climate change, such as increase in salinity and surface water temperature. Investigating how these variables affect the DOM-microbes interaction may give some clues to understand potential future changes in DOM-microbes relationship and therefore in the marine C cycle.