Effect of an extreme cold event on the metabolism of planktonic microbes in the northernmost basin of the Mediterranean Sea

In the northern Adriatic Sea (the northernmost basin of the Mediterranean Sea) wind forcing plays a key role inshaping the water column structure, circulation patterns and dense water formation. The latter process might beenhanced during winter by local north-easterly wind, the Bora, responsible for intense and long-lasting episodicoutbreaks. Although outbreaks of Bora are extensively studied in the context of thermohaline circulation, little isknown about their effects on planktonic microbial communities. From late February to early March 2018, apersistent and strong Bora event caused an unusual drop of seawater temperature (down to 5.7 °C) in the Gulf ofTrieste. To investigate the effect of this extreme meteorological event on the coastal planktonic microbes, weevaluated changes in their abundance and cytometric fingerprinting, heterotrophic carbon production rates andthe activity of six exoenzymes (β-glucosidase, β-galactosidase, chitinase, lipase, alkaline-phosphatase and leucine aminopeptidase). Additionally, we set up temperature manipulation experiments to investigate the potential limiting effect of the temperature drop on microbial degradation and uptake of organic matter. Thedataset clustered in two distinct groups: a cold-affected period (26th February - 8th March), characterised by areduced particulate organic matter availability and slow metabolic rates, and a recovery one, (12th - 16thMarch) defined by an overall increase of substrate availability and enhanced heterotrophic carbon productionand hydrolysis rates. Our findings revealed that the exoenzymatic activities were constrained at their lowerthermal limit during the cold-affected period and that, deviating from the Arrhenius’ linear response to temperature, the prokaryotic growth was limited by the interactive effect of temperature and substrate availability.By limiting the microbial processing of organic matter, extreme meteorological events could alter the functioning of the entire planktonic compartment functioning for several days during and after the onset of the phenomenon, affecting the structure of local trophic webs.