Marine heterotrophic prokaryotes degrade, transform, and utilize half of the organic matter (OM) produced by photosynthesis, either in dissolved or particulate form. Microbial metabolic rates are affected by a plethora of different factors, spanning from environmental variables to OM composition. To tease apart the environmental drivers underlying the observed organic matter utilization rates, we analysed a 21 year-long time series from the Gulf of Trieste (NE Adriatic Sea). Heterotrophic carbon production (HCP) time series analysis highlighted a long-term structure made up by three periods of coherent observations (1999-2007; 2008-2011; 2012-2019), shared also by OM concentration time series. Temporal patterns of HCP drivers, extracted with a random forest approach, demonstrated that a period of high salinity anomalies (2002-2008) was the main driver of this structure. The reduced river runoff and the consequent depletion of river-borne inorganic nutrients induced a long-term Chl a decline (2006-2009), followed by a steady increase until 2014. HCP driving features over the three periods substantially changed in their seasonal patterns, suggesting that the years following the draught period represented a transition between two long-term regimes. Overall, temperature and particulate organic carbon concentration were the main factors driving HCP rates. The emergence of these variables highlighted the strong control exerted by the temperature-substrate co-limitation on microbial growth. Further exploration revealed that HCP rates did not follow the Arrhenius’ linear response to temperature between 2008 and 2011, demonstrating that microbial growth was substrate-limited following the draught event. By teasing apart the environmental drivers of microbial growth on a long-term perspective, we demonstrated that a substantial change happened in the biogeochemistry of one of the most productive areas of the Mediterranean Sea. As planktonic microbes are the foundation of marine ecosystems, understanding their past dynamics may help to explain present and future changes.

海洋异养原核生物以溶解或颗粒形式降解，转化和利用光合作用产生的一半有机物质（OM）。微生物代谢率受多种不同因素的影响，从环境变量到OM组成。为了区分观察到的有机物利用率的潜在环境驱动因素，我们分析了里雅斯特湾（东北亚得里亚海）的一个长达21年的时间序列。异养碳产量（HCP）时间序列分析强调了由三个相干观测期（1999-2007； 2008-2011； 2012-2019）组成的长期结构，OM浓度时间序列也具有这种结构。通过随机森林方法提取的HCP驱动程序的时间模式，结果表明，高盐度异常时期（2002-2008年）是这种结构的主要驱动力。河流径流的减少以及由此带来的河道无机养分的枯竭导致长期的Chl一种下降（2006-2009年），然后一直稳定增长到2014年。这三个时期的HCP驱动特征在季节性模式上发生了显着变化，表明该时期之后的年份代表了两个长期政权之间的过渡。总体而言，温度和颗粒有机碳浓度是驱动HCP速率的主要因素。这些变量的出现突出了温度-底物共同限制微生物生长的强大控制力。进一步的研究表明，在2008年至2011年之间，HCP速率未遵循Arrhenius对温度的线性响应，这表明在事件草案之后微生物的生长受到底物的限制。通过长期研究微生物生长的环境驱动因素，我们证明了地中海最富产地区之一的生物地球化学发生了重大变化。由于浮游微生物是海洋生态系统的基础，因此了解浮游微生物的过去动态可能有助于解释当前和未来的变化。