Synergistic interactions between biogenic organic matter and microbial dynamics during simulated senescent cyanobacterial blooms in freshwater mesocosms.

One of the major consequences of a phytoplanktonic bloom is the massive release of autochthonous organic matter (OM) into the water column, stimulating heterotrophic microbial activity and disrupting the trophic web. To better understand the consequences of such biotic stress, the senescent phase of a bloom was simulated under semi-controlled conditions through the addition of cyanobacterial-derived OM from Microcystis aeruginosa and Aphanizomenon gracile (30% enrichment as eq C) into lake water mesocosms. By following both the autochthonous OM and the microbial communities during 28 days, we observed that both cyanobacterial-derived OM differed qualitatively (displaying different levels of lability), but enhanced a rapid bacterial mineralization within 2-7 days of incubation. During this early response, we observed an enrichment of Alphaproteobacteria in both the particle-attached (PA) and free-living (FL) fractions, followed by Gammaproteobacteria during the late-response stage. Specifically, in the Microcystis-derived OM supply, the emergence and persistence of Bacilli members were detected. At the class level, differences according to cyanobacterial species were also detected, suggesting specific ecological niches within the PA fraction, likely driven by the quality of the cyanobacterial-derived OM pools. Concomitantly, the supply of cyanobacterial-derived organic matter promoted the accumulation of less bioavailable, chemically complex, and persistent compounds, a pattern that was more pronounced for Aphanizomenon-derived OM than for Microcystis-derived OM. The refractory nature of the remaining OM explained the rapid decline in microbial abundances and activities. The chemical imprint of cyanobacterial-derived OM persisted even after microbial mineralization over the 28-day experiment. Our study highlights that different cyanobacterial OM pools induce specific microbial responses, while also exerting a long-term effect on both OM recycling and microbial community composition.