Quality matters: Response of bacteria and ciliates to different allochthonous dissolved organic matter sources as a pulsed disturbance in shallow lakes

• Published in: The Science of the total environment 2024-03, Vol.916, p.170140-170140, Article 170140
• Main Authors: Yalçın, Gülce, Yıldız, Dilvin, Calderó-Pascual, Maria, Yetim, Sinem, Şahin, Yiğit, Parakatselaki, Maria-Eleni, Avcı, Feride, Karakaya, Nusret, Ladoukakis, Emmanuel D., Berger, Stella A., Ger, Kemal Ali, Jeppesen, Erik, Beklioğlu, Meryem
• Format: Article
• Language: English
• Subjects: Bottom-up control; Brownification; Mesocosm experiment; Nutrients; Top-down control
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.170140

Shallow lake ecosystems are particularly prone to disturbances such as pulsed dissolved organic matter (allochthonous-DOM; hereafter allo-DOM) loadings from catchments. However, the effects of allo-DOM with contrasting quality (in addition to quantity) on the planktonic communities of microbial loop are poorly understood. To determine the impact of different qualities of pulsed allo-DOM disturbance on the coupling between bacteria and ciliates, we conducted a mesocosm experiment with two different allo-DOM sources added to mesocosms in a single-pulse disturbance event: Alder tree leaf extract, a more labile (L) source and HuminFeed® (HF), a more recalcitrant source. Allo-DOM sources were used as separate treatments and in combination (HFL) relative to the control without allo-DOM additions (C). Our results indicate that the quality of allo-DOM was a major regulator of planktonic microbial community biomass and/or composition through which both bottom-up and top-down forces were involved. Bacteria biomass showed significant nonlinear responses in L and HFL with initial increases followed by decreases to pre-pulse conditions. Ciliate biomass was significantly higher in L compared to all other treatments. In terms of composition, bacterivore ciliate abundance was significantly higher in both L and HFL treatments, mainly driven by the bacterial biomass increase in the same treatments. GAMM models showed negative interaction between metazoan zooplankton biomass and ciliates, but only in the L treatment, indicating top-down control on ciliates. Ecosystem stability analyses revealed overperformance, high resilience and full recovery of bacteria in the HFL and L treatments, while ciliates showed significant shift in compositional stability in HFL and L with incomplete taxonomic recovery. Our study highlights the importance of allo-DOM quality shaping the response within the microbial loop not only through triggering different scenarios in biomass, but also the community composition, stability, and species interactions (top-down and bottom-up) in bacteria and plankton. [Display omitted] •Labile allo-DOM boosts bacterial biomass; HuminFeed has no impact.•Only labile allo-DOM triggers a ciliate composition shift, favoring bacterivores.•Ciliate and bacterial responses differ; bacteria show stronger bottom-up effects.•Bacteria but not ciliates recover fully under the effect of labile-DOM.•Allo-DOM quality, and not just quantity, is crucial for microbial dynamics and st