Impacts of lake browning on composition and function of phytoplankton microbiomes (IMPACT-BIO)

• Applicant: Clara Martínez-Pérez, Ph.D.
• Subject Area: Ecology and Biodiversity of Plants and Ecosystems ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management ; Microbial Ecology and Applied Microbiology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 554773884

As climate change and human activities intensify, terrestrial inputs of dissolved organic matter (DOM) to lakes are expected to increase. This elevated input leads to water browning and reduced light availability in the water column, posing challenges to lake ecosystems and impacting their ecological health and societal value. Aquatic microorganisms can be particularly vulnerable to lake browning, with consequences for primary production, food webs, and the occurrence of toxic algal blooms. However, our ability to predict the ecological consequences of lake browning is hindered by limited knowledge about the mechanisms underlying community responses to it, as well as the resilience of these communities in the face of environmental changes. We propose that the response to environmental stress in aquatic microorganisms is strongly influenced by biotic interactions. Therefore, this project will investigate ecological interactions among lake unicellular algae (phytoplankton) and bacteria experiencing increased DOM and reduced light availability. Interactions between phytoplankton and associated bacteria occur within the immediate vicinity of the algal cell, or "phycosphere," similar to the rhizosphere around plant roots. While microbial interactions have primarily been studied in simplified systems focusing on specific algal species and bacteria, the sensitivity of algal-associated bacterial communities to environmental stressors and their impact on algal physiology remain largely unexplored. To address this gap, we will conduct experiments with freshwater algae and their associated bacteria under controlled laboratory conditions, manipulating DOM and light levels. Our goal is to investigate how water browning influences: 1. the physiological responses of phytoplankton, 2. the transfer of algal-derived DOM to associated bacteria, and 3. the composition of algal-associated bacterial communities. With this, we aim to disentangle reciprocal influences between phytoplankton and associated bacteria, as well as the carbon uptake of bacterial taxa of interest under varying light and DOM availability. Natural isotope abundance measurements and stable-isotope labeling experiments will be used to quantify primary productivity, respiration, and uptake of algal-derived carbon, respectively. In addition, we will use microscopy and genomic data to assess the spatial structuring and community composition of algal-associated bacteria. Our experiments will test whether overall community functionality remains stable despite shifts in composition and identify which bacterial taxa and traits are likely to respond stronger to change. This project will integrate the knowledge on microscale phycosphere interactions into an ecosystem-wide perspective of freshwater lakes.

• DFG Programme: Research Grants
• International Connection: Austria
• Co-Investigator: Dr. Alexander Frank
• Cooperation Partner: Dr. Marc Mußmann