In the dilute planktonic ocean, point source release of dissolved organic matter (DOM) from individual phytoplankton cells – either through slow exudation or sudden lysis – generates nutrient rich microenvironments, known as phycospheres, which heterotrophic bacteria may exploit through chemotaxis. The chemically-mediated interactions between phytoplankton and bacteria played out within these microenvironments are predicted to have profound impacts on microbial growth and chemical cycling in the pelagic ocean. However, examining the ecological interactions taking place within the phycosphere in situ has been precluded by technological limitations, restricting our capacity to decipher the microscale relationships between marine microbes in nature.
Here we describe the application of a purpose-designed microfluidic chip, the In Situ Chemotaxis Assay (ISCA), to assess the ability of marine microbes to respond to microscale chemical cues in their natural environment. When deployed in the ocean, the ISCA generates 25 different diffusing microplumes of phytoplankton-derived DOM, and trap motile microbes attracted to these chemicals. Flow-cytometry and metagenomic analyses subsequently determine the strength of chemotaxis and reveal the identity and metabolic capabilities of the responding microbes.
We used the ISCA to investigate the response of marine microbes to microscale patches of DOM derived from twelve globally-distributed and diverse phytoplankton species. We observed specific phytoplankton-bacteria associations underpinned by chemotactic behaviours, as well as the identity of important phytoplankton-derived chemical cues that likely drive this specificity. By mimicking phycospheres, our approach has begun to tease apart the chemical interactions that underpin phytoplankton-bacteria interactions and support the trophic foundation of marine ecosystems.