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 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, these interactions have never been studied in situ because of technological limitations, restricting our capacity to decipher the microscale relationships between marine microbes in the environment. 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 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 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 attract these specific bacteria. By unveiling a rich tapestry of microbial interactions through in situ microscale observations, our results provide the basis for quantifying the role of chemotaxis in accessing microscale hotspots in marine systems, and an opportunity to scale up the impact of these processes on the ocean’s biogeochemistry.

Dr JB Raina grew up in the south of France and, prior to joining the University of Technology Sydney (UTS), he completed his PhD in 2014 at James Cook University. He is an ARC Future Fellow and the deputy team leader of the Ocean Microbiology group in the Climate Change Cluster (C3). He works on marine symbiotic interactions (e.g., corals, phytoplankton) and study these processes at spatiotemporal scales relevant to microbial cells, using a range of molecular and analytical chemistry techniques, high-resolution imaging and custom-made in situ devices. These approaches are intended to bridge the gap between the micrometer scale - where microbial processes occur - and ocean basin scales - where the effects of these processes are typically reported.