High-dimensional characterization of phototroph-heterotroph interactions

Phototrophic and heterotrophic microbes are ubiquitous in ecosystems from soils to lakes and oceans. Interactions between them lie at the heart of global biogeochemistry, ecosystem productivity, and biofuel generation. However, relatively little is known about how the chemical nature of the environment affects their interactions. Using a high throughput droplet-based microfluidic platform, we have assayed phototroph-heterotroph interactions across a wide range of availability of the non-substitutable nutrients - carbon, nitrogen, and phosphorus. The system under study comprises the alga Chlamydomonas reinhardtii as the phototroph and Escherichia coli as the heterotroph. In contrast to the prevailing view that microbial interactions are governed by nutrient competition, we have found that the interactions in our system are governed by the initial pH and buffering capacity which are themselves set by the nutrient availability. Low pH and buffering capacities result in nutrient-dependent interactions whereas high pH and buffering capacities result in nutrient-independent inhibition of the heterotroph. Our work presents a new view of how the chemical environment impacts phototroph-heterotroph interactions.