Direct exchange of vitamin B12 is demonstrated by modelling the growth dynamics of algal-bacterial cocultures. Single cell imaging and quantification of cellular processes.

Growth dynamics of populations of interacting species in the aquatic environment are of great importance, both for understanding natural ecosystems and in efforts to cultivate these organisms for industrial purposes. Here we consider a simple two-species system wherein the soil bacterium /Mesorhizobium loti/ supplies vitamin B12 (cobalamin) to the freshwater green alga /Lobomonas rostrata/, which requires this organic micronutrient for growth. In return the bacterium receives photosynthate from the alga. Mathematical models are developed that describe minimally the interdependence between the two organisms, and that fit the experimental observations of the consortium. These models enable us to distinguish between different mechanisms of nutrient exchange between the organisms, and provide strong evidence that rather than undergoing simple lysis and release of nutrients into the medium, /M. loti/ regulates the levels of cobalamin it produces, resulting in a true mutualism with /L. rostrata/. Over half of all microalgae are dependent on an exogenous source of cobalamin for growth, and this vitamin is synthesized only by bacteria; it is very likely that similar symbiotic interactions underpin algal productivity more generally. While we have managed to quantify robustly the behavior of this system at the population level, we have little insight into the way the exchange mechanism works at the molecular and cell levels. This challenge, which we are now addressing by developing bespoke microfluidics platforms, is taken as a case study to illustrate the potential of using microfabricated chambers, single cell imaging, and controlled flow conditions to unravel biological processes. Other examples, from the work of our lab on gene regulation in bacteria, on the response of immune system cells, and on host-pathogen interactions, will be presented. http://www.bss.phy.cam.ac.uk/~pc245/