Micro-scale interactions between chemotactic bacteria and algae

Abstract

Traditional views of marine environments describe the ocean pelagic zone as a homogeneous nutrient-poor environment. Heterotrophic marine bacteria that have evolved high-energy mechanisms for swimming abilities and sensing nutrient gradients would gain no survival advantage under this model. Recent identification of microscale (<1cm) nutrient patches, such as those produced by algal exudates, explain a potential for these evolved functions. With this new model for the pelagic zone, bacteria, through chemotaxis and motility, can sense and respond to microscale carbon patches exuded from growing algae. This study examines possible conditions necessary under which it is advantageous to swim. As an initial step to test this hypothesis, we developed a system to investigate bacterial chemotaxis to algal exudates. Two algae from the genus, Thalassiosira, which differed in size, were grown in artificial seawater and filtered, with the use of a novel instrument, to generate nutrient heterogeneity at the microscale. Pseudoalteromonas haloplanktis was added to algal cultures with varying algae:bacteria ratios of 1:250 to 1:50,000 and bacterial chemotaxis was observed by localization around individual algae. P. haloplanktis exhibited chemotaxis towards the larger algae Thalassiosira rotula within seconds but not Thalassiosira weissflogii suggesting larger algae elicit a chemotactic response. Results provide evidence of real time clustering in response to the presence of live algae and suggest a mechanism that provides a fitness advantage over non-motile bacteria.