Bacterial coaggregation in waters: when cooperation support growth

Aggregative behaviour is a well known anti predator strategy of many animals (primates, herbivores, birds, fish, zooplankton) in air, water and soil. The advantages against predation given by grouping are various, and in several cases compensate or exceed the disadvantages given by living in a spatially limited and overcrowded environment (e.g., intraspecific competition, increased costs of motility, higher transmission of diseases and parasites, reduced oxygen availability). Aggregation is also a successful anti-predation strategy for free-living aquatic bacteria to escape from grazing by heterotrophic nanoflagellates. Coaggregation of different species has been detected between organisms from a variety of taxa (algae, bacteria, protozoa); it may influence a number of ecological variables (from diversity to the control of C fluxes in oceans). We demonstrate that a bacterium with limited aggregation potential (Brevundimonas sp) could form large aggregates only with the participation of a second strain (Arthrobacter sp). The coaggregation of the two strains provided Brevundimonas with a means to better avoid predation by a flagellate, whereas Arthrobacter seemed to profit from reduced competition. Moreover, the co-aggregates apparently provided a surface for the attachment of the predator, potentially allowing for a more efficient foraging on planktonic cells, as reflected in substantially higher flagellate densities. Altogether, the cooperative 3 species experimental system supported the formation of significantly more total biomass then every single two species competitive or predator-prey system.