Do microbial coexistence and cooperation preserve diversity and productivity in aquatic systems?

Potential competitors can raise their fitness, and their chances of ecological success, by developing common strategies to face sudden limitations. By applying artificial experimental systems it is possible to use microbes as model organisms to powerfully test ecological theories and draw back experimental trends to the natural environment. The interactions between microbial species common (and ecologically successful) in waters and occasional species belonging to the rare biosphere are often more complex than expected. Apparent cooperation, as well as necessary mutualism can be established between potentially competing bacterial species, for example in order to escape from predation (by co-aggregation, for the first time here presented as an anti-predator strategy) and concomitantly survive the uneven competition for resources in open waters. The result of the positive interaction between the two strain is the survival of both strains, even when ecological stressors could drive them to extinction when grown alone. Predator-prey interactions are highly impacted by the formation of co-aggregates: predators gain in terms of numbers and biomass when fed on mixed cultures, allowing speculations about the actual quality (and complementarity) of the different bacterial strains. The potential function of the co-aggregates composed by prey of different palatability as proxy of lake and marine-snow and thus as additional sources of nutrients in periods of food limitation, supports larger (predator) populations at higher trophic levels. Thanks to co-aggregates potential competitors survive to environmental stressors, incrementing their ecological relations. At the same time, the more complex prey population can support a higher predator population, resulting fundamental for the correct functioning of the trophic pyramid in limiting aquatic environments.