Public good exploitation in natural bacterioplankton communities

Microorganisms such as bacteria often interact with their environment through extracellular molecules that increase access to limiting resources. These secretions can act as public goods, creating incentives for exploiters, a.k.a cheaters, to invade and steal public goods away from producers. This phenomenon has been studied extensively in microbiology due to its implications for the evolution of cooperation, but little is known about the occurrence and impact of public good exploiters in the environment. Here, we develop a new genomic approach to systematically identify bacteria that can exploit public goods produced during the degradation of polysaccharides. Focusing on chitin - the second most abundant biopolymer on the planet, we show that public good exploiters are active in natural marine microbial communities that assemble on chitin particles, invading during early stages of colonization and potentially hindering degradation. Unlike in classical studies of social evolution, exploiters and polysaccharide degraders are not isogenic and instead belong to distant lineages, facilitating their coexistence. Our approach opens novel avenues to use the wealth of genomic data available to infer ecological roles and interactions among microbes.