Rival phytoplankton contribute to the cross protection of from oxidative stress.

The marine cyanobacterium numerically dominates the phytoplankton communities in all lower latitude, open ocean environments. Having lost the catalase gene, is highly susceptible to exogenous hydrogen peroxide (HO) produced at the ocean's surface. Protection by HO-scavenging heterotrophic "helper" bacteria has been demonstrated in laboratory cultures and implicated as an important mechanism of survival in the ocean. Importantly, some other phytoplankton can also scavenge HO, suggesting these competing microbes may inadvertently protect . In this study, we assessed the ability of co-occurring phytoplankton, the cyanobacterium and picoeukaryotes and , to protect from HO exposure when cocultured at ecologically relevant abundances. All three genera could significantly degrade HO and diminish mortality during HO exposures simulating photochemical production and rainfall events. We suggest that these phytoplankton groups contribute significantly to the HO microbial sink of the open ocean, thus complicating their relationships with and perhaps contributing to the evolutionary history of .IMPORTANCEThe marine cyanobacterium is the most abundant photosynthetic organism on the planet and is crucially involved in microbial community dynamics and biogeochemical cycling in most tropical and subtropical ocean waters. This success is due, in part, to the detoxification of the reactive oxygen species hydrogen peroxide (HO) performed by "helper" organisms. Earlier work identified heterotrophic bacteria as helpers, and here, we demonstrate that rival cyanobacteria and picoeukaryotic phytoplankton can also contribute to the survival of during exposure to HO. Whereas heterotrophic bacteria helper organisms can benefit directly from promoting the survival of carbon-fixing cells, phytoplankton helpers may suffer a twofold injury: production of HO degrading enzymes constrains already limited resources in oligotrophic environments, and the activity of these enzymes bolsters the abundance of their numerically dominant competitor. These findings build toward a better understanding of the intricate dynamics and interactions that shape microbial community structure in the open ocean.