Physiological characterization of <i>Prochlorococcus</i> under abiotic stressors temperature and hydrogen peroxide

Cyanobacteria of the genus Prochlorococcus are the smallest and most abundant phytoplankters in the ocean. Temperature is a major influence on Prochlorococcus abundance and distribution in the ocean, but the physiological basis for this relationship is not well understood. In other microbes, lipid and fatty acid composition have been shown to be influenced by temperature, and temperature has also been proposed as a relevant factor for setting the elemental allocation in marine phytoplankton. In this study, we found that percentage of fatty acids unsaturation was negatively related with temperature in some Prochlorococcus strains, but this was not universal. Temperature had a significant linear positive effect on nitrogen and carbon cell quotas across all strains. As temperature increased 10˚C, nitrogen and carbon quotas rose by 40.0% and 34.6%, respectively. Individual strains displayed negative relationships between growth rate and phosphorus quota.

Studies in other photosynthetic organisms suggested that HOOH and temperature extremes act together as stressors. Importantly, it also has been shown that Prochlorococcus is highly susceptible to hydrogen peroxide (HOOH) and co-occurring heterotrophs such as Alteromonas sp. facilitate the growth of Prochlorococcus at the ocean surface by scavenging HOOH. To address the potential synergistic effects of temperature and HOOH on Prochlorococcus, we monitored the growth of environmental-relevant concentrations of cold-adapted (MED4) and warm-adapted (MIT9312) Prochlorococcus strains with different initial concentrations of HOOH under a range of temperatures. While not impacting the temperature optima for growth, higher concentrations of HOOH severely diminished the permissive temperature range for growth of both Prochlorococcus strains. At the permissive temperatures, the growth rates of both Prochlorococcus strains decreased as a function of HOOH, and temperature extremes increased susceptibility of photosystem II to HOOH-mediated damage. While these effects were manifest in both strains, they were more pronounced in the warm-adapted strain. Heterotrophic bacteria, serving as a proxy for the natural community, increased the Prochlorococcus growth rate under these temperatures and increased the growth temperature range (MED4), and this was attributed in part to their ability to remove HOOH from the medium.