Prochlorococcus Exudate Stimulates Heterotrophic Bacterial Competition with Rival Phytoplankton for Available Nitrogen.

The marine cyanobacterium Prochlorococcus numerically dominates the phytoplankton community of the nutrient-limited open ocean, establishing itself as the most abundant photosynthetic organism on Earth. This ecological success has been attributed to lower cell quotas for limiting nutrients, superior resource acquisition, and other advantages associated with cell size reduction and genome streamlining. In this study, we tested the prediction that Prochlorococcus outcompetes its rivals for scarce nutrients and that this advantage leads to its numerical success in nutrient-limited waters. Strains of Prochlorococcus and its sister genus Synechococcus grew well in both mono- and cocultures when nutrients were replete. However, in nitrogen-limited medium, Prochlorococcus outgrew Synechococcus but only when heterotrophic bacteria were also present. In the nitrogen-limited medium, the heterotroph Alteromonas macleodii outcompeted Synechococcus for nitrogen but only if stimulated by the exudate released by Prochlorococcus or if a proxy organic carbon source was provided. Genetic analysis of Alteromonas suggested that it outcompetes Synechococcus for nitrate and/or nitrite, during which cocultured Prochlorococcus grows on ammonia or other available nitrogen species. We propose that Prochlorococcus can stimulate antagonism between heterotrophic bacteria and potential phytoplankton competitors through a metabolic cross-feeding interaction, and this stimulation could contribute to the numerical success of Prochlorococcus in nutrient-limited regions of the ocean. IMPORTANCE In nutrient-poor habitats, competition for limited resources is thought to select for organisms with an enhanced ability to scavenge nutrients and utilize them efficiently. Such adaptations characterize the cyanobacterium Prochlorococcus, the most abundant photosynthetic organism in the nutrient-limited open ocean. In this study, the competitive superiority of Prochlorococcus over a rival cyanobacterium, Synechococcus, was captured in laboratory culture. Critically, this outcome was achieved only when key aspects of the open ocean were simulated: a limited supply of nitrogen and the presence of heterotrophic bacteria. The results indicate that Prochlorococcus promotes its numerical dominance over Synechococcus by energizing the heterotroph's ability to outcompete Synechococcus for available nitrogen. This study demonstrates how interactions between trophic groups can influence interactions within trophic groups and how these interactions likely contribute to the success of the most abundant photosynthetic microorganism.

Degradation of hydrogen peroxide at the ocean's surface: the influence of the microbial community on the realized thermal niche of Prochlorococcus.

Prochlorococcus, the smallest and most abundant phytoplankter in the ocean, is highly sensitive to hydrogen peroxide (HOOH), and co-occurring heterotrophs such as Alteromonas facilitate the growth of Prochlorococcus by scavenging HOOH. Temperature is also a major influence on Prochlorococcus abundance and distribution in the ocean, and studies in other photosynthetic organisms have shown that HOOH and temperature extremes can act together as synergistic stressors. To address potential synergistic effects of temperature and HOOH on Prochlorococcus growth, high- and low-temperature-adapted representative strains were cultured at ecologically relevant concentrations under a range of HOOH concentrations and temperatures. 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 cold temperature increased susceptibility of photosystem II to HOOH-mediated damage. Serving as a proxy for the natural community, co-cultured heterotrophic bacteria increased the Prochlorococcus growth rate under these temperatures, and expanded the permissive range of temperature for growth. These studies indicate that in the ocean, the cross-protective function of the microbial community may confer a fitness increase for Prochlorococcus at its temperature extremes, especially near the ocean surface where oxidative stress is highest. This interaction may play a substantial role in defining the realized thermal niche and habitat range of Prochlorococcus with respect to latitude.The ISME Journal advance online publication, 31 October 2017; doi:10.1038/ismej.2017.182.

A Student's Guide to Giant Viruses Infecting Small Eukaryotes: From Acanthamoeba to Zooxanthellae.

The discovery of infectious particles that challenge conventional thoughts concerning "what is a virus" has led to the evolution a new field of study in the past decade. Here, we review knowledge and information concerning "giant viruses", with a focus not only on some of the best studied systems, but also provide an effort to illuminate systems yet to be better resolved. We conclude by demonstrating that there is an abundance of new host-virus systems that fall into this "giant" category, demonstrating that this field of inquiry presents great opportunities for future research.

The Role of Ocean Currents in the Temperature Selection of Plankton: Insights from an Individual-Based Model.

Biogeography studies that correlate the observed distribution of organisms to environmental variables are typically based on local conditions. However, in cases with substantial translocation, like planktonic organisms carried by ocean currents, selection may happen upstream and local environmental factors may not be representative of those that shaped the local population. Here we use an individual-based model of microbes in the global surface ocean to explore this effect for temperature. We simulate up to 25 million individual cells belonging to up to 50 species with different temperature optima. Microbes are moved around the globe based on a hydrodynamic model, and grow and die based on local temperature. We quantify the role of currents using the "advective temperature differential" metric, which is the optimum temperature of the most abundant species from the model with advection minus that from the model without advection. This differential depends on the location and can be up to 4°C. Poleward-flowing currents, like the Gulf Stream, generally experience cooling and the differential is positive. We apply our results to three global datasets. For observations of optimum growth temperature of phytoplankton, accounting for the effect of currents leads to a slightly better agreement with observations, but there is large variability and the improvement is not statistically significant. For observed Prochlorococcus ecotype ratios and metagenome nucleotide divergence, accounting for advection improves the correlation significantly, especially in areas with relatively strong poleward or equatorward currents.