Contrasting Prochlorococcus temperature niches in the lab and across ocean basins

Prochlorococcus, a diverse group of marine cyanobacteria, are the most abundant photosynthetic organisms in the world, accounting for up to 48% of global primary production. Previous studies have shown that different strains of Prochlorococcus prefer different optimum temperatures for maximum growth and thus can coexist along temperature gradients through niche partitioning. However, the temperature distributions for individual strains measured in the field (i.e. realized temperature niches) are not always the same as the temperature distributions predicted from laboratory growth rate experiments (i.e. fundamental temperature niches). While recent studies have shown evidence of these niche differences, the expected and measured temperature distributions for strains of Prochlorococcus have not been quantified and compared across ocean basins. We aggregated both lab-derived growth rates and field-derived abundance distributions across temperatures for four genetically distinct groups of Prochlorococcus (eMED4, eMIT9312, eMIT9313, and eNATL2A). Using statistical modeling methods, we quantified the fundamental and realized temperature niches for each of these four groups. We found that optimum temperature for growth in the lab is generally higher than the temperature at which the Prochlorococcus groups are most abundant in the field. The temperature niche widths in the field are also generally wider than estimated in the lab. In addition, we found that the realized niches for each of the four distinct groups of Prochlorococcus differ across ocean basins suggesting some level of either phenotypic plasticity, local adaptation, or variation in community dynamics.