UC Irvine

Prochlorococcus, the smallest known photosynthetic bacterium, is abundant in the ocean’s surface layer despite large environmental variation. There are several phylogenetically distinct lineages within Prochlorococcus and considerable gene gain and loss throughout its evolutionary history. However, the extent to which vertical versus horizontal inheritance shapes its genome diversity across the global oceans is unknown. We observed that Prochlorococcus field populations from a global circumnavigation had a significant relationship between phylogenetic and gene content diversity including regional differences in both phylogenetic composition and gene content that were related to environmental factors. Overall we showed that the environment determines the functional capabilities of successful Prochlorococcus.

We know Prochlorococcus has extensive genetic diversity, including the presence of multiple major clades, its sister taxa Synechococcus displays similar levels of genetic diversity. Prochlorococcus has a clear phylogeography relating to environmental selective pressures, while the biogeography and environmental drivers of Synechococcus clades are more difficult to define. To better characterize Prochlorococcus and Synechococcus genetic diversity we used high throughput sequencing of the marker gene rpoC1 from 339 samples across the Pacific Ocean and Atlantic Oceans. At multiple taxonomic scales (lineage, clade, and SNP) we observed clear parallel biogeography between these two lineages. Overall, this parallel biogeography suggests similar evolutionary selective pressures for these important marine Cyanobacteria.

Oceans are warming and will continue to increase over the next 100 years due to global climate change. Adaptation will likely play a role, but it is unclear how it will impact microbial distributions and processes. To address this unknown, we experimentally evolved a member of the prevalent marine Roseobacter clade to high temperature for 500 generations. We found that this evolved Roseobacter shifted from its usual planktonic growth mode to creating more biofilm at the surface of the culture. Furthermore, this altered lifestyle was coupled with a suite of genomic changes linked to biofilm formation and increased growth in low oxygen transfer environments.