Prochlorococcus growth responses to light and oxygen regimes

Abstract

Prochlorococcus is a genus of very small marine picocyanobacteria characterized by an unusual green pigmentation, because they lack the phycobilisomes characteristic of most cyanobacteria. Prochlorococcus belongs to the phylogenetically diverse functional group of photosynthetic picophytoplankton and is the most abundant photosynthetic organism on earth. In aquatic environments, phytoplankton, and specifically Prochlorococcus, metabolism represents a major source of extracellular reactive oxygen species (ROS). If left un-scavenged ROS can have detrimental effects on marine phytoplankton growth and survival. Different Prochlorococcus strains grow stratified in marine aquatic environments from the surface to the bottom of the illuminated euphotic zone. Strains of Prochlorococcus at different depths exhibit diverse responses to light, however, they are slow growing and difficult to culture, with limited analyses of their growth responses. During this project I monitored growth trajectories of diverse Prochlorococcus strains across a matrix of light levels, light colors and oxygen levels. I found that each strain shows a distinct pattern of growth rates across the environmental matrices, with Prochlorococcus MED4 thriving under a high light, fully oxygenated niche; Prochlorococcus SS120 preferring a low light, oxygenated niche and Prochlorococcus MIT9313 preferring a low oxygen, high light niche. The combination of blue light and oxygenation is particularly stressful to Prochlorococcus MIT9313. We hypothesize that the oxygen intolerance of Prochlorococcus MIT9313 relates to strong sensitivity to ROS. Therefore, we took samples for subsequent analyses of cross linking of the major RbcL protein, a proxy for cumulative effects of ROS exposure. The results presented in this project can be applied to climate change research because it highlights the environmental responses of the foundational organism in marine food webs and bottom-up factors in ecosystem change within marine environments.