Hidden Worlds: Biodiversity of Marine Microbes

Every drop of seawater contains hundreds of fantastically diverse groups of microbes that together control key biogeochemical processes in the ocean and help determine the habitability of our planet. The challenge is to scale from this world of individual cells to ecosystem function and ultimately to ocean basin processes. We focus on microscopic marine diatoms because they are responsible for about one-fifth of the photosynthesis that occurs on Earth each year, they form the base of highly productive marine food webs, and they help regulate past and current atmospheric levels of CO₂. To begin to dissect how diatoms interact with other member of the microbial community, we developed a model microbial system consisting of a common coastal diatom, Pseudo-nitzschia multiseries, and a member of the bacterial genus, Sulfitobacter.  In this system, the bacterium is entirely dependent on the diatom for production of organic carbon.  In co-culture, the synergistic bacterium produces the plant hormone auxin and the diatom produces tryptophan, with ammonia and different organosulfur compounds exchanged between partners. These interactions significantly increase the growth rate and productivity of the diatom. We subsequently detected auxin in seawater samples collected from coastal and open ocean sites in the northeast Pacific Ocean.  We then used metatranscriptomic approaches to detect transcripts associated with the biosynthesis of auxin in these same samples and find that uncultured bacteria appear to generate the detected auxin.  We hypothesize that these types of specific interactions are common features of microbial communities, influencing ecosystem function.