Members of the Roseobacter clade are among the most abundant and ecologically relevant marine bacteria. Some roseobacters live on the surface of small marine algae (phytoplankton, dinoflagellates), and the long-term goal of this research is to understand how they establish and maintain their symbiosis with these phytoplankton cells. These bacteria have a biphasic lifestyle that includes a motile phase that is attracted to the dinoflagellate, and a sessile stage that loses flagella, synthesizes adhesive proteins, and forms biofilms on the host. The sessile cells produce an antibacterial compound (TDA) that likely inhibits competing bacteria and promotes the growth of the host. The Belas lab has used genetic and molecular techniques to discover two genes that that play major roles in this "swim-or-stick" life-style. One gene complex controls motility, while the other controls production of the antibiotic. Understanding how these genes operate and are regulated will significantly increase our understanding of how bacterial motility and colonization are controlled, key issues for both the marine environment and for understanding the spread of pathogens. On a broader scale, the dinoflagellates produce a compound (DMSP) that the bacteria convert to dimethyl sulfide (DMS); DMS in the atmosphere is thought to play a role in climate control. The results of this research will be incorporated in educational programs to train high school teachers, graduate, undergraduate and high school students, many of whom are underrepresented minorities and women, as well as to provide community "K-to-gray" outreach, involving students of all ages.
Marine microorganisms, such as bacteria and unicellular microalgae, live in tight-knit associations and symbioses. One group of bacteria in particular, the roseobacters, is particularly noteworthy as these bacteria are among the most abundant, ecologically relevant marine bacteria that form symbioses with photosynthetic algae, which are major producers of the world’s oxygen and extremely important to the health of oceans. Roseobacters, such Silicibacter sp. TM1040 (our model bacterium), exhibit close physical and physiological relationships with microalgae suggesting that these bacteria are highly adapted to engage in this symbiosis. It is our long-term goal to understand the genetic mechanisms used by the roseobacters that allow them to establish and maintain their symbiosis with phytoplankton. Through this NSF-funded research project, we discovered that roseobacters have a biphasic "swim-or-stick" lifestyle that includes a motile phase characterized by highly motile cells that sense and are drawn to potential host cells, and a sessile stage during which the roseobacters attach to the alga forming a biofilm. Roseobacter biofilms produce an antibacterial compound whose activity is critical during symbiosis, as it is both an antibiotic that kills nonroseobacteria and a chemical that promotes the symbiosis by positively affecting the health of both the roseobacters and the alga. As algal cells die they release chemicals that trigger the roseobacters to produce yet another chemical compound that flips the bacterial "swim-or-stick" switch back towards motility. This compound, called a roseobacticide, has two functions. First, it lyses the algal host, releasing nutrients that the roseobacters use – this is particularly important, as bacterial swimming motility is energy intensive. Second, the roseobacticide induces the bacteria to leave the biofilm and swim elsewhere to find a new algal cell, thereby restarting the symbiosis anew. Our results indicate that production of this chemical is common in Roseobacter clade. Thus, during this research project’s term, we identified and characterized chemical signaling components of both sides of the swim-or-stick switch, and added valuable new knowledge that scientists studying bacteria, algae, symbioses, and marine ecosystems will find invaluable. Our results show that small molecules, such as the compounds discovered as part of this NSF-funded research, may be considered the lingua franca that helps coordinate bacterial activities with those of the algal host in the marine environment. In so doing, these chemicals determine the productivity of the algae, and in turn influence nutrient cycles, oxygen production, and carbon sequestration in the ocean, and (perhaps) may affect global climate by modulating algal physiology. From a biotechnological standpoint, the discovery of these chemical signals that control the health and productivity of the microalgal hosts may also be exploited for use in commercial industries that rely on algae for the production of biofuels, pharmaceutical compounds, algal-derived human supplements (for example, omega-3 fatty acids), and food sources for aquaculture.
