Scientists have detailed a lifestyle switch that occurs in marine bacteria, as they change from coexisting with their algae hosts in a mutually beneficial interaction to suddenly killing them. The results were published today eLife.
Details of this key lifestyle could provide new insights into the regulation of algal bloom dynamics and their impact on large-scale biogeochemical processes in marine environments.
Single-celled algae, better known as phytoplankton, make up ocean flora responsible for about half of the photosynthesis that occurs on land, and form the basis of marine food webs. Therefore, understanding the factors that control phytoplankton growth and death is critical to maintaining a healthy marine ecosystem. Marine bacteria of the Roseobacter group are known to mate and coexist with phytoplankton in a mutually beneficial interaction. Phytoplankton provide Roseobacter with organic substances useful for bacterial growth, such as sugar and amino acids, and Roseobacter bacteria in return provide B vitamins and growth-promoting factors.
However, recent studies have revealed that Roseobacter bacteria undergo a lifestyle change from commensal to pathogenic, killing their phytoplankton hosts. A chemical compound called DMSP is produced by algae and is hypothesized to play a role in this switch.
“We previously identified that the bacterium Roseobacter sulfate D7 displays a lifestyle switch when interacting with an algae plant Emilia Huxley,” says first author Noa Barak Gavish, a PhD graduate in the Department of Plant and Environmental Sciences, Weizmann Institute of Science, Israel. However, our knowledge of the factors that determine this switch is still limited.”
To characterize this lifestyle shift, Barak Gavish and colleagues conducted a transcription experiment, allowing them to compare genes that are differentially expressed by sulfate D7 in commensal or pathogenic stages.
Their experimental setup showed that sulfate D7 grown in pathogenic media have higher expression of metabolite transporters such as amino acids and carbohydrates than those grown in symbiont medium. These transporters maximize the uptake of metabolites released from death Emilia Huxley (E. huxleyi). Moreover, in pathogens sulfate D7, the team observed increased activation of flagellate genes responsible for the movement of bacteria. These two factors allow it sulfate D7 to use the “eat and run” strategy, where they beat competitors with the items they have been issued E. huxleyi Cells die and swim away in search of another suitable host.
The team confirmed the role of DMSP in inducing the switch to this lethal behavior by mapping active genes in sulfate D7 in response to the presence of DMSP and other algae-derived compounds. However, when only DMSP was present, lifestyle switching did not occur. This means that although DMSP mediates lifestyle switching, it also depends on the presence of others E.huxleyi-Derived informational chemicals – compounds that organisms produce and use to communicate. DMSP is an informational chemical produced by many phytoplankton, so it is likely that other informational chemicals required would allow bacteria to recognize a specific phytoplankton host. In natural environments, where many different microbial species co-exist, this specificity ensures that bacteria only invest in altering gene expression and metabolism when the right algal partner is present.
The study also reveals the role of algae-derived benzoates in sulfate D7 f E. huxleyi interactions. Even in high concentrations of DMSP, benzoate works to maintain a commensal lifestyle. Benzoate is an effective growth factor and is provided by E. huxleyi to sulfate D7 during coexistence. The authors suggest that as long sulfate D7 benefits from symbiosis by receiving materials for growth, and will maintain mutual interaction. When less benzoate and other growth substrates are provided, the bacteria undergo a lifestyle switch and kill their phytoplankton host, engulfing any remaining beneficial materials.
The exact mechanism of sulfate D7 Pathogenicity vs E. huxleyi They remain to be discovered, and the authors call for more work in this area. The type 2 secretion system of the cellular machinery—a complex used by many bacteria to transport materials across the cell membrane—is more prevalent in sulfate D7 compared to other Roseobacters, suggesting a unique mode of pathogenicity that requires further investigation.
“Our work provides a contextual framework for the transition from commensal to pathogenic in Bacteriobacteria and Phytoplankton interactions,” concludes senior author Asaf Vardi, Professor in the Department of Plant and Environmental Sciences, Weizmann Institute of Science. “These interactions are an underappreciated component in regulating algal bloom dynamics and further study in this area could provide insights into their impact on the fate of carbon and sulfur in the marine environment.”