A study sheds new light on chemical processes that cause marine bacteria to switch from coexistence with an algal host to killer mode – Zoo House News
Scientists have described a lifestyle shift occurring in marine bacteria, in which they switch from coexisting with algal hosts in a mutually beneficial interaction to suddenly killing them. The results are published today in eLife.
Details of this lifestyle shift 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 called phytoplankton form oceanic blooms, which are responsible for about half of the photosynthesis that occurs on Earth, 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 from the Roseobacter group are known to mate and coexist with phytoplankton in a mutually beneficial interaction. The phytoplankton provide Roseobacter with organics such as sugars and amino acids that are useful for bacterial growth, and Roseobacter in turn provides B vitamins and growth-promoting factors.
However, recent studies have shown that Roseobacter undergoes a lifestyle shift from coexistence to pathogenicity, where they kill their phytoplankton hosts. A chemical compound called DMSP is produced by the algae and is thought to play a role in this switch.
“We previously found that the roseobacter Sulfitobacter D7 exhibits a lifestyle change when interacting with the phytoplankter Emiliania huxleyi,” says first author Noa Barak-Gavish, a PhD student in the Department of Plant and Environmental Sciences at the Weizmann Institute of Science, Israel. “However, our knowledge of the factors driving this switch was still limited.”
To characterize this lifestyle change, Barak-Gavish and colleagues performed a transcriptomics experiment that allowed them to compare the genes differentially expressed by Sulfitobacter D7 in coexistence or pathogenicity states.
Their experimental setup showed that Sulfitobacter D7 grown in a pathogenicity-inducing medium has higher expression of transporters for metabolites such as amino acids and carbohydrates than those grown in a coexistence medium. These transporters serve to maximize uptake of metabolites released by dying Emiliania huxleyi (E. huxleyi). In addition, the team observed increased activation of flagellar genes, which are responsible for bacterial movement, in the pathogenic Sulfitobacter D7. These two factors allow Sulfitobacter D7 to employ an “eat-and-run” strategy, where they beat competitors for the material released upon E. huxleyi cell death and swim away in search of another suitable host.
The team confirmed the role of DMSP in inducing the switch to this killer behavior by mapping the genes that were activated in Sulfitobacter D7 in response to the presence of DMSP and other algal-derived compounds. However, when only DMSP was present, the lifestyle change did not occur. This implies that DMSP, while mediating lifestyle change, is also dependent on the presence of other E. huxleyi-derived infochemicals – compounds produced by organisms and used for communication. DMSP is an infochemical produced by many phytoplankton, so it is likely that the other required infochemicals allow the bacteria to recognize a specific phytoplankton host. In natural environments where many different microbial species live together, this specificity would ensure that bacteria only invest in altering gene expression and their metabolism when the right algal partner is present.
The study also reveals the role of algal-derived benzoates in the interactions between Sulfitobacter D7 and E. huxleyi. Even in high concentrations of DMSP, benzoate serves to sustain the coexistence lifestyle. Benzoate is an efficient growth factor and is supplied to Sulfitobacter D7 during E. huxleyi coexistence. The authors suggest that as long as Sulfitobacter D7 benefits from coexistence by preserving materials for growth, it maintains mutual interaction. With less benzoate and other growth substrates provided, the bacteria make the lifestyle switch and kill their phytoplankton host, swallowing any remaining beneficial materials.
The exact mechanism of pathogenicity of Sulfitobacter D7 against E. huxleyi has yet to be discovered and the authors call for further work in this area. The type 2 secretory system of the cellular machinery — a complex that many bacteria use to move materials across their cell membrane — is more common in Sulfitobacter D7 than other Roseobacter, indicating a unique mode of pathogenicity that warrants further study.
“Our work provides a contextual framework for the shift from coexistence to pathogenicity in Roseobacter-phytoplankton interactions,” concludes senior author Assaf Vardi, professor in the Weizmann Institute of Science’s Department of Plant and Environmental Sciences. “These interactions are an underappreciated component in regulating algal bloom dynamics, and further studies in this area could provide insights into their impact on the fate of carbon and sulfur in the marine environment.”