Inorganic nitrogen exchange discovered to be pathway for microbial communication in marine ecosystems

A recent study has revealed that the interactions between algae and bacteria in oxygen-rich marine environments depend on the exchange of inorganic nitrogen compounds, such as nitrite and nitric oxide. These compounds act as signaling molecules, initiating a cascade that triggers algal cell death. This process could potentially contribute to the sudden collapse of oceanic algal blooms. The research was conducted by Dr. Adi Abada, Dr. Roni Beiralas, among others, and published in the ISME Journal.

According to the study, marine biogeochemistry is heavily influenced by microbial interactions. Traditionally, the exchange of organic molecules has been considered the primary communication pathway. However, this research has uncovered a novel inorganic route of microbial communication in oxygenated marine environments. The study specifically examines the interactions between Phaeobacter inhibens bacteria and Gephyrocapsa huxleyi algae. It demonstrates that inorganic nitrogen compounds like nitrite and nitric oxide (NO) play a significant role in mediating these interactions. The study reveals that under oxygen-rich conditions, aerobic bacteria utilize denitrification—a well-studied anaerobic respiratory mechanism—to reduce algal-secreted nitrite to NO. This bacterial NO then triggers a cascade within algae resembling programmed cell death, leading to the collapse of algal populations and mimicking the abrupt demise of oceanic algal blooms.

The researchers also shed light on the unexpected presence of denitrification genes in various marine bacteria inhabiting oxygenated surface waters. According to them, denitrification is typically associated with low-oxygen conditions and allows microorganisms to maintain their bioenergetics through a series of nitrogen species reductions—including nitrate (NO3¯) to nitrite (NO2¯), NO to nitrous oxide (N2O), and ultimately to dinitrogen (N2). Abada's team found that denitrification intermediates, including nitrite and NO, accumulate in the marine environment. However, knowledge about the short-lived NO intermediate has been limited. This research suggests that NO may play a more significant role in marine microbial ecology than previously acknowledged.

Furthermore, the study uncovers that these algal-bacterial interactions involving inorganic nitrogen compounds are not unique to the G. huxleyi—P. inhibens model system. Beiralas and his team detected denitrification genes and transcripts in oxygen-rich regions beyond this specific interaction, indicating the potential ecological importance of this phenomenon. While facultative anaerobes like P. inhibens T5 might contribute to this process, aerobic bacteria with subsets of denitrification genes were also identified. Previous research has hinted at the expression of denitrification genes and active denitrification in oxygen-rich conditions; however, their ecological implications were not previously explored. The research suggests that microbial NO could have overlooked roles in microbial ecology, warranting further investigation into the prevalence and significance of microbial denitrification in oxygen-rich marine environments.

Source: Springer Nature: Adi Abada, Roni Beiralas, et al., Aerobic bacteria produce nitric oxide via denitrification and promote algal population collapse, ISME (2023). https://doi.org/10.1038/s41396-023-01427-8