Resilience of a marine, methylotrophic denitrifying biofilm to environmental changes in a lab-scale denitrification system.

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In closed circuit water ecosystems, such as aquarium or aquaculture tanks, rapid increases in nitrate is a common problem, often reaching toxic levels for the ecosystem's inhabitants. Wastewater biofiltration units to closed ecosystems requires addition of a denitrification step. Denitrification takes place in bacterial cells where nitrate is reduced in N2 and serve as terminal electron acceptors instead of oxygen (O2) for energy production during O2 depletion. Although the engineering component of denitrification systems is well mastered, our understanding of how the microbial community operates in these systems is incomplete, especially under marine/salty conditions. My laboratory has been characterizing the microbial community of marine methylotrophic denitrifying biofilm obtained from a denitrification system. We found that the community is composed of 60% to 80% of methylotrophic bacteria affiliated to Methylophaga and Hyphomicrobium. We isolated two strains, Methylophaga nitratireducenticrescens JAM1 and Hyphomicrobium nitrativorans NL23, from this biofilm that are responsible for the denitrifying activities. Strain JAM1 has a partial denitrification pathway. It is capable of reducing nitrate to nitrite, but not reducing nitrite to nitric oxide. Strain NL23 can accomplish the full denitrification pathway, but cannot grow in pure cultures under saline conditions, which is peculiar because it originates from a marine system. However, co-culturing these two strains allows full denitrification to occur under marine conditions, suggesting cooperation between the two strains. We also showed that the denitrifying biofilm was very resilient to changes as it could sustain denitrifying activities at different pH and temperatures, and at different concentrations of methanol, nitrate and NaCl. We also demonstrated the presence of heterotrophic non-methylotrophic denitrifying bacteria in the biofilm suggesting that the biofilm has the potential to adapt to heterotrophic, non-methylotrophic environments. Taken together, these results demonstrate the plasticity of our marine methylotrophic denitrifying biofilm in adapting to changing environments. The general aim of my research program is to determine how changes in the operating conditions of a denitrification system can affect strains JAM1 and NL23, and to assess the importance of other members of the community in supporting the resilience of the system. In-depth studies on denitrification under marine conditions remain relatively scarce, even more so those focusing on methylotrophic bacteria under these unique conditions, which makes my research program highly original. Better understanding of how the microbial community in a biofilm adapts to changes will lead to the development of more stable and efficient denitrification systems, such as those found in closed-circuit systems or in marinocultures.