Physiological responses to nitrogen availability in Hydrogenovibrio marinus

The ability for microorganisms to adapt to changes in resource availability is critical for their survival in nature. A common adaptation among microbes is the accumulation of glycogen as intracellular energy reserves during times when carbon availability exceeds that of other important nutrients, such as nitrogen. In addition, some microbes form biofilms under resource limitation to trap and concentrate nutrients. A fundamental understanding of these metabolic processes and what controls them is critical for utilizing microorganisms for societal benefit. Here, we investigated how the obligately chemolithoautotrophic hydrogen-oxidizing bacterium Hydrogenovibrio marinus responds to differences in nitrogen availability. Using either ammonium or urea as nitrogen sources, we assessed H. marinus' capacity for growth, substrate uptake, glycogen production, and biofilm formation at three different nitrogen concentrations. Growth was generally stronger on ammonium, regardless of the concentration tested, although we did observe concentration-dependent effects on both growth and gaseous substrate uptake when urea was provided. While there were few differences in glycogen production between the tested conditions, we found that low ammonium concentrations (0.1 g NH₄-N/L) promoted robust biofilm formation, a phenotype not yet described for this organism. Taken together, these results further our understanding of the ecophysiology of marine chemolithoautotrophs and how nitrogen availability influences their survival. Understanding the basic microbial physiology will enable us to harness these organisms to valorize CO₂ into usable products in fermentation processes.IMPORTANCEIn order for microbes to survive in nature, they must develop strategies to handle stresses caused by nutrient limitation due to changes in environmental chemistry. There are several adaptations that microbes can use to cope with nutrient limitation, including accumulating internal energy reserves during times of excess, or forming biofilms to more effectively capture and distribute nutrients. This study examines the strategies that hydrogen-oxidizing bacteria use to deal with changes in nutrient availability, using Hydrogenovibrio marinus as a representative case. Since nitrogen is critical for growth and metabolism, we examined the physiological responses of H. marinus to different concentrations of either ammonium or urea. We found that growth was generally more robust on ammonium than on urea, and H. marinus is also capable of forming biofilms when provided 0.1 g-N/L from ammonium. These results contribute to understanding how environmental factors influence the survival of marine microbes in nature toward harnessing them for biotechnology.