Static osmolyte concentrations in microbial biomass during seasonal drought in a California grassland

Microbes cope with environmental changes by physiologically adapting to cues such as moisture and temperature. These responses happen at the molecular scale, but ultimately serve as controls on ecosystem level biogeochemical cycles. One example of this response is the accumulation of osmolytes, cellular compounds used to prevent desiccation under osmotic stress. We explored microbial biomass levels of nitrogenous osmolytes including amino acids glutamate and proline, and the tertiary amine glycine betaine, monthly in a seasonally dry California grassland using high performance liquid chromatography and either fluorescence or mass spectrometry-based detection. We also examined the behavior of amino acids in soil to analyze our ability to accurately measure putative osmolytes and determine contributions of microbial consumption and abiotic adsorption during extraction. We determined there was limited microbial consumption or abiotic adsorption of amino acids during extraction, and found no accumulation of expected nitrogenous osmolytes during the summer dry period with glutamate levels consistently 2–3% of the microbial biomass pool, and below detection limit levels of proline and glycine betaine. The lack of osmolyte use suggests the physiological response expected based on pure culture studies of individual microorganisms, and that of mixed soil consortia may be different due to a lack of sufficient osmotic stress or resource limitation in an environmental system.