Comparative physiology of two poikilohaline Picochlorum strains

Abstract

The Picochlorum genus was first circumscribed in 2004, and has since had numerous species and strains added and characterized. This study uses batch and semicontinuous chemostat culture modes to investigate the physiology of Picochlorum oklahomense and P. sp. SENEW3, in relation to salinity acclimation and shock. Though these two strains are closely related with >99% rRNA identity, they show marked differences in growth rate, nitrogen uptake, and lipid content in response to salinity stress. Additionally, while each strain is shown to accumulate free proline as an organic osmolyte, the timing and magnitude of proline accumulation differs significantly between strains. While both strains utilize proline in a similar manner in salinity shock, P. oklahomense relies more heavily on proline in salinity acclimation, indicating that P. sp. SENEW3 must have additional salinity tolerance mechanisms. P. oklahomense was also shown to have an increased lipid content at high salinity but grew slower than P. sp. SENEW3. The results of this physiological characterization support past transcriptomic evidence and help to explain how the environment shapes the physiology and stress response of organisms.