Physiological responses to pH in the freshwater microalga Limnomonas gaiensis

The ecological niche of the recently described limnic microalga Limnomonas gaiensis (Chlamydomonadales) in Northern Europe remains unknown. To decipher the species tolerance capacity to pH, the effects of hydrogen ions on the physiological response of L. gaiensis were investigated. Results showed that L. gaiensis could tolerate exposure from pH 3 up to pH 11, with an optimal survival at pH 5-8. Its physiological response to pH was strain specific. Globally the southernmost strain was more alkaliphilic, had a slightly rounder shape, a slowest growth rate, and a lowest carrying capacity. Despite strain discrepancies among lakes, Swedish strains exhibited similar growth rates, faster at more acidic conditions. The extreme pH conditions affected its morphological features such as the eye spot and papilla shape, especially at acidic pH, and the cell wall integrity, at more alkaline pH. The wide range tolerance of L. gaiensis to pH would not be a hindrance to its dispersal in Swedish lakes (pH 4-8). Notably, the storage of high-energetic reserves over a wide range of pH conditions, as numerous starch grains and oil droplets, makes L. gaiensis a good candidate for bioethanol/fuel industrial production and a key resource to sustain aquatic food chain and microbial loop.