The mechanisms whereby the green alga Chlorella ohadii, isolated from desert soil crust, exhibits unparalleled photo damage resistance.

Acquired ability to acclimate to extreme environments is usually accompanied by reduced performance under optimal conditions. However, the green alga Chlorella ohadii, recently isolated from a harsh biological sand crust (BSC) in the desert, does not obey this rule. When grown under controlled laboratory or outdoor facility it exhibits the fastest growth rate ever reported for an alga, showing ultradian rhythm that reflects metabolic shifts during continuous growth in the light. The cultures perform one of the highest photosynthetic rates ever reported and can reach very high densities, over 109 cells/mL. Unlike other photosynthetic organisms, C. ohadii is undamaged by irradiances as high as twice full sun light and, unlike other photosynthetic organisms, shows no damage to protein D1 in the core center of photosystem II. Instead the cells undergo major structural, compositional and physiological changes including a large rise in the thylakoid abundance and packing, pyrenoids development, increased photosynthetic Vmax and affinity to inorganic carbon, and a 2-3 fold rise of lipid and carbohydrate contents. Exposure to light intensities higher than required to saturate CO2 fixation increased the O2 evolution rate while forming lipids whereas variable fluorescence ceases raising doubts about use of fluorescence to assess photosynthetic activity. The wide dynamic range of C. ohadii growth and its unique performance under stressing and optimal conditions may help to uncover growth limiting processes in photosynthetic organisms. C. ohadii may be used in algal farms for biomass or energy production or as a gene source for the improvement of crop plants