Haim Treves - Chlorella ohadii - fastest growing, photodamage and desiccation tolerant alga from desert crusts

The unparalleled performance of Chlorella ohadii clearly indicated that we lack information on the photosynthetic machinery and what sets the upper growth limits. When grown under optimal laboratory or controlled outdoor conditions, this alga, recently isolated from one of the harshest environments (a biological desert sand crust), exhibits the fastest growth rates ever reported for an alga, division times shorter than 2 h were recorded. The cultures perform very high photosynthetic rates and reach high cell densities (1.3*109 cells/mL). Unlike other photosynthetic organisms, C. ohadii productivity is unaffected by irradiances twice full sun light; and the level of protein D1, encoded by a single gene, is hardly affected. Rather than succumbing to photodamage C. ohadii undergoes major structural and compositional changes (including 2-3 fold increase of the lipid and carbohydrate contents and a large rise in the abundance of the thylakoids), emphasizing the importance of its unique PSII functioning as well as highly efficient reductant utilization downstream of the photosynthetic reaction centers. Comparing its genome sequence with those of other algae shed light on the unique genetic potential of C. ohadii, its growth and photosynthetic performance and its ability to withstand salinity and desiccation in its natural environment. RNA-Seq revealed regulation of genes networks under changing light and trophic regimes, and provided novel insights on the mechanism underlying its exceptional photodamage resistance. In spite of its enormous growth and photosynthetic capabilities, C. ohadii is found in low abundance in its natural environment and was originally isolated as a contamination in a decaying culture of the filamentous cyanobacterium Leptolyngbya ohadii, an important primary producer in the BSCs. Using a tailor-made chamber simulating crust conditions, we show that ability of C. ohadii to revive after desiccation depends on close contact with L. ohadii, through a unique and novel mode of interspecies association. In view of the unparalleled growth and photosynthetic performance, C. ohadii may be used to clarify the processes that rate-limit growth and productivity of photosynthetic organisms. The biotechnological potential and uses are self-evident.