Multi-omics approach provides insights into extreme illumination stress response

The unparalleled performance of Chlorella ohadii (Treves et al., 2016), clearly indicated that we lack essential information on the photosynthetic machinery and what sets the upper growth limits. Unlike other photosynthetic organisms, C. ohadii productivity is unaffected by irradiances of twice full sun light; Rather than succumbing to photodamage C. ohadii undergoes major structural and compositional changes emphasizing its unique PSII functioning.

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. Growth of batch cultures under continuous high light (3000 µmol photons m^-2 s^-1) combined with metabolome analyses revealed a highly coordinated metabolic switch, regulated by specific signaling molecules of the Polyamines group (Treves et al., 2017).

This remarkable resistance allowed us to investigate, for the first time, the systems response of photosynthesis and growth to extreme illumination in a metabolically active cell. Using redox proteomics, transcriptomics, metabolomics and lipidomics, we identified key response regulators and explored the cellular mechanisms underlying inorganic carbon concentration, excess redox dissipation, protein S-glutathionylation, lipid and starch accumulation and increased thylakoid stacking. Frequently, the response in C. ohadii deviated from those reported in model species, reflecting its life history in desert sand crusts. These, together with a transformation system developed these days for C. ohadii will allow to dissect what distinguishes this alga from its more sensitive counterparts.