Green gold from the desert

The surface of sandy deserts is one of the most inhospitable environments on the planet, yet some organisms have evolved remarkable adaptations that allow them to survive such harsh conditions. This is the story of one of them - a humble single-celled alga.

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By Haim Treves and Mark Stitt

Desert biological sand crusts are one of the harshest environments for life on earth, with wide temperature amplitudes from subfreezing during winter nights to 60°C in mid-summer days, frequent hydration by morning dew and dehydration by rising sunlight and temperature, and extremely high light. Nevertheless, some life forms manage to grow in this harsh environment.  Photosynthetic members of the so-called ‘crust consortium’ have adapted to carry out photosynthesis and to grow for a short time in the morning, after the sun has risen and before morning dew has evaporated.

Figure: Schematic view of desert BSC diurnal cycle (Extended Data Figure 10 in the applying publication). Main water supply is by dew formation during the night, followed by morning dehydration. Through most of the day, crusts are dry and exposed to extreme illumination (~2000 µmol photons m-2 s-1).

Working in the Negev desert, one of the driest and hottest in the world, a small green alga was found which resists all these stressors combined [1]. This alga, thereafter named Chlorella ohadii after the late famous Israeli plant physiologist Itzhak Ohad, can maintain productivity in the face of twice full sunlight [2], and exhibits the fastest growth rates reported for any photosynthetic cell [3] under optimal laboratory conditions.

The photosynthetic apparatus can be visualized as an electric cable that transfers current from a power supplier (sunlight) to a receptor (CO2). Situations where the power source is too strong or there are too few electron acceptors increase the likelihood of a potentially destructive current overload on the cable. Technically this is termed photoinhibition. It involves destruction of Photosystem II protein core and significantly lowers global photosynthetic productivity.

In our study, we showed that rather than succumbing to the destructive effects of excess light, C. ohadii is able to rapidly repoise its redox status, aided by rapid post-translational redox regulation of protein kinases and reactive oxygen- and heat-shock- factors, and novel thylakoid remodeling. We also showed that C. ohadii maintains surplus metabolic and growth capacity in low light, allowing a rapid increase in photosynthesis and growth to utilize the energy when it is suddenly exposed to extremely high light - which is important in the ecological niche in which it grows with only brief periods when a combination of light and water availability allow photosynthesis and growth.

Our work emphasizes that the potential of exploiting C. ohadii as a model for basic research is enormous, addressing a wide span of fields including photosynthesis research, stress response, algal cell biology and growth, crop plant improvement and microbial interactions. Less than a decade after its discovery, C. ohadii is emerging as an exciting new algal model with a growing international community exploring its eco-physiology, cell and molecular biology. C. ohadii is being used as a gene source to develop improved-yield crop plants. It may prove to be a "game-changer" in the ongoing quest to meet increasing global demand for food and energy supply. In an era where many are seeking for ways to increase biomass for energy and feed stock and facing global climate change, this potential cannot be overestimated.

Who would have thought that from the dwellings of the pristine sand dunes, facing extreme challenges and struggling adversity, would emerge a model alga with the promise of a glittering career ahead of it. Chlorella ohadii - a name to watch out for in the future.

Figure: Multi-omics illustration of C. ohadii temporal responses of photosynthesis and central C metabolism to EIL treatments. Genes, proteins, metabolites and lipids presented showed significantly higher (red) or lower (blue) levels of expression, redox-response, and accumulation, respectively. Arrows illustrate potential positive (red) and negative (blue), direct (solid line), and indirect (dashed line) effects.

References

  1. Treves, H., et al., A newly isolated Chlorella sp. from desert sand crusts exhibits a unique resistance to excess light intensity. FEMS Microbiology Ecology, 2013. 86: p. 373-380
  2. Treves, H., et al., The mechanisms whereby the green alga Chlorella ohadii, isolated from desert soil crust, exhibits unparalleled photodamage resistance New Phytologist 2016. 210: p. 1229-1243.
  3. Treves, H., et al., Metabolic flexibility underpins growth capabilities of the fastest growing alga. Current Biology, 2017. 27: p. 2559-2567.
  4. Treves, H., et al., Multi-omics reveals mechanisms of total resistance to extreme illumination of a desert alga. Nature Plants, 2020. doi: 10.1038/s41477-020-0729-9.

Haim Treves

Post-doctoral fellow, Max Planck Institute of Molecular Plant Physiology

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