REIGNING IN THE HORSES: INTERSPECIES INHIBITION KEEPS THE WORLD`S FASTEST GROWING PLANT CELL AT LOW ABUNDANCE

Chlorella ohadii is a small unicellular green alga isolated from desert crusts in the Negev. It is unique in many ways, not least of which is its exceptional growth rate. At~1.5 hours, it exhibits the shortest generation time ever reported in a photosynthetic organism. Given optimal conditions, C. ohadii cultures can reach a very high cellular density. Despite these facts, discovery of this organism was long delayed due to a very low abundance in its natural habitat. This is surprising, as in addition to fast growth, C. ohadii is a very resilient organism capable of withstanding illumination as high as 3500µmolphotons*m^-2*s^-1. These traits have led us to wonder what has kept this organism from dominating freshwater systems worldwide, and its scarcity in its natural habitat is a curiosity in itself.

One possible explanation to this conundrum is inter-species inhibition. When grown in co-culture with Leptolyngbya ohadii, a cyanobacterial strain abundant in the same desert crusts, little to no growth of C. ohadii is observed. Here we report on initial efforts to characterize and understand this interaction. Our experiments show conclusively that inhibition is produced by a soluble excreted factor (or group thereof), of a size smaller than 10kD. Furthermore, excretion of this factor is activated by initial exposure of L. ohadii to C. ohadii, and is not constitutively active. This factor, in addition to inhibiting growth, is shown to impact the physiological well-being of C. ohadii.

The discovery and characterization of a highly effective inhibiting factor utilized by a slow-growing prokaryote against a fast-growing eukaryote may have great significance is in achieving a fuller understanding of desert sand crusts, a unique system of global importance.