Keep your friends close and your competitors closer: novel interspecies interaction in desert biological sand crusts

The green alga Chlorella sp. and the filamentous cyanobacterium Leptolyngbya sp., regularly cope with various stressors including frequent hydration-desiccation cycles. When grown in axenic cultures, Chlorella sp. is unable to resurrect even after very slow desiccation unless desiccation takes place in the presence of Leptolyngbya sp. or a lysate thereof. When Chlorella sp. was provided with extracellular polysaccharides from Leptolyngbya sp., or its main sugar constituents, it was able to be resurrected after mild desiccation. These findings suggest that desiccation tolerance of Chlorella sp. in the field depends on the presence of Leptolyngbya sp. or components thereof. Chlorella sp. was originally isolated as a contaminant in a decaying stationary culture of Leptolyngbya sp. that was established by the addition of a cyanobacterial growth media to biological soil crusts. Co-incubation with a dialysis tube containing Leptolyngbya sp. severely inhibited growth of Chlorella sp. These data suggest that secondary metabolites excreted by Leptolyngbya sp., the nature of which is yet unknown, inhibit growth of Chlorella sp. Thus, though Leptolyngbya sp. enables the survival of Chlorella sp. in a desert habitat, it simultaneously inhibits growth of this potential competitor. The combination of positive and negative effects of Leptolyngbya sp. on fitness of Chlorella sp. creates a scenario in which the former tightly controls the abundance of the latter.