Cyanobacteria in the Australian northern savannah detect the difference between intermittent dry season and wet season rain

This research was conducted in the northern Australian savannah at Boodjamulla National Park where cyanobacterial crusts dominate the soil and rock surfaces in between tussock grasses. It is widely accepted that terrestrial cyanobacteria are drought tolerant and rapidly recommence photosynthesis once moisture is available. Initial tests at the research site indicated that cyanobacteria did not respond to rehydration during the dry season, even after several days. We hypothesised that resurrection had not taken place and new growth from survival cells had to take place during the follow-up wet season. To further understand the desiccation-resurrection processes we tested photosystem II (PSII) responses both during the dry and wet seasons. In the 2009 dry season after 125 days without rain, crust samples were regularly rehydrated. Over the 10 day trial cyanobacteria did not recover PSII activity or CO2-uptake. Although new colonies of Nostoc grew other cyanobacteria remained inactive, even though liverworts and lichens in the same crusts had responded within 24 h. Dry season cyanobacterial crusts were collected in 2010 then reintroduced into their natural environment and exposed to rainfall during the 2011 wet season. Within 24 h PSII in cyanobacteria from a range of crust types had resurrected and CO2-uptake was verified, although different crust types responded at significantly different rates. These are the first studies that have demonstrated that PSII does not respond to rainfall during the dry season and cyanobacterial function appears controlled by other environmental conditions. It is likely that mass extracellular polysaccharide (EPS) production during the wet season, once dry, protects cyanobacteria from premature resurrection in the dry season. We propose that EPS regulates moisture penetration, thus the resurrection of PSII at the onset of the wet season, at which time moisture and humidity alters the rheological properties of EPS permitting rehydration.