PHOSPHORUS LIMITATION INHIBITS CYANOBACTERIAL GROWTH BUT NOT EXOPOLYSACCHARIDES PRODUCTION

Cyanobacteria are adapted to environments with low phosphorus (P) availability such as desert soils. They are key to the formation of biological soil crusts (biocrusts hereafter), pivotal mini ecosystems in deserts. Studies have demonstrated that P limitation in lakes increases exopolysaccharides (EPS) production in aquatic cyanobacteria, impacting their fitness. However, the role of phosphorus limitation on EPS production by biocrust cyanobacteria remains poorly understood. Here, we evaluated the impact of P limitation on the growth, photosynthetic activity, and EPS production of six biocrust cyanobacterial strains obtainedfrom biocrusts in the Chihuahuan and Mojave Deserts, and belonging to three different orders (Nostocales, Oculatellales and Gomontiellales). All strains had lower biomass under P limitation than under control conditions. Quantum photosynthetic yield measurements did not differ between control and P limitation, suggesting that photosynthetic activity was not severely impacted. However, the EPS-to-biomass concentration (total EPS normalized to chlorophyll a) remained consistent between treatments throughout the experiment. These results indicate that low phosphorus availability limits the growth of biocrust cyanobacteria. However, they maintain EPS production at a proportional level to biomass. This suggests that alternative phosphorus acquisition strategies, such as polyphosphate storage, may compensate for reduced external P availability. The stable EPS-to-biomass ratio, even under P limitation, likely acts as a survival mechanism, facilitating biofilm formation and cell aggregation.