Niche partitioning with temperature among heterocystous cyanobacteria (Scytonema spp., Nostoc spp., and Tolypothrix spp.) from biological soil crusts

Heterocystous cyanobacteria of biocrusts are key players for biological fixation in drylands, where nitrogen is only second to water as a limiting resource. We studied the niche partitioning among the three most common biocrust heterocystous cyanobacteria sts using enrichment cultivation and the determination of growth responses to temperature in 30 representative isolates. Isolates of Scytonema spp. were most thermotolerant, typically growing up to 40^◦C, whereas only those of Tolypothrix spp. grew at 4^◦C. Nostoc spp. strains responded well at intermediate temperatures. We could trace the heat sensitivity in Nostoc spp. and Tolypothrix spp. to N₂-fixation itself, because the upper temperature for growth increased under nitrogen replete conditions. This may involve an inability to develop heterocysts (specialized N₂-fixing cells) at high temperatures. We then used a meta-analysis of biocrust molecular surveys spanning four continents to test the relevance of this apparent niche partitioning in nature. Indeed, the geographic distribution of the three types was clearly constrained by the mean local temperature, particularly during the growth season. This allows us to predict a potential shift in dominance in many locales as a result of global warming, to the benefit of Scytonema spp. populations.