Cyanosphere community composition of M. vaginatus bundles from the Chihuahuan Desert

Microcoleus vaginatus plays a prominent role as both primary producer and pioneer species in biocrust communities that inhabit dryland soils. And yet, M. vaginatus lacks the ability to fix nitrogen from N2, an essential nutrient often deficient in dryland soils. A recent study has shown that a unique bacterial community containing diazotrophs exists in the cyanosphere of M. vaginatus, hinting at beneficial metabolic exchanges between autotrophs and diazotrophs. Here, we provide evidence that symbiotic interactions between M. vaginatus and its cyanosphere microbes, driven by nitrogen availability, are the solution to this apparent paradox. We determined that nitrogen limitation was crucial in keeping nitrogen-fixing cyanosphere bacteria associated to M. vaginatus, as increasing nitrogen availability decoupled the resource trading relationship. We also found that co-culturing of cyanosphere bacteria with M. vaginatus not only resulted in copious growth of the cyanobacterium in nitrogen limiting conditions, but also induced bundling, a morphogenetic trait rarely observed in culture. Our findings provide direct evidence for a symbiotic relationship between M. vaginatus and its cyanosphere and help explain how M. vaginatus can be a global pioneer species in spite of its physiological shortcomings.