A urea-based symbiotic transfer of nitrogen in biological soil crusts

Crucial to the establishment of biological soil crusts is a mutualistic exchange of C for N between pioneer filamentous non-heterocystous cyanobacteria of the Microcoleus type and a selected group of heterotrophic diazotrophs (for example, Massilia sp.) that come together in the so-called “cyanosphere”. In other such C for N mutualisms, N is transferred between species in the form of amino acids, and potential losses to adventitious bacteria are prevented by sequestering the diazotrophs into specialized structures. Yet, in the Microcoleus symbiosis no such structures exist, and only proximity achieved through chemotaxis is known to optimize the exchange. How is specificity then achieved?  We posited that the exchange might occur through urea, because it is the preferred N source for growth in Microcoleus. We show using cultures that members of the cyanosphere excrete urea when growing in symbiosis with Microcoleus at low concentration but sustained rates, but not when growing on their own diazotrophically. Consistently, Microcoleus can grow on urea down to the micromolar range, unlike other cyanobacteria that need much higher concentrations to grow. We also show that Massilia overexpresses the genes for the urea cycle (production of urea) when in symbioses, but not otherwise, and that Microcoleus overexpresses genes for urea uptake and utilization when in symbiosis. Microcoleus contains a rare set of enzymes for urea utilization (allophanase and urea carboxylase), that, unlike the widespread urease, allow it to process urea at very low concentrations. Hence, it seems that with the combination of these unique biochemical and regulatory capacities, low concentration urea can effectively transfer N in a partner-specific way, dodging potential losses of N to most other bacteria that can only use urease. We discuss the importance of these findings for the ecology and restoration of biological soil crusts.