Even after years without their legume hosts, enough rhizobia can survive to nodulate the host when it’s grown again, without inoculation.  But is that due to reproduction of rhizobial cells in the soil replacing some of the cells that die?  Or could individual cells survive for years, in a dormant or semidormant state?

My PhD student, Katherine Muller, used published metabolic rates and her own measurements of resources that field-collected rhizobial strains acquire in soybean nodules (specifically, amount per cell of the energy-rich lipid, PHB) to show that a sufficiently-dormant rhizobial cell could survive for many years on stored resources alone.  See Figure 2B in our paper, “Resource acquisition and allocation traits in symbiotic rhizobia with implications for life-history outside of legume hosts”, just published in Royal Society Open Science.  This research was supported mainly by Minnesota’s Long-Term Agricultural Research Network.

But can rhizobia cells really become dormant enough to make their stored resources last for years?  Figure 2C shows a bimodal distribution of PHB during nodule senescence, consistent with earlier work by Will Ratcliff on the rhizobia that nodulate alfalfa.  Will showed that this pattern was due to the mother cell keeping almost all the PHB after dividing, and then going dormant, which would make the PHB last longer.  So if rhizobia can escape risks other than starvation (for example, by hiding in crevices too small for predatory protozoa), maybe the rhizobial cells that nodulate this year’s soybean crop are not the descendants of those released from nodules by soybeans two or more years ago, but the original, nodule-escaping cells themselves.

“It seems impossible,” he stammered. “And yet I can’t help thinking of it — it’s astonishing — and extraordinary — and quite incredible — and yet not absolutely beyond my powers of belief —”

“What is, my son?”

And Conway answered, shaken with an emotion for which he knew no reason and which he did not seek to conceal: “THAT YOU ARE STILL ALIVE, FATHER PERRAULT.”

— Lost Horizon (which is not actually about soil erosion, despite the title)