Mutualistic relationship between mangrove plants and diazotrophs

Trees need nitrogen. When they shed leaves with all the nitrogen in them it would be a big loss, were it not for the army of creepy-crawlies that recycle the leaves back into the soil. Life is tougher for mangroves. At the edge of the ocean, leaves don’t lie around to decay. They are swept away by the tide, taking their nitrogen with them. So how can mangroves get the nitrogen they need from the soil?

Tomomi Inoue and colleagues have examined the chemistry and biology of the mangrove rhizosphere, the part of the soil connected to the mangrove roots. They looked at how the mangroves interact with diazotrophs, microorganisms that can take atmospheric nitrogen and convert it into chemical compounds that the trees can use.

Specifically they looked for an enzyme, nitrogenase, that converts nitrogen N₂ to ammonia NH₃. They wanted to see if there was more enzyme activity near the roots compared to further away. They also sequenced DNA from soil samples, looking at how it varied between the soil by the trees and the soils elsewhere. They were looking for tell-tale signals that diazotrophs were living with the trees.

“The results of this study support the hypothesis of a mutualistic interaction between the diazotrophic community and mangrove trees,” write Inoue and colleagues. “This mutualism was evidenced in our study by the observed increase of soil nitrogenase activity with higher live root biomass and detrital organic matter content. The hypothesis was also supported by the change in community composition of the microbiota at different stages of forest development: key anaerobic taxa were associated with the bulk soil, which lacked fine root biomass, whereas aerobes were more common in the inner forest and close to stems and prop roots.”

In younger trees the microbe community in the soil was closer to the bulk soil microbial community. The authors argue this difference is due to the trees recruiting microbes as they grow. If this is the case, then understanding how the diazotrophs live, and conserving them will help mangrove conservation and restoration projects.

Note: 24 February 2020: An earlier image of a cypress swamp was replaced with the current image.