As­ton­ish­ing My­c­or­rhizal Net­works

Note­wor­thy

Wel­come to the world of ar­bus­cu­lar my­c­or­rhizal (AM) fungi. These ben­e­fi­cial mi­crobes go mostly un­no­ticed, but they pro­duce mas­sive net­works of un­der­ground thread-like hy­phae. Here at STC, where we love to no­tice the un­no­ticed, these fungi have long fas­ci­nated us.

AM fungi co­op­er­ate with ap­prox­i­mately 80% of plants, form­ing one of the most an­cient and wide­spread mu­tu­al­is­tic as­so­ci­a­tions on Earth. Through their hy­phal net­works, these fungi en­able the ex­change of nu­tri­ents and car­bon. My­c­or­rhizae pro­vide plants with es­sen­tial nu­tri­ents like phos­pho­rus and ni­tro­gen, and the fungi re­ceive sug­ars from plants in re­turn. 

But my­c­or­rhizae also pro­vide ad­di­tional ben­e­fits. Their mere ex­ten­sion and bi­o­log­i­cal func­tion mean they are im­por­tant in terms of cy­cling nu­tri­ents and car­bon on our planet, i.e., mov­ing el­e­ments be­tween liv­ing or­gan­isms and the en­vi­ron­ment and en­sur­ing their sus­tained use. On a global scale, they are es­ti­mated to move about ~1 bil­lion met­ric tons of car­bon into soils an­nu­ally. But just how ex­ten­sive is this hy­phal in­fra­struc­ture? 

In a re­cent study, re­searchers com­piled data from unique quan­tifi­ca­tions of AM hy­phal den­si­ties from over 16,000 soil cores across nine bio­mes, large ar­eas de­fined by the species and spe­cific en­vi­ron­men­tal con­di­tions. Us­ing ma­chine learn­ing, they pre­dicted an av­er­age global hy­phal den­sity of 4.4 me­ters per cu­bic cen­time­ter. If stretched end to end, these tiny thread-like struc­tures could ex­tend ap­prox­i­mately 110 quadrillion kilo­me­ters (i.e., 1.1 x 10¹⁷). To put this in per­spec­tive, this length is equiv­a­lent to thou­sands of light-years and al­most a bil­lion times the dis­tance be­tween Earth and the Sun. That is a lot of fungi in the top 15 cm of Earth's soil!

Global pre­dicted den­sity of AM hy­phae. Dark col­ors (blue) rep­re­sent soils pre­dicted to have low den­si­ties, and bright col­ors (yel­low) rep­re­sent those pre­dicted to have high den­si­ties. Earth's dens­est AM fun­gal hy­phae are pre­dicted in grass­lands, with par­tic­u­larly high den­si­ties in the Sudd wet­lands, the Ever­glades, and the Ti­betan steppe. Gray ar­eas, where there is low host veg­e­ta­tion (e.g., deserts), are ex­cluded from the model. Ecosys­tems with high hy­phal den­si­ties that are pre­dicted to hold >5.0 m/cm3 are shown. Source.Fron­tispiece: Liv­ing AM fun­gal mycelium. Source 

We have known for some time that these un­der­ground fun­gal ecosys­tems are un­ques­tion­ably im­por­tant for nu­tri­ent and car­bon cy­cles. The mag­ni­tude of their bio­mass now un­der­scores just how vi­tal they are for global soil health.