The Gunnera-Nostoc symbiosis

Here at O2 Landscapes, we reserve the right to go deep-nerd with some of our journal entries, and this short account of the symbiosis between a flowering plant and a blue-green alga may just be our deepest foray yet.

That said, there is a certain blockbuster appeal to the title of the phenomenon exhibited by members of the genus Gunnera. And if blockbuster botany ever manages to gain traction, this weird relationship is one of the more interesting stories to be told about the interconnectedness between nature’s seemingly disparate characters.

The primary catalyst for this minor diversion into the symbiotic relationship between an angiosperm and a cyanobacterium was research for an upcoming plant profile about our native species of Gunnera – as well as a recent trip to a rare Auckland population (pictured below) of one of our Gunnera species (G. monoica).

We will describe this symbiosis to a greater extent in the impending plant profile, but the short version is that it centres around nitrogen fixation (especially as far as the Gunnera is concerned).

The transaction is comparatively straightforward. The cyanobacterium provides the vascular plant with the majority of its nitrogen requirements, and it receives carbohydrates in return (as a result of Gunnera‘s photosynthetis).

The oddest part of this particular symbiosis is that the Nostoc lives within the stems of the Gunnera, which it enters via glands at the base of the leaf. If one cuts the stem, Nostoc is discernible as dark blue-green patches distributed around the circumference of Gunnera‘s vascular system. Although Nostoc is capable of photosynthesis, it gives up on this almost completely, and devotes itself studiously towards fixing nitrogen.

Aside from the fascinating role that cyanobacteria play in providing nitrogen to rice plants (Oryza sativa), this is the only example in the plant kingdom of this kind of relationship between an angiosperm and a cyanobacterium – and has therefore been the subject of many studies focussed on how it works and potential reasons for the evolution of this highly unusual symbiosis.