Phytoremediation with Geosiphon-like symbiosis?

Dear Editor, In their recent article published in Environmental Science and Pollution Research, Anna Ogar et al. (2015) have shown, thanks to a careful experimental design, that the addition of diazotrophic bacteria together with mycorrhizal fungi significantly improves plant growth and performance. Strikingly, the addition of inoculum containing only diazotrophs but not mycorrhizal fungi had contrary effect and led to the reduced shoots and roots biomass and lower photosynthesis efficiency of Medicago sativa and Hieracium pilosella, as compared to experimental plants co-inoculated with mycorrhizal fungi and nitrogen-fixing bacteria. The authors gave their credit to many features of microbes used in the study that might have resulted in such a visible change. Reasons for growth inhibition by diazotrophic bacteria not accompanied by mycorrhizae seem well justified. However, positive impact of combined addition of mycorrhizae and diazotrophs deserves, in our opinion, examining of another relevant explanation. As hypothesized by Pirozynski and Malloch (1975) and evidenced later by many (Selosse and Le Tacon 1998; Smith and Read 2008), mycorrhizal fungi enabled plants’ ancestors to conquer terrestrial habitat. It is also more and more often suggested, that mycorrhizal fungi led to the evolution of roots themselves, in a similar way symbiotic microbes drove the evolution of animals’ gut (as argued by Margaret McFall-Ngai in Velasquez-Manoff 2015). The comparison between root and gut is not a simple analogy, as in both cases symbiotic microbes significantly contribute to nutrition and protection of their hosts, and several developmental processes are mirrored in those seemingly different organs making their mode of evolution follow the same pattern (Selosse et al. 2014). Rhizophagus irregularis (syn. Glomus intraradices), an arbuscular mycorrhizal fungus (phylumGlomeromycota), was used by Ogar and her co-workers. Glomus sensu lato descended from the hypothetical, primordial-plant mycorrhizal symbionts and as all members of this group are unable to synthesize simple sugars, nor to take them up from the environment. For this, symbiosis with appropriate photosynthetic partner(s) needs to be established. Despite such dependency, scientists are quite sure that the presence of mycorrhizal fungi in soils preceded and was a prerequisite for green plants’ Binvasion^ (Redecker et al. 2000). While occupying land before green plants’ emergence,mycorrhizal fungimust have been involved in associations with other phototrophs. One of the Glomeromycota members does so until nowadays: Geosiphon pyriforme is a unique microorganism that has a rather stormy history of taxonomical record. First described as an alga, it is now known to be a truly Glomeromycota and the only fungus known to endocytobiotically host cyanobacteria (Schüßler 2012). Geosiphon associates with various members of cyanobacterial order Nostocales. However, it may also form symbiosis with plants (Schüßler 2012). Accordingly, the existence of interaction between other Glomeromycota with cyanobacteria, just like Geosiphon, cannot be ruled out. Quite recently, physically andmetabolically close associations have been artificially induced between unicellular green alga— Chlamydomonas and a hyphae of a mold-like, Bnon-symbiotic^ fungus—Aspergillus, as well as between Chlamydomonas and yeasts (Hom and Murray 2014). This phenomenon was suggested to prove latent capacity for mutualistic symbiosis in * Grzegorz Wojtczak [email protected]