It is a gas lab: New conceptual work proposes the use of species-area relationships to model microbial mutualisms

The authors introduce the article by highlighting that many mutualisms, including mycorrhizas and gut systems, are mediated by communities of microorganisms that interact with much larger hosts, and the identity of the microbes involved is a key determinant of the outcome of the symbiosis. By contrast, most studies describe the ecology of mutualisms in relation to biogeographical descriptors that suit best the much larger partner. Thus, there is a growing need to improve our understanding of endosymbiont ecology, particularly at fine scales, and our paper addresses this gap. Recent advances in upscaling with species are relationships (SARs) offer opportunities to address a range of exciting questions on endosymbionts.

Fig. 1 Overview of some open questions which we could address via integrating species-area relationships (SARs) into core mycorrhizal ecology, some likely applications of SARs and a set of three hypotheses. To describe SARs (top left panel), we use two parameters, the slope z and the intercept c (both describing the power law between area and richness). At fine spatial scales within a single root system, however, spatial aggregation of species, arising from the continuity of AM fungal structures in the roots (bottom right panel depicting a root of Fragaria vesca colonized with Rhizophagus irregularis and featuring arbuscules), leads to deviations from a perfect power law. To date most studies are concerned with the overall richness, to which they refer as α-diversity, of the AM fungal communities and do not try to decompose it to its two constituents. Because the sampling intensity and the laboratory approaches vary across studies, the estimates as a result are not comparable and it is hard to synthesize across studies. We believe that integrating species-area approaches in mycorrhizal ecology has a range of advantages: (i) we can assess the degree of spatial aggregation in fine spatial scales and compare across mycorrhizal systems (Hypothesis A); (ii) we can get mechanistic on the way scaling parameters change over time and across systems (Hypothesis B); (iii) we can cross synthesize with other phyla of fungi (Hypothesis C) or develop a new generation of null hypotheses for large-scale biogeographical studies targeting AM fungi.

The authors present an overview of SARs and how some assumptions (e.g. contingency of areas) have been relaxed in existing implementations on microbial systems. They then discuss likely benefits from abstracting mutualisms through SARs. These include (Fig. 1) generating null expectations of the global endosymbiont richness and combining observations from otherwise unstandardized surveys into larger syntheses, and forming questions at a micro-scale where we can observe spatial aggregations (e.g. differences across hosts). They finally support the ideas with the existing state of the art in relation to mycorrhizas and through generating additional questions specific to arbuscular mycorrhizas.

Article Link: https://www.sciencedirect.com/science/article/pii/S0966842X23001610