The colonisation of land masses – what controls community assembly and habitability?

The interactions between life and the geosphere are complex, yet poorly understood and modelled. They play a crucial role in biogeochemical cycling, defining the limits to life and ecosystem energy flow. We currently have a limited understanding of how the planetary crust selects for different microbial communities in the process of colonisation. We implemented an experiment to investigate the emergent complexity of microbial communities, by studying colonisation of two end-member igneous rock types representing the two major chemical compositions of the bulk of the Earth’s crust, granite and gabbro, during the course of two years. The experiment was exposed to the atmosphere to allow for atmospheric inoculation. Microbial community composition is determined by 16S rDNA analysis each month, and abundance measured by cell and CFU counts. The study addresses the process of colonisation using the metacommunity concept, elucidating the role of species sorting and neutral assembly in defining the course of colonisation. After six months, the communities cluster according to rock type in a principal coordinate analysis, indicating that the geochemical environments selects for the resident community. However, the dispersal of the communities suggests a role for stochastic or neutral processes in community assembly. This experiment yields new insights into the factors that determine how new planetary crust becomes colonised and whether the bulk composition of a planet’s surface has a significant role in determining the types of microbial communities that become established. Furthermore, by elucidating the link between the microbial populations and substrate the experiment shows the extent of coupling between the geosphere and biosphere.