Diversity of microbial phototrophs and heterotrophs in Icelandic biocrusts and their role in phosphorus-rich Andosols

Abstract

Icelandic biocrusts are unique micro-ecosystems, considering their origin from volcanic material as a mineral base. However, little is known about their chemistry and diversity of microorganisms. Being phosphorus (P)-rich, Icelandic soils contribute to the global P cycling. Therefore, here, we focused on investigating P content, P pools, microbial biomass P (Pmic) and potential phosphatase activity along an altitude gradient (11–157 m a.s.l.) stretching away from the sea coast. Total P (Ptotal) content in the studied biocrusts and soils varied between 982 and 1571 mg kg−1 and Pmic, measured in biocrusts, constituted only a minor fraction <0.2% of Ptotal. Sequential P fractionation revealed the dominance of moderately labile P in biocrusts across the sites. Additionally, basic physicochemical properties were measured in biocrusts, and contents of total carbon (TC) and total nitrogen (TN) were positively correlated to Pmic and potential phosphatase activity.Diversity and abundance of microbial phototrophs in the studied biocrusts were assessed by morphological observations using light and epifluorescence microscopy. Microphototrophic communities were dominated by eukaryotic microalgae (e.g. Chlorella, Coccomyxa, Stichococcus) including diatoms (e.g. Pinnularia and Eunotia) and the highest cell biovolume was recorded in the site located at the sea coast. Furthermore, the majority of cyanobacteria belonged to filamentous forms, such as Microcoleus, Leptolyngbya, Phormidesmis and Stenomitos. In addition, the majority of fungal strains isolated from the studied biocrusts belonged to Ascomycota (e.g. Penicillium, Fusarium, Truncatella) and Zygomycota (e.g. Mucor, Mortierella).Overall, this study provides for the first time a comprehensive description of microbial community composition and P distribution in Icelandic biocrusts, which were previously overlooked.