Kinetics and Pathways of Sub-Lithic Microbial Community (hypolithon) Development

Type I hypolithons are microbial communities dominated by Cyanobacteria. They adhere to the underside of semi-translucent rocks in desert pavements, providing them with a refuge from the harsh abiotic stresses found on the desert soil surface. Despite their crucial role in soil nutrient cycling, our understanding of their growth rates and community development pathways remains limited. This study aimed to quantify the dynamics of hypolithon formation in the pavements of the Namib Desert. We established replicate arrays of sterile rock tiles with varying light transmission in two areas of the Namib Desert, each with different annual precipitation regimes. These were sampled annually over 7 years, and the samples were analysed using eDNA extraction and 16S rRNA gene amplicon sequencing. Our findings revealed that in the zone with higher precipitation, hypolithon formation became evident in semi-translucent rocks 3 years after the arrays were set up. This coincided with a Cyanobacterial 'bloom' in the adherent microbial community in the third year. In contrast, no visible hypolithon formation was observed at the array set up in the hyper-arid zone. This study provides the first quantitative evidence of the kinetics of hypolithon development in hot desert environments, suggesting that development rates are strongly influenced by precipitation regimes. Analysis of the microbial composition of desert sub-lithic hypolithon communities growing under semi-translucent rocks over a period of 7 years has provided the first quantitative evidence of a timeline for early hypolithon development. Data from this study has led us to propose a hypolithon development involving initial recruitment of specific soil Cyanobacteria to the illuminated soil surface immediately underneath a translucent rock followed by a transition to an adherent state triggered by wet-dry cycles, and further maturation to hypolithon biofilm. image