Elevational Gradients in Microbial Diversity: A case study of Mt. Fuji and Mt. Halla

Abstract

Little is known of how microbial diversity and community ecology behaves along elevational gradients. We chose to study Mount Fuji of Japan as a geologically and topographically uniform mountain system along with Mt. Halla of Jeju Island, South Korea, a massive shield volcano, consisting of a mosaic of slightly different volcanic types (mainly trachybasalt and basalt) of different ages. PCR-amplified soil DNA for the archaeal and bacterial 16S rRNA gene was pyrosequenced and taxonomically classified against EzTaxon-e microbial database for a wide range of elevational zones on both the mountains. Previous studies on soil bacteria/archaea have variously found either a diversity decline, or no trend. However most of these studies did not control for confounding geological factors. Here we studied how microbial diversity and community composition varies in relation to elevation. There was a significant peak in total bacterial/archaeal diversity at certain elevations for both the mountain ranges except for the Yeongsil transect on Mt. Halla which had a hollow (U-shaped) trend with elevation, rarely observed in elevation studies in nature. Individual bacterial/archaeal phyla show distinct trends with elevation—increase, decrease, or a mid-elevational bulge in diversity. Elevation, together with the closely related parameters of mean annual temperature and mean annual precipitation, was clearly the best predictor of variation in community composition on both the mountains. These variables exceeded the explanatory power of all other measured variables such as pH, organic C, N and P on Mt. Halla whereas on Mt. Fuji microbial soil communities were also highly responsive to soil environmental gradients, in terms of both their diversity and community composition. Distinct communities of archaea and bacteria specific to each elevational zone on Mt. Fuji suggest that many microbes may be quite finely niche-adapted within the range of soil environments. A further interesting finding is the presence of a mesophilic component of archaea at high altitudes on a mountain that is not volcanically active. This emphasizes the importance of microclimate – in this case solar heating of the black volcanic ash surface for the ecology of soil archaea. A hollow trend that has not been found before in studies of microbial diversity on mountains, and set alongside with the other diversity trends we found here on Mt. Halla, emphasizes that no simple rule can be generalized for the worlds mountain systems. Apart from elevational soil chemistry and climatic factors, stochastic processes involving complex environmental mosaics may also be playing a role in shaping the displayed patterns observed on both the mountains.