1National Institute of Genetics, 2JSPS Research Fellow, 3Lake Akan Eco-Museum Center, 4National Institute of Polar Research
A rare freshwater green alga, Aegagropila linnaei, forms ball-like aggregations called “Lake Ball” or “Marimo” in Lake Akan, Hokkaido, Japan. It has long been a biological mystery as to why this alga that is known as a mat-former forms such a unique structure. Marimo is likely to provide habitats of various microorganisms, and some of them are considered to play a key role in nutrient cycling and recycling within the Marimo structure. However, the detailed identification of microbial community structure has yet to be carried out. This study analyzed the biodiversity of the exterior and interior of small-, medium-, and large-sized Marimo using high-throughput sequencing of prokaryotic 16S rRNA gene amplicons. The most frequently detected sequences in the studied specimens belonged to the phyla (Alpha-, Beta-, Gamma-, Delta-) Proteobacteria and Bacteroidetes. In addition, the photoautotrophic Cyanobacteria were characteristically detected in the exterior of medium- and large-sized Marimo, while the proportion of nitrite oxidizer Nitrospira significantly increased from the interior of small- to that of large-sized Marimo. Moreover, archaeal sequences were minor but relatively dominant in the interior, and these were assigned to the ammonia-oxidizer ‘Candidatus Nitrosoarchaeum limnia’ in the phylum Thaumarchaeota. Our study thus indicates that some prokaryotic phylogenies differed among the Marimo sections. As this study is currently in progress, we plan to report more phylogenetic data in the presentation. These findings will shed light on the mechanisms underlying the growth and maintenance of Marimo.
keywords:freshwater ecosystem,phylogeny,biodiversity,prokaryote,16S rRNA gene,high-throughput sequencing