[BCG06-P04] Search for microbial CH4 production processes in lake sediments and surface water associated with cyanobacterial bloom
Keywords:Microbial methane production, Methane paradox, Freshwater lake, Methanogenic archaea, Cyanobacterial bloom
Microbial CH4 production is still regarded by many as a process limited to anoxic environments. Nevertheless, over the past few decades, increasing evidence of CH4 accumulation in oxygen-saturated lake and marine surface water has emerged . To explain the enigmatic issues, it has been proposed that CH4 production by methanogenic archaea takes place in anoxic microenvironments such as detritus and animals’ gut. Alternatively, recent studies suggest that some bacteria break down methylphosphonate (MPn) or dimethylsulfoniopropionate (DMSP) and release CH4 as a biogenic byproduct. These novel microbial CH4 production processes may play a potential contribution to the total atmospheric carbon cycle.
Oxic CH4 peaks have been found to be closely associated with phytoplankton dynamics across multiple lakes. Grossart et al.  found potentially methanogenic archaea associated with cyanobacteria such as Aphanizomenon and Microcystis in oxygenated surface water. Additionally, Berg et al.  showed that hydrogen produced during nitrogen fixation by cyanobacteria can be consumed by methanogenic archaea. These studies suggest the presence of a CH4 production process by methanogenic archaea associated with cyanobacteria in lake surface water. However, it is unclear whether the CH4 production is taking place in actual environments.
In this study, we investigated microbial CH4 production in sediments and surface water of Lake Suwa in Nagano Prefecture, Japan, a typical hypertrophic shallow lake, using culture-independent methods. Especially, we focused on cyanobacterial bloom and tried to identify a CH4 production process by methanogenic archaea associated with cyanobacteria. 16S rRNA gene analyses revealed the presence of methanogenic archaea closely related to the order Methanobacteriales, Methanomicrobiales, and Methanosarcinales in sediment core samples from 30 cm depth. On the other hand, 16S rRNA genes and mcrA genes of methanogenic archaea were not detected in bulk DNA from cyanobacterial bloom sample (mainly Microcystis). In order to estimate the potential for methanogenic archaea, we extracted Coenzyme F430, a biomarker for methanogenic archaea, from sediment and bloom samples and determined by using HPLC . F430 was detected from sediment samples, but that from bloom sample was currently below detection limit (<1 pmol).
These results suggest that methanogenic archaea actively produce CH4 in the sediments of Lake Suwa, however, its contribution in oxygenated surface water seems to be small. We are currently conducting a more sensitive analysis of F430 using LC-MS/MS  with the pre-treatment procedure . The analysis may identify the CH4 production process by methanogenic archaea associated with cyanobacteria in the surface water. Although this study did not cover other CH4 production processes such as MPn and DMSP decomposition, our results may also be applicable to estimate the in situ contribution of these novel CH4 source.
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