Isotopic physiological signatures of anaerobic methanotrophic archaea in the sediment of the East Siberian Sea

Abstract

The East Siberian Sea was investigated to understand existence of methane-related microbes and their methane-metabolic process in the methane seepage sites through quantification of amino acids, archaeal lipids, and carbon stable isotope analysis. The purpose of this study is to (1) provide evidence of the presence of anaerobic methanotrophic archaea (ANME), (2) determine the physiological activity of the methane removal process, and (3) quantify the capacity of methane removal. The results showed that archaeol and sn-2-hydroxyarchaeol were present and confirmed that anaerobic methane oxidation (AOM) by ANME may occur in the methane seepage site. The carbon stable isotopes of amino acids (alanine, valine, isoleucine, and leucine) were progressively depleted according to the metabolic process of amino acid synthesis in methanotroph. This is because methane with a depleted isotopic ratio is used as a carbon source, and the carbon isotope ratio of methane would be reflected in the amino acids and archaeal lipids. Suggesting a new indicator of anoxic oxidation of methane, this study used the depletion of carbon stable isotope ratio in pyruvate family amino acids to estimate the methane removal rate of over 72%. As a result, this study provides evidence for anaerobic methane oxidation in East Siberian Sea sediments and confirms the potential for physiological studies using stable isotopes.