Microorganisms and electron acceptors affecting methane oxidation in freshwater and marine systems

Abstract

Water column stratification is one of the main factors determining electron acceptor availability. In a well-mixed water column, water masses, particulate and dissolved matter and gasses can move freely through the water column, resulting in homogeneous temperature, nutrient, oxygen and other gas concentration profiles, as is illustrated in Fig. 3. Stratified systems, by contrast, consist of two or more layers separated by a steep gradient in temperature or dissolved substances, which limits vertical transport of water, particles and gasses (Fig. 3). The diffusion of atmospheric gasses such as oxygen is limited to the upper water layer, whilst gasses that diffuse from the sediment, such as methane, get trapped in the bottom water layer. The bottom water layer can become oxygen depleted, creating an oxygen gradient within the water column with distinct niches for aerobic and anaerobic microorganisms, including methane producers and oxidizers. Stratification can occur seasonally, as is observed in many lakes, or be permanent, like in the Black Sea. The microbial community within the anoxic zone of the Black Sea is highly adapted to the permanently anoxic conditions (Deuser, 1971). In seasonally stratified systems, however, microorganisms that are able to adapt to the changing conditions may have a competitive advantage, e.g. facultative anaerobes and fast-growing microorganisms. tion of a seasonally stratified aquatic system. In the mixed situation (left side), the water column is homogeneous and well-mixed. The oxic-anoxic interface is located in the sediment. In the stratified situation (right side), two or more water masses exist, separated by an area that is characterized by a steep oxygen gradient, called the oxycline, which forms the oxic-anoxic interface. Methane is produced by methanogenic microorganisms (i.e. archaea) in the anoxic sediments (1), where it may partly already be oxidized. The remaining methane diffuses into the water column, both in the mixed and stratified situation. In the stratified situation, methane may also be produced in the anoxic water column. In the mixed situation, the dissolved, diffused methane that escaped sedimentary methane oxidation can be transported through the whole water column (2), and will reach the surface waters, from where it can be emitted to the atmosphere (3). Water column methane oxidation in the oxic, mixed situation is generally considered to be inhibited by high oxygen concentrations. In the stratified water column, the oxycline acts as a barrier for the dissolved methane, limiting diffusion and trapping methane in the bottom water layer, resulting in an increase in methane concentration over time (4). Methane may be oxidized in the anoxic water column or at the oxycline, or can be released when mixing of the water layers occurs, ending stratification. Microorganisms involved in methane consumption Methane oxidizing bacteria Methane oxidizing organisms were first detected in 1906 by N.L. Söhngen (Söhngen, 1906). In 1970, Whittenbury, Phillips, and Wilkinson c...