Methanotrophy in geothermal soils, an overlooked process: The example of Nisyros island (Greece)

A multidisciplinary field campaign was carried out at Nisyros Island (Greece). Hydrothermal gases were sampled and analysed, and CH and CO fluxes from the soils were measured with the accumulation chamber method. The sampling area (Lakki plain) covers an area of about 0.08 km, and includes the main fumarolic areas of Kaminakia, Stefanos, Ramos, Lofos and Phlegeton. Flux values measured at 130 sites range from -3.4 to 1420 mg m d for CH and from 0.1 to 383 g m d for CO. The fumarolic areas show very different CH degassing patterns, Kaminakia showing the highest CH output values (about 0.8 t a from an area of about 30,000 m) and Phlegeton the lowest (about 0.01 t a from an area of about 2500 m). The total output from the entire geothermal system of Nisyros should not exceed 2 t a. Previous indirect estimates of the CH output at Nisyros, based on soil CO output and CH/CO ratios in fumarolic gases, were more than one order of magnitude higher. The present work further underscores the utmost importance of direct CH flux data because indirect methods totally disregard methanotrophic activity within the soil. Ten soil samples were collected for CH consumption experiments and for metagenomic analysis. Seven of the soil samples showed small but significant CH consumption (up to 39.7 ng g h) and were positive for the methanotrophs-specific gene (pmoA) confirming microbial CH oxidation in the soil, notwithstanding the harsh environmental conditions (high temperature and HS concentrations and low pH).