Methane Carbon Supports Aquatic Food Webs to the Fish Level

Large amounts of the greenhouse gas methane (CH4) are produced by anaerobic mineralization of organic matter in lakes. In spite of extensive freshwater CH4 emissions, most of the CH4 is typically oxidized by methane oxidizing bacteria (MOB) before it can reach the lake surface and be emitted to the atmosphere. In turn, it has been shown that the CH4-derived biomass of MOB can provide the energy and carbon for zooplankton and macroinvertebrates. In this study, we demonstrate the presence of specific fatty acids synthesized by MOB in fish tissues having low carbon stable isotope ratios. Fish species, zooplankton, macroinvertebrates and the water hyacinth Eichhornia crassipes were collected from a shallow lake in Brazil and analyzed for fatty acids (FA) and carbon stable isotope ratios (delta C-13). The fatty acids 16:1 omega 8c, 16:1 omega 8t, 16:1 omega 6c, 16:1 omega 5t, 18:1 omega 8c and 18:1 omega 8t were used as signature for MOB. The delta C-13 ratios varied from -27.7 parts per thousand to -42.0 parts per thousand and the contribution of MOB FA ranged from 0.05% to 0.84% of total FA. Organisms with higher total content of MOB FAs presented lower delta C-13 values (i.e. they were more depleted in C-13), while organisms with lower content of MOB signature FAs showed higher delta C-13 values. An UPGMA cluster analysis was carried out to distinguish grouping of organisms in relation to their MOB FA contents. This combination of stable isotope and fatty acid tracers provides new evidence that assimilation of methane-derived carbon can be an important carbon source for the whole aquatic food web, up to the fish level.