Light hydrocarbon ratios as biosignatures of microbial activity

The River Colne, UK, is a temperate estuary with large expanses of anoxic mudflats, referred to as a “microbial observatory”. Methane and ethane emissions were characterised over three sites spanning from the head of the estuary (Hythe), mid-estuary (Alresford) and the mouth (Mersea) which were sampled for dissolved hydrocarbons, in-situ fluxes, and hydrocarbon production from ex-situ sediment slurry incubations. Dissolved methane concentrations were highest at Mersea (85.88 ± 17.36 nM CH4) followed by Alresford (27.75 ± 14.29 nM CH4). Dissolved ethane concentrations at Hythe were highest with 631.05 ± 315.89 pM, followed by Mersea (605.69 ± 302.84 pM), with Alresford dipping to 0 pM. Methane fluxes were highest at Hythe for both the sediment-air (56.37 ± 12.40 nM min-1 cm-2) and water-air interfaces (134.73 nM min−1 cm−2). Sediment-air ethane fluxes at Hythe averaged 439.91 ± 19.98 pM min-1 cm-2, decreasing at Alresford (407.81 pM min−1 cm−2), while water-air at Hythe produced 892 pM min−1 cm−2, double that of Alresford (483.22 ± 388.71 pM min−1 cm−2) with no ethane fluxes at Mersea. Incubations from Hythe, Alresford and Mersea increased by 55.67 ± 72.094, 0.04 ± 0.158 and 0.69 ± 0.489 mM d-1 in headspace CH4 while mean ethane concentrations were 215.97 ± 12.373, 141.10 ± 59.839 and 182.86 ± 66.897 pM, respectively. Methane concentrations were significantly lower in slurries incubated with 2-bromoethanesulfonic acid (2-BES) or after autoclaving, while ethane concentrations were unaffected. These results, coupled with calculated methane – ethane ratios, can be used to inform future studies, particularly with regard to astrobiological research.