They mention in the article non-biological process that can affect isotope ratios. This seems much more plausible. Suggesting that there were large enough amounts of terrestrial chemotrophs to change isotopic ratios is not supported by anything else we know about this time period, or life on Earth in general. The burden of proof is still in their camp to demonstrate:<p>1. Any evidence to suggest biological origin
2. Reasons why biological process are more likely to account for the observed iostope ratio that non-biological.<p>Until then, this is only an interesting, though unlikely, explanation.