Layered Microbial Communities and the Search for Life in the Universe

Searching for life when we do not know what it is like is at best a difficult task, and at worst, an exercise fraught with frustration and potential failure. In developing methods that will have general applicability, one must ascertain first what the general features of life might be that, when present, would provide strong inference of the presence of active life. We propose here that the tendency of life to be associated with energetic gradients, in fact to create its own gradients via the catalytic consumption of nutrients and oxidants, and the production of waste products, is one of the general features of active extant life. As life seeks out and creates such gradients, layered microbial communities, or LMCs, result that are in their own way, prima facie biosignatures. These LMCs are abundant, perhaps ubiquitous in physically stratified environments on our own planet. If energy flows through a given system, and if life has developed efficient catalytic methods to utilize this energy, then LMCs are predicted to be present. Here we discuss the general issues of what can be gleaned from the study of LMCs in terms of energy flow, carbon flow, and carbon storage in earthly ecosystems, as well as the philosophy of using LMCs as biosignatures in the search for life on Earth, and perhaps, in extraterrestrial sites.