Extreme environmental gradients as natural laboratories for studying microbial culturability, life cycle and evolution

Extreme environments may experience strong temporal and/or spatial gradients that dramatically change the physico-chemical conditions affecting the organisms living in there. Microbial communities are the most important life form assemblages in these environments, showing a surprising ability to cope with the sharp changes, even having a selective advantage when able to enter in a survival life style mode. Gradients influence the structure, function and culturability of the associated microbiome and extreme environments showing strong changes in a short spatial/temporal interval are suitable candidates to study microbial life cycle and adaptation. Deep anoxic hypersaline brines are an important marine ecotone where microorganisms face strong meter-scale halo- and chemoclines, challenging and enhancing their biology and level of associations. Semiterrestrial mangrove crabs are important ecosystemic keystone species that are undergoing a landward evolution establishing a symbiontic relationship with bacteria that help them cope with a nitrogen poor and high salinity intertidal environment; crabs, and thus their symbionts, continually switch between air and seawater, and are exposed to salinity fluctuations, fuelling a unique and complex symbiotic community. In polar deserts, when glaciers retreat release a barren mineral substrate subjected to the processes of primary colonization, generating ‘chronosequences’, exhibiting kilometer-scale spatial gradients of ecosystem development, where microorganisms cope with different physico-chemical gradients shaping the microbial community associated with soil substrate and the pioneer plants growing there. We propose these ecotones as model systems to explore the response of microorganisms to variable conditions which influence their life cycle, evolutionary pattern and the culturable state.