Life on Earth. Extremophiles Continue to Move the Goal Posts

Abstract

Environmental Context.Since the discovery of a diverse array of microbial life associated with hydrothermal vents on the ocean floor, where conditions are hot, reducing and acidic, scientists have been seeking insight into the mechanisms used by ‘extremophilic’ organisms (those that reside permanently under environmental extremes of temperature (hot or cold), pH (acid or alkaline), salinity, or pressure) to thrive under such seemingly inhospitable conditions. Abstract.The discovery of an abundance of microorganisms that flourish in a diverse range of environments, from the frigid waters of the Antarctic, to the superheated waters of the hydrothermal vents, at the bottom of 11-km deep ocean trenches and in salt-saturated lakes, has fuelled research aimed to understand the novel survival strategies evolved by these extreme-loving (extremophilic) organisms. Adaptations of biomolecules (proteins, nucleic acids, membranes and small molecules) evolved by extremophiles are wide ranging. Compared with a protein from a ‘regular’ organism, the extremophilic analogue might feature changes to the relative frequencies of amino acid residues that modulate the properties (e.g. conformational flexibility and stability) of the protein under conditions of the specific environmental challenge. The integrity of RNA and DNA from extremophiles may be maintained by subtle structural changes to RNA nucleobases and, in the case of (hyper)thermophiles, the expression of the enzyme reverse gyrase, which catalyses positive DNA supercoiling. The expression of small molecular weight heat-shock or related caretaker proteins also features as a common adaptive strategy for maintaining cell viability at environmental extremes. Membrane architecture in extremophiles can be modulated by the environmental temperature, with additional thermal stability in membranes from some hyperthermophiles conferred by novel (cyclised) lipid chains. In addition, a selection of osmolytes and small molecules are biosynthesised or sequestered by extremophilic organisms that have adapted to conditions of high salt and/or micronutrient deprivation.