The upper temperature limit of life under high hydrostatic pressure in the deep biosphere

Abstract

Abstract High hydrostatic pressure (HHP) is a common factor in the deep sea and provides an ignorable parameter of consideration in all studies related to deep life. High-temperature environments in the deep sea, mainly including hydrothermal vents and deep sediments in the subseafloor, support enormous amounts of biomass, productivity, and diversity of life. Many microbes living there are usually (hyper)thermophilic piezophiles, which cope with the dual stresses of high temperature and HHP at the same time and playing significant roles in geochemical elemental cycling. Knowledge of the upper temperature limit of life under deep-sea HHP conditions can help us to estimate the boundary of the biosphere and explore its habitability on Earth and in extraterrestrial areas, but uncertainties remain. Here, we have summarized the current known knowledge of physiological correlations between high temperature and HHP, as well as the effects of HHP on cells at high temperature. These effects mainly comprise two aspects: biological integrity and metabolic feasibility. The former has been investigated in many studies on various microorganisms, from which we can draw a general conclusion that HHP helps cells maintain biological integrity under high temperature. For the latter, existing studies have provided clues suggesting that both high temperature and HHP challenge metabolic feasibility, but it is still difficult to draw conclusions on the additive effects on metabolism due to the lack of systematic analysis. Here, we also propose a series of questions for further investigation and called for more attention on metabolic responses to high temperature and HHP; this could provide a direct bridge between geochemistry and ecology, help us to understand the microbial functions in the deep biosphere and allow us to estimate the boundaries of life and habitats.