The biokinetic spectrum for temperature: astrobiological significance

It has been known for some time that the growth rates of assemblages of phytoplankton have an upper limit known as the Eppley Curve. Recently, we have established that the maximum rates of growth of all life have a distinct upper limit, even when grown under optimal conditions, and which varies predictably with temperature. We term this sharp boundary the Biokinetic Spectrum for Temperature (BKS) and we were the first to identify and describe this boundary. No species from any biological Domain is known that grows at a rate exceeding the BKS, but the causative factors that determine the limit are unknown.

The BKS has a distinctive triangular appearance with a sharp peak at around 42°C followed by a steady decline in maximum rates thereafter. We used Bayesian quantile regression to describe the BKS and explore variation of model parameters with temperature and within subgroups, making use of the growth rates of a large number of species spanning the whole temperature range over which life is known to exist to obtain realistic limits for growth. The BKS possibly arises from a trade-off between catalytic activity and stability of enzymes involved in a rate-limiting Master Reaction within the cell.

Here, we report that by examining the apex of the BKS we can estimate the posterior probability of the maximum rate of growth of any form of life on Earth. In addition, we can estimate the maximum rate of growth that may be expected at, and near, the presumed limits of life (-20° C, 130° C), or the temperatures at which growth rates becomes arbitrarily close to zero. The BKS also has evolutionary and ecological implications. The Master Reaction is likely to have been heavily conserved since the Last Universal Common Ancestor. This means it can serve as a signature summarising the nature of life in environments beyond Earth, or to characterise species arising from an alternative biogenesis on Earth.

Our approach is different from that of previous studies, which have usually considered cardinal temperatures such as T_min, T_opt, and T_max. We suggest that biological growth rates are more informative than limits when considering how temperature variations affect ecophysiology, biodiversity, and evolutionary adaptation. The cardinal temperatures are defined as those temperatures where growth is observed to cease or is maximised, an approach that ignores the rich information content represented by a species' temperature-dependent growth curve.