Evolutionary adaptation vs. physiological acclimation
Acclimation responses, whereby individuals’ physiological mechanisms buffer the kinetics effect of warming and maintain homeostasis in key life-history traits such as growth and maintenance, are sometimes referred to as ‘adaptation’ (e.g. of microbial carbon use efficiency in response to warming ) (Allisonet al. 2010; Wieder et al. 2013; Allison 2014). However, this is phenotypic plasticity at the individual level, rather than evolutionary adaptation at the population level. In our model, constant mortality (in the baseline and temperature-dependent MGE scenarios) and constant MGE (in the baseline and temperature-dependent mortality scenarios) can be interpreted as manifestations of microbial acclimation (Allison et al.2010; Hagerty et al. 2014). Our results show that such plasticity, whereby individual cells buffer growth efficiency and/or mortality against temperature variation, does not necessarily impede or overwhelm evolutionary adaptation to climate warming. It is precisely under the assumptions that MGE and mortality remain constant with respect to temperature that the strongest adaptive change in exoenzyme allocation is predicted (see Supplementary Note 7 for further discussion of implications regarding ‘adaptation’ of microbial carbon-use efficiency).
The exoenzyme allocation fraction itself might be plastic. Indeed, Steinweg et al. (2013) interpreted the positive effect of temperature on allocation to enzyme production as a cell-level response to larger nutrient needs driven by higher maintenance costs. In our model, warming causes larger rates of nutrient uptakes (higher\(v_{\max}^{U}\)) and this can lead to an adaptive increase of the enzyme allocation fraction (equation (8)). From the general evolutionary theory of phenotypic plasticity (Scheiner 1993), we expect the evolutionary optimal reaction norm of enzyme allocation fraction to follow the same pattern, i.e. φ increasing with temperature, provided that the cost of plasticity is not too high and does not alter the selection gradient too markedly. This warrants future investigation.
An interesting way of testing the role of evolutionary adaptationvs. plasticity would be to monitor the effect of warming on experimentally evolving bacterial communities at different levels of medium diffusivity or porosity (Rebolleda-Gómez & Travisano 2018). Our model predicts that the diffusion of resources (DOC), that is likely dependent on physical properties of the soil medium, has a strong influence on the adaptive evolution of microbial exoenzyme production in response to warming, but not on the ecosystem (non-evolutionary) response driven by enzyme kinetics and physiological plasticity. Thus, comparing population-scale decomposition and respiration across treatments that factorially cross temperature and medium physical properties may help disentangle the effects of adaptation vs. plasticity.