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.