Disentangling the multiple drivers of ecological adaptation in a microorganism

Abstract

AbstractThe survival and reproduction of living organisms depend on their ability to achieve an adequate balance between energy intake and energy expenditure. Multiple quantities contribute to this energetic balance, such as the feeding rate, and the allocation of available energy to growth, maintenance, movement and reproduction. Given that many of these quantities scale in a predictable way with the size of the organism and with environmental parameters, by quantifying these scaling relations it should be possible to predict how organisms may adapt to novel environmental conditions in terms of adjusting their morphology, physiology, and behaviour. In a series of experiments, we adapted axenic experimental populations of the ciliateTetrahymena pyriformisto different environmental conditions of temperature (15°C, 20°C and 25°C) and resource levels (50%, 100%, and 200% of a standard protein solution). We measured population growth, metabolic rate (from respiration), cell size, and movement speed (from video-tracking). On a very short time scale, movement speed and metabolic rate increased with environmental temperature in a way that can be predicted from simple physical scaling relations such as the Boltzmann-Arrhenius equation and the ‘viscous drag’ impacting movement. However, soon after the introduction of tetrahymena into a novel environment, all the measured quantities were further modulated in a direction that likely provided better biological fitness in the new environment. We discuss our results in terms of both theoretical and experimentally observed scaling relations of individual quantities, combined with reasonable optimisation principles.