Øystein Nordeide Kielland, Claus Bech, Sigurd Einum
Animals need oxygen to support life, as most of the chemical reactions in the cells of the body require oxygen to function properly (referred to as aerobic metabolism). In water, oxygen is increasingly less soluble with increasing temperatures. At the same time, diffusion of oxygen occurs faster due to thermodynamics, making more of it available for water-breathing individuals. The question then is: is this increased availability of oxygen enough to sustain the (increased) metabolism at higher temperatures? This is because, being ectothermic, their body temperature and thus metabolism also increases in parallel with oxygen availability.
With future increasing temperatures, do animals risk having a deficit of oxygen to sustain their metabolism? Can we expect that animals can acclimatize to these changes without evolving (referred to as plasticity)?
We measured the oxygen uptake as well oxygen consumption in the freshwater water flea, Daphnia magna. In addition, we measured the threshold at which oxygen levels become critical at various temperatures. This water flea is quite special, as it reproduces through cloning, thus keeping the setup of genes more or less constant through time in our lab. We acclimated a single clone to three temperatures around normally experienced and extremely high temperatures. We then used our empirically measured values to model how the imbalance between oxygen uptake and consumption changes between the different temperatures.
We found that Daphnia do increase their ability to absorb oxygen from their environment as temperatures increase. This response is strongest within the highest temperature range (22-28 °C), where the “oxygen challenge” is expected to be most pronounced. However, the observed ability to increase oxygen supply failed to compensate for the increased demand. Thus, the ability to acclimatize oxygen uptake and demand is not sufficient to cope with the increasing deficiency of oxygen that is expected with warmer waters.
Functional Ecology: Plain Language Summaries 
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