Authors: Colin Bruce Jack
In principle, many animals could obtain significant metabolic energy direct from their environment, additional to that from food.
Photosynthesis requires a large area of modified skin and imposes behavioural constraints: few animals are photosynthetic.
An alternative is to use the Carnot cycle, exploiting temperature differences. The maximum efficiency with which work can be extracted is ~ΔT/T, where terrestrially T~300 K: comparable to photosynthesis. For a cold blooded animal which moves frequently between environments at significantly different temperatures, this energy harvest could be substantial: its entire body can act as a thermal reservoir. The energy harvesting machinery might however be hard to spot, much as ‘brown fat’ in human adults was overlooked until recently. It could be based on any temperature-sensitive chemical equilibrium.
In the temperate and tropical oceans, animals could gain Carnot cycle energy very easily as they swim up and down through the thermocline during diel vertical migration. In so doing, they would transfer significant heat from near-surface waters to the mid-depths. Such behavior would be increasingly favored as surface waters become warmer, as has recently occurred. This could conceivably have contributed to the recent ‘pause’ in the anthropogenic greenhouse effect.
At the origin of life, diurnal thermal harvesting by non-motile organisms could evolve far more easily than photosynthesis, and might have preceded both photosynthesis and the use of external chemical energy.
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