Cooler equals bigger in even the tiniest organisms

Size matters, and colder temperatures make things bigger! This is true not just for most large furry animals and for birds, but also for the microscopic plants and animals that are at the base of the ocean’s food chain.

Scientists have long known that animals and plants are usually larger when they grow in colder environments. Now, for the first time David Atkinson, Ben Ciotti, and David Montagnes, from the University of Liverpool’s School of Biological Sciences, have found that this observation applies to microscopic organisms, whose entire body is just a single cell (known as “protists”).

These researchers have made a further remarkable discovery: a single temperature-size “rule” applies to all the protists. Even more surprising is that it doesn’t matter how or where the protists “make their living”; the rule still applies. Protists can be plant-like, depending on sunlight for energy, or can be animal-like, eating food; they can come from the sea, from lakes, or even puddles on the pavement: they all get larger in cold environments and smaller when they live in warm environments. The rule that they have found is that protists decrease by 2.5% of their average size with each increase of one degree Celsius; hence if the water is warmed by 10 ºC, the single-celled creatures may shrink by about one quarter of their original size.

However, temperature not only changes the size of protists, it influences how fast they grow. Protists grow by dividing from one cell to two, to four, etc, and temperature speeds up this process. In a second, controversial, study, David Montagnes, Susan Kimmance, and David Atkinson re-investigated the effect of temperature on growth of protists, and their new findings suggest that the previous temperature-growth rules are wrong.

The ecological impact of these two investigations is wide-ranging. According to Dr Montagnes: “As protists form the base of many aquatic food webs, our new temperature-size rule and the revised temperature-growth rule will allow scientists to better predict how environmental warming affects processes influencing food production in nature.”

The findings are likely to stimulate further research, in particular to determine the cause of these size-temperature relationships. According to Dr Atkinson: “It is very useful to find and describe these widespread patterns, but what is even more exciting is to understand the underlying causes. Our future work is aimed ultimately at that goal.”

This research was partially funded by a British Ecological Society Small Ecological Project Grant.