Sizing-up the impacts of fluctuating resources on body size

Recent work in the Centre for Geometric Biology has found that smaller algal cells had slower growth, lower storage of phosphorous and poorer recovery from phosphorous depletion but, interestingly, there was no effect of size when nitrogen was limiting.

Resource levels, such as food or nutrients, are rarely constant in nature but tend to fluctuate through time and across space. Such fluctuations in resources might have different impacts on organisms of different sizes but current ecological theories differ in their predictions of how the evolution of body size will be influenced by pulse inputs of food or nutrients.

While this is theoretically interesting, there is also a more pressing need for improving our understanding of such geometric biology. Phytoplankton cells are becoming smaller as a result of increased temperature and ocean acidification and we need to be able to better predict the consequences of this size shift under varying levels of resources.

Martino Malerba, Maria Palacios and Dustin Marshall have been able to test predictions from three different models of resource-use by using algae that have been genetically modified. They used 280 generations of artificial selection to create larger and smaller phytoplankton cells differing by as much as 1000% in mean body size.  

Cells were exposed to various resource levels by manipulating nitrogen (N) and phosphorous (P) in the growth media, to quantify how size can influence the ability of a species to cope with unpredictable nutrient conditions. 

Martino and his colleagues considered three different ecological theories that differ in their predictions on how size should mediate responses to fluctuating resources.  

  1. The ‘Fasting Endurance Hypothesis’ would predict that larger cells are more buffered against periods of nutrient limitation.
  2. The classic ‘r-K Selection Theory’ predicts that smaller cells with faster generation times will be better placed to take advantage of a nutrient pulse and so recover quickly from periods of nutrient limitation.  
  3. The ‘Metabolic Theory of Ecology’ would predict that tolerance to nutrient deprivation would decrease with increasing mass specific metabolic rate of an organism.

For the phytoplankton species used in this study (Dunaliella tertiolecta), the mass specific metabolic rate increases with size which means that larger cells should grow faster but be less tolerant to nutrient depletion than smaller cells. 

So which theory turned out to be correct?  The team found that periods of P depletion had a greater negative effect on smaller cells as predicted by the ‘Fasting Endurance Hypothesis’ but there was no effect of size on response to N depletion which was not predicted by any of the theories. 

Overall Martino, Maria and Dustin were able to determine that size interacts with stored resources in different ways. Increasing size can promote the ability to use stored P to supplement growth in D. tertiolecta, whereas the ability to store and utilise N does not change across sizes.

Transmission Electron Microscopy (TEM) photos with false coloring showing different densities of internal nutrient reserves between evolved algal cells from small-selected and large-selected lineages.
Mean population growth rate (with 95% confidence intervals) for each size-selected lineage of Dunaliella tertiolecta after experiencing nutrient-replete (red), nitrogen-deplete (green) or phosphorous-deplete conditions (blue) pre-trial conditions.

This research was published in Proceedings of the Royal Society B.