Net-energy gain in phytoplankton cells is dependent on cell size, light, and population density

While there have been many studies on the effects of organism size on energy use, there are few studies that also examine rates of energy acquisition. The overall effect of size on the energy budget of an organism will depend on how size affects both energy gain and use, that is, the net-energy gain.

To improve our understanding on this topic, postdoctoral fellow Martino Malerba has been working with colleagues Craig White and Dustin Marshall to investigate how size affects net-energy gain in single celled phytoplankton species at different light intensities and population densities.

They used 21 species of phytoplankton from seven phyla, spanning four orders of magnitude in cell volume. All species were measured across six light intensities and four different population densities. Net-energy gain was calculated by fitting non-linear models to changes in percentage oxygen saturations across all treatment combinations, which then allowed researchers to calculate rates of photosynthesis and respiration.

The team examined 21 species of phytoplankton from seven phyla.

Martino and colleagues found that increasing the size of a cell or decreasing population density produced similar proportional increases in both rates of energy production (via photosynthesis) and respiration.

Larger cells produced more energy but also had higher energy costs due to respiration. Increasing population density decreased both photosynthesis and respiration rates. While the reduction in photosynthetic activity may be explained by an increase in shading by suspended cells at higher population densities, this does not explain why increased population density also decreased respiration rates in the dark.

It is possible that reducing metabolic rates at increasing population densities is an adaptive strategy that reduces the minimum requirements of a cell and improves fitness when resources are limited.

Finally, it is increasingly apparent that global temperature increases are reducing body sizes worldwide, particularly of aquatic organisms. Results from this study suggest that future phytoplankton communities dominated by smaller species should display higher size-specific rates of net-energy gain. This would mean current rates of carbon storage in aquatic environments are likely to increase.

This research is published in the journal Ecology.