Do good things come in small packages?

Good things don’t always come in small packages: large-evolved phytoplankton cells had greater oxygen production and growth performance than small-evolved cells.

Biologists have been striving to understand the basic rules regulating the physiology and ecology of phytoplankton species for decades.  Now, more than ever, this is important because temperature increases may be changing the geometry (shape and size) of phytoplankton throughout the world’s oceans.

Post-doc Martino Malerba and his colleagues have used the artificially evolved large and small plankton cells to assess how efficiently cells of different sizes can utilise light, and if differences are predicted by current theories.

Previous studies have focused on looking at different species that range in size and have often showed physical constraints in the way cells use light. The long-standing theory of the ‘package effect’ says that as cells get bigger their ability to absorb light decreases, due to increasing self-shading of pigments within a cell.

Martino and his colleagues were interested in disentangling the underlying reasons for differences in size – are species a certain size because of specific features of their photosynthetic apparatus, or, are there ways to get around the physical limitation of increasing in size? The team wanted to find out what would happen when they only manipulated the size of a cell; would other things co-evolve in a way that is predicted by current theory? Or would the evolution of size take an unexpected path?

The research team monitored a number of photosynthetic traits in evolved cells that differed in size by up to 1,500%.  Small evolved cells were more efficient at producing oxygen (per unit of chlorophyll) which is what the physics behind the package effect predicts.  But the researchers also found that large-evolved cells not only showed a massive increase in chlorophyll content but also greater photosynthetic and growth performance (both per-cell and per-volume). This was possible because of a decrease in ‘sunscreen’ pigments (such as β-carotene) and a reorganization of the photosynthetic apparatus (more light receptors but of smaller sizes), which optimised light penetration within larger cells by reducing physical constraints.

The research team found that the evolution of size results in changes to many of the organelles within the cell, with many changing in direct proportion to the size of the cell. Some traits such as chlorophyll increased disproportionally in relation to the size of the cell while others such as β-carotene decreased disproportionally to the size of the cell. The photos show cells of large-evolved and small-evolved cells taken with a scanning electron microscope and coloured to emphasise some of the cells structures.

So while the ‘package effect’ successfully predicts the chlorophyll-standardised performance in cells of different sizes, this research shows that it doesn’t translate to whole cell performance.  This is because the loss of efficiency per chlorophyll pigment is more than made up for by the increased chlorophyll content and other changes that allow greater light penetration in larger cells.

Overall, Martino and his colleagues showed that altering the size of a cell profoundly alters many fundamental traits of algal physiology and ecology, with potentially serious impacts on global carbon cycles.