Illuminating results on plant plankton

Posted on March 27, 2012

Trait-based approaches are increasingly used to explain how environmental gradients influence the organization of ecological communities.  Because light is a key resource for plant plankton, a large number of studies have explored how growth rates of phytoplankton species respond to changing light levels.  However, there have been few quantitative comparisons of the light responses of major phytoplankton groups.  Ecologists in the USA and Japan drew on databases and experiments to compile light utilization traits for 56 species of freshwater phytoplankton, and used these traits to explain differences in major group distributions in 527 U.S. lakes.   The traits in question were maximum growth rate, the initial slope of the growth-light curve, and the optimum light level (i.e., the level beyond which the efficiency of photosynthesis falls off due to light-induced damage).  The main plankton groups showed significant differences in their light use traits and patterns of distribution.  Initial curve slopes were highest for cyanobacteria, reflecting their adaptation to low light levels, but low for green algae, indicating high-light adaptation.  Groups with high curve slopes tended to have high maximum growth rates but low optimum light levels.  A lot of the variation in curve slopes was explained at the highest taxonomic levels, which suggested that there had been a fundamental divergence in the early evolution of light use strategies employed by phytoplankton.  The high light-use efficiency of cyanobacteria could be linked to their lack of chloroplasts.  Because their photosynthetic membranes are located in the cytoplasm, light has only to cross the cell wall to reach them; in contrast, in plants, light is lost by the need to pass through two or more additional membranes in the chloroplasts.  Initial curve slopes and maximum growth rates both tended to be high in small-celled plankton, presumably at least partly because of lower levels of internal light loss.  These relationships help to explain why size distributions of phytoplankton communities vary with the light conditions.

Reference:  Schwaderer, A.S., Yoshiyama, K., Pinto, P., Swenson, N.G., Klausmeier,, C.A. & Litchman, E.  2011.  Eco-evolutionary differences in light utilization traits and distributions of freshwater phytoplankton.   Limnology & Oceanography 56(2), 589–598.

Posted in: physiology, plankton