Parasite of the month

Cyanophages

I visited relatives in France last August and, when my plane took off, I photographed the archipelagos surrounding Helsinki. When we passed the shore of the neighbouring city, Espoo, the reed line suddenly became quite evasive. The colour and structure were somehow different from usual reeds and it was only when the plane flew over the open sea that it became clear to me: these were cyanobacterial blooms (also known as blue-green algae). Later on, I could see large blooms offshore. Although I had observed these in small lakes before, I had only seen pictures of such large cyanobacterial blooms that occur in the Baltic Sea. The boat on the picture looks very tiny...

Among the diversity of cyanobacteria that can contribute to the bloom, Nodularia spumigena is the most common and troublesome in the Baltic Sea. Nitrogen and phosphorus concentrations have rised in the last century in the Baltic Nutrient as a result of human activities that increased productivity (i.e. generation of organic matter) - i.e. eutrophication - in marine ecosystems. High availability of resources for primary producers favours the growth of fast-growing organisms, such as cyanobacteria. In addition to directly competing for light and nutrients with other organisms, N. spumigena produces toxins that can accumulate through the food-web either when the cyanobacteria is consumed by grazers (e.g. copepods, daphnia) or in their dissolved form, when the bloom decays.

Cyanophages are important players in bloom termination. Although most of our knowledge is restricted to marine phages, virus of cyanobacteria have also been detected in freshwaters but their role in freshwater ecosystems remains unclear. Current work aims at controlling blooms with grazers (e.g. great pond snail, Lymnaea stagnalis) and cyanophages. Viral lysis has been notably discussed to affect dissolved nitrogen concentration not only by limiting its consumption by regulating cyanobacteria populations but also by inducing the release of nitrogen from lysed cells. One of the best examples of how parasites can alter whole ecosystems.