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.

Behavioural manipulation: 10 bedtime stories!

Behavioural manipulation is one of the most impressive consequences of parasitic infections. In order to increase their transmission rate, some parasites alter the behaviour of their host, sometimes leading to its death.

Suicides of hairwom-infected crickets (Thomas et al. 2002) and of mice infected with protozoa (Berdoy et al. 2000) are among the best-documented examples where host death is necessary for parasite reproduction or transmission. Such manipulations can be very simple, such as for the eye fluke Diplostomum pseudospathaceum, which encysts in the eyes of its fish host to cause blindness and increase predation by the fish-eating bird final host (Karvonen et al. 2003). Other parasites, such as the trematode Schistosoma mansoni (Kavaliers et al. 1999), use more sophisticated manipulation by secreting of neuroactive substances to alter host behaviours.   

Ten bedtime stories of behavioural manipulation by parasites. 

Among these is notably mentioned the tapeworm Schistocephalus solidus, which is known to deeply affect the behaviour of its secondary intermediate host - the threespine stickleback Gasterosteus aculeatus. More about this parasite here!