Virus Decimates Algal Blooms

The pest alga species Phaeocystis globosa has attacked the North Sea coast each spring and summer for many decades. The slimy colonies can be up to one centimetre in size and contain tens of thousands of brown-coloured algal cells. Strong winds whip the slime up into foam. This leads to a metres-thick, stinking mess on the beach. In the water the slime remnants block the gills of shellfish, herbivorous plankton and fish.

In a large-scale controlled experiment, biologists from the Netherlands Institute for Sea Research investigated how viruses influence the growth of the pest algae. During the period of rapid growth the algae scarcely seemed susceptible to viruses. However, the algae weaken once the supply of phosphate and nitrate has been depleted.

In particular, the free-living, non-colonising cells appear to be highly susceptible to viruses. The cells in the slime-forming colonies initially suffer less. However, in the end the lack of nutrients ensures that these colonies also fall apart. At this point the viruses can perform their fatal work. They destroy the entire algal bloom within a matter of days. Herbivorous plankton which always graze on the algae, assist the viruses in their destructive work.

The researchers foresee possibilities for controlling algae with the help of viruses. However, this will only be possible in isolated areas, for example in fiords or in rearing areas for oysters. The spreading of viruses along the entire North Sea coast would be met with a barrage of practical and ethical objections.

Grazing by herbivorous plankton leads to a different fate for the algae than that caused by viruses. After grazing the material remains in the food chain, whereas after the virus infection the entire cell content is dissolved in the seawater where it subsequently decomposes. The bacteria responsible for this decomposition process use 'costly' oxygen and produce carbon dioxide as opposed to absorbing it.