Dependence of cyanobacteria defense mode on grazer pressure

In a series of experiments we investigated the ability of a cyanobacterium of the genus Phormidium to defend itself against the ciliate grazer Furgasonia blochmanni. The experiments showed that the cyanobacterium can employ different modes of defense, depending on grazer density. Under high grazer pressure, each mat of Phormidium instantly formed a dense clump surrounded by a mucilage layer. Trichomes remaining inside such clumps were completely inaccessible to ciliates. Under medium pressure, the defense reaction was slower and consisted in enhanced sheath production by individual trichomes. The effectiveness of such a defense was also very high, as clearly reflected in the condition of the ciliates, When the pressure was low, the cyanobacteria did not exhibit any form of defense, dispersing evenly all over the bottom in a pattern similar to that observed in control mats. We conclude that the cyanobacterium is able to modify its defense reaction according to the real grazing risk.