Palmelloids formation in Chlamydomonas reinhardtii : defenceagainst rotifer predators?

Abstract

The green alga Chlamydomonas reinhardtii usually occurs in cultures as single, biflagellated cells. However, C. reinhardtii is known for its ability to form gelatinous and palmelloid stages that arise as a result of an interaction with its environment. Exponentially growing unicellular C. reinhardtii formed palmelloid colonies rapidly within 25 h when cultured together with their enemy the rotifer Brachionus calyciflorus. Consequences of palmelloid formation for population dynamics of both C. reinhardtii and B. calyciflorus were examined in continuous flow systems. Palmelloids were only formed in a one-stage system where B. calyciflorus grazers and C. reinhardtii prey were cultured together, but not in a two-stage system in which mainly unicellular C. reinhardtii was pumped into a rotifer culture placed in darkness. The rotifer abundance was lower and the algal biomass higher in the one-stage system compared to the grazing unit of the two-stage system. Inasmuch as palmelloids seemed to give C. reinhardtii cells resistance to grazing, we suggest that at least one of the reasons why C. reinhardtii is capable of forming palmelloids is to cope with herbivory.