Altered cell wall morphology in nutrient‐deficient phytoplankton and its impact on grazers

Grazing experiments were performed with the zooplankters Daphnia pulex and Daphnia magna feeding on nitrogen‐ and phosphorus‐limited cells of two green algae ( Chtamydomonas reinhardtii and Selenastrum capricornutum ). To analyze the role of the cell wall structure in digestibility of the algae by Daphnia , the same experiments were carried out with both wild‐type C. reinhardtii and a cell wall‐deficient mutant. The nonlimited algae were efficiently assimilated, whereas P‐ and N‐limited algal cells were not. Especially P‐limited cells passed mostly intact and viable through the gut and were thus spared from heavy grazing pressure. In life‐history experiments, D. pulex grazing on nonlimited algae reached the largest body size, whereas animals fed N‐ or P‐limited algae exhibited reduced growth. Cells of the wall‐deficient mutant of Chlamydomonas , grown under both nutrient‐limited and nonlimited conditions, were efficiently ingested and digested by Daphnia . Morphological changes in the cell wall of nutrient‐limited cells most probably reduced their digestibility. This phenomenon might be considered a defense mechanism by the algae to reduce grazing pressure when their growth rates are low.