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 o f two green algae (Chlamydomonas reinhardtii and Selenastrum capricorn utum). To analyze the role of the cell wall structure in digestibility of the algae by Daphnia, the same experiments were carried out with b oth wild-type C. reinhardtii and a cell wall-deficient mutant. The non limited algae were efficiently assimilated, whereas P- and N-limited a lgal cells were not. Especially P-limited cells passed mostly intact a nd viable through the gut and were thus spared from heavy grazing pres sure. In life-history experiments, D. pulex grazing on nonlimited alga e reached the largest body size, whereas animals fed N- or P-limited a lgae exhibited reduced growth. Cells of the wall-deficient mutant of C hlamydomonas, grown under both nutrient-limited and nonlimited conditi ons, were efficiently ingested and digested by Daphnia. Morphological change's in the cell wall of nutrient-limited cells most probably redu ced their digestibility. This phenomenon might be considered a defense mechanism by the algae to reduce grazing pressure when their growth r ates are low.