Van Donk, E., M. Lurling, D.O. Hessen, B. Lokhorst
Abstract:
Grazing experiments were performed with the zooplankters Daphnia pulex and Daphnia magna, feeding on nitrogen- and phosphorus-limited cells of two green algae (Chlamydomonas reinhardtii and Selenastrum capricornutum). To analyse the role of the cell wall structure in digestibility of the algae by Daphnia, the same experiments were carried out with a cell wall-deficient mutant of Chlamydomonas reinhardtii.
The non-limited algae with cell wall were efficiently assimilated, while P- and N-limited algal cells were not. Especially P-limited cells passed for the most part intact and viable through the gut and were thus spared from heavy grazing pressure. In life table experiments D. pulex grazing on non-limited algae, reached the largest size, while when fed N- or P-limited algae, the increase in size was significantly reduced. Cells of the wall-deficient mutant of Chlamydomonas were under both nutrient-limited and non-limited conditions 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 defence mechanism by the algae to reduce grazing pressure on them when their growth rates are low.
(Agricultural University Wageningen, Dept. of Water Quality Management and Aquatic Ecology, P.O. Box 8080, 6700 DD Wageningen, The Netherlands)
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