Differential sensitivity of Synchaeta and Daphnia to nucleosides from Anabaena affinis

Abstract

The cyanobacterium Anabaena affinis contains two nucleosides responsible for its toxicity: 9-deazaadenosine 5'α-D-glucopyranoside (compound 1) and 9-deazaadenosine (compound 2). As expected, a strain of Daphnia pulex inhibited by A. affinis also was inhibited by these nucleosides. Surprisingly, however, a strain of D. pulex coexisting with A. affinis, and not inhibited by it, was equally or more inhibited by the nucleosides. LC50 values for compounds 1 and 2 were, respectively, 1.33 and 0.56 μg ml-1 for the former D. pulex and 0.79 and 0.54 μg ml-1 for the latter. The resistant D. pulex, which benefits from the ingestion of A. affinis, may have evolved a mechanism to detoxify the nucleosides in its intestine. In contrast, Synchaeta pectinata, which is unaffected by A. affinis, was not inhibited by the nucleosides. High concentrations of compounds 1 and 2 (3.6 and 2.2 μg ml-1, respectively) reduced neither survivorship nor fecundity. The resistance of this rotifer to the dissolved nucleosides may be due to its inability to absorb them across its surface membranes, to its inability to metabolize them into more toxic compounds, or to its lack of a receptor for them. An evolved resistance seems unlikely, as S. pectinata probably does not ingest A. affinis. The effect of A. affinis on natural zooplankton communities should be very different from that of strains of Anabaenaflos-aquae producing the alkaloid, anatoxin-a. The A. affinis should be ingested by many cladocerans but not rotifers, and it contains toxins which inhibit cladocerans but not S. pectinata and perhaps other rotifers. The A. flos-aquae is ingested by rotifers as well as cladocerans, and its toxin inhibits both rotifers and cladocerans.