Integration of molecular with higher-level effects of dietary zinc exposure in Daphnia magna.

We exposed Daphnia magna for 21 days to dietary Zn, incorporated in a diet of the green alga Pseudokirchneriella subcapitata at 720 microg Zn/g dry wt and compared its response to D. magna fed with a control diet (116 microg Zn/g dry wt). Exposure to dietary Zn resulted in an increased body burden of D. magna (93.7 microg/g dry wt vs. 61.3 microg/g dry wt in the control) but did not affect survival, growth, or feeding rate. Only reproduction was significantly reduced from the 2nd brood onwards. Gene expression analysis, using microarray analysis and RT-PCR, showed that dietary Zn exposure resulted in the differential expression of several genes involved in molting-associated processes (i.e., chitin binding, cuticle metabolism), especially after 6 days of exposures (but not after 13 or 21 days of exposure). Monitoring of time to molt and intermolt-period confirmed this molting effect at the organism level in the first week of exposure. The data suggest a possible link between Zn-induced effects on molting-related processes and reproductive inhibition, but this link is only obvious for effects on the 2nd brood size and not for later broods. Reproductive inhibition in later broods may also be explained by a disturbed mitochondrial function, but more research is clearly needed to give a more definitive integrated explanation of the observed effects at the molecular and organism level.