Parasite prevalence and nanosilver affect benthic nutrient cycling and productivity

Background/Question/Methods

Parasites are ubiquitous members of ecological communities, but have only recently been recognized as key players in broad ecological interactions and ecosystem dynamics.  Likewise, organisms in natural systems are exposed to an increasing array of potential chemical contaminants including manufactured nanomaterials with unknown ecological consequences.  Trematodes are flatworm parasites that infect gastropods as intermediate hosts, often altering snail body and excretory elemental ratios.  We explored the effects of trematodes on benthic ecosystem processes through their effects on snail nutrient recycling and grazing in the presence of realistic concentrations of a potential contaminant, nanosilver.  Specifically, we investigated the relationships among the prevalence of the trematode Trichobilharzia, their host snails (Physa acuta), and ecosystem processes with and without nanosilver.  In an outdoor mesocosm experiment, we measured periphyton productivity, N:P ratio, algal community structure, and snail populations across a range of trematode prevalence.

Results/Conclusions

Trematode prevalence negatively affected benthic algal productivity and altered algal elemental composition, but the relationships were altered in the presence of 0.03µg/L nanosilver.  Specifically, algal N:P ratios were linearly and positively dependent on trematode prevalence in the presence of nanosilver, but nonlinearly related in the absence of nanosilver.  Nanosilver reduced periphyton productivity by 20% likely due to its antibacterial properties.  Physa biomass was reduced in highly infected mesocosms.  However, nanosilver stimulated Physa egg production resulting in 27% more snail individuals after 28 days of exposure compared to mesocosms without nanosilver.  Thus, greater trematode prevalence reduced algal productivity and altered algal N:P elemental composition, but the effects depended on the presence of realistic concentrations of nanosilver. Overall, these results indicate that trematodes modify benthic ecosystem processes and the net result of these parasite-ecosystem interactions is affected by anthropogenic chemical pollution.