Biological Interactions between Hosts, Parasites, and Mercury

Coastal communities, including regions along the North Carolina coast, have seen impacts of sea level rise. These impacts include wetland salinization, loss of nursery habitat, and decreased protection from major storm events. Wetlands provide ecosystem services for our environment, including filtration of runoff from land pollutants. One of those major pollutants which affects North Carolina biota is the heavy metal, mercury. Mercury pollution in wetlands is mainly due to anthropogenic sources, including the burning of fossil fuels, and also natural sources, such as volcanic activity. Inorganic mercury is released into the atmosphere, where it eventually settles to the ground. Mercury can also be taken up by plants and stored in leaves which eventually fall to the ground. These processes of mercury deposition can ultimately make their way to wetlands through surface water runoff. Inorganic mercury, which is deposited in wetlands, can be converted through sulfate reducing bacteria within the sediment to a bioavailable form known as methyl-mercury. Methyl-mercury (MeHg) production may also depend on salinity concentrations in wetlands\; therefore, sea level rise could affect mercury levels in wetlands, possibly leading to increases in MeHg in some locations. In general, mercury is a major health concern for North Carolina residents, especially those living near the coast. In particular, mercury poses a major health concern to animals, and can bioaccumulate and biomagnify through trophic levels, eventually reaching humans. Previous work has shown that parasites can take in pollutants, like mercury, from their host tissues and store those pollutants within their own tissues, thus acting like a sponge to pollutants in the host, and in some cases, lowering the levels of pollutants in the host. However, salinity may also have an impact on the amount of MeHg which gets produced within wetland sediments. One of my objectives looked at the mercury levels of uninfected host tissues compared to infected host tissues, and how this might change over a salinity gradient. In order to determine this, parasite life-cycles were taken into consideration and how parasites and mercury might transfer to different hosts. This study took place in two North Carolina estuaries, the Pamlico and Neuse Rivers. I had five sites along each river, from oligohaline to mesohaline localities. Over a one-year period, I collected three host species (two resident species: naked gobies and white-fingered mud crabs, and one mobile species: blue crabs) and recorded their parasite diversity and abundance. I then preserved host and parasite tissues and analyzed them for total mercury (THg) content. In addition, water samples were collected to measure DOC levels because DOC has also been shown to correlate with THg in biota. Also, sediment samples from each site were measured for THg and MeHg. Finally, I took abiotic measurements (most importantly salinity and temperature) as these measures were predicted to be important physical factors of MeHg concentrated in my study system. I predicted that hosts which were parasitized would have lower levels of THg in their tissues compared to hosts which were unparasitized, and that number of parasites would positively increase with salinity. For one type of parasite in the mud crabs, this prediction was upheld, but I did not observe this pattern for the other parasites in my study. In fact, results from THg analysis showed higher mercury levels in hosts which were parasitized compared to unparasitized hosts. This result could suggest that parasites are influencing the health of these hosts making them more prone to infection by parasites (i.e., susceptibility is higher when mercury levels are higher). In addition, I found THg and MeHg levels in sediments were higher at lower salinities than at higher salinities. In naked gobies, THg levels in tissues from the Neuse River were highest at the lowest salinity site. In the Pamlico River, THg in naked goby tissues showed a slight increase with increasing salinity. With increasing salinity, DOC decreased. Finally, for naked gobies in the Neuse River, parasites were found consistently throughout my sites, and in the Pamlico River parasites were most abundant in the second lowest salinity and at the highest salinity site. Altogether, this study provides important information for how parasite communities and THg levels change throughout an estuary\; however, more investigation is needed to determine the THg levels of uninfected and infected hosts, and their parasites