DOM consumption and demethylation of MeHg as potential drivers of low MeHg in Mediterranean Sea sponges and benthic fish: a modeling perspective

Abstract

Methylmercury (MeHg) is a bioaccumulative neurotoxin that poses a risk to human health through seafood consumption. Sponges have unique mercury (Hg) profiles. Measurements show an unusually high inorganic Hg (iHg) content in Low Microbial Abundance (LMA) sponges and an even higher iHg content in High Microbial Abundance (HMA) sponges, while MeHg concentrations remain low, particularly in HMA sponges. Combined with the recently improved understanding of the important ecological role of sponges as a food source for other biota, this suggests that their low MeHg content may influence MeHg transfer within benthic food webs. In this study, we used a 1D water-column model to investigate the bioaccumulation of MeHg in sponges. It has been hypothesized that the low MeHg content in HMA sponges may result from active MeHg demethylation. Our model results indicate that the consumption of dissolved organic matter (DOM) can already explain the low observed MeHg content in LMA sponges, and higher DOM consumption in HMA sponges can account for the even lower MeHg levels in HMA species. Alternatively, if MeHg demethylation occurs, a low rate of 1 % d−1 could explain the differences between LMA and HMA sponges. Although DOM consumption increases iHg bioaccumulation in both sponge types, it does not explain the extremely high iHg concentrations observed. Finally, our model suggests that HMA sponges could potentially reduce MeHg concentrations in benthic fish by up to 53 % when they dominate at the base of the food web. These findings highlight the potentially important role of sponges in Hg cycling and indicate that sponge-dominated systems could help reduce MeHg accumulation in benthic food webs.