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

Abstract. Methylmercury (MeHg) is a bioaccumulative neurotoxin that poses a risk to human health through seafood consumption. Sponges play a complex role in mercury (Hg) cycling, with measurements showing an unusually high inorganic Hg (iHg) content in Low Microbial Assemblage (LMA) sponges and an even higher iHg content in High Microbial Assemblage (HMA) sponges. At the same time, the MeHg content remains low, particularly in HMA sponges. In this study, we used a 1D water column model to investigate the bioaccumulation of MeHg in sponges. It has been hypothesized that this low MeHg content is due to active demethylation in HMA sponges. Our model results suggest that the consumption of dissolved organic matter (DOM) in LMA sponges can explain the low observed MeHg content, and higher DOM consumption in HMA sponges can account for the even lower MeHg content in HMA sponges. If demethylation occurs, a low demethylation rate of 1 % per day can account for the observed difference between LMA and HMA sponges. Although DOM consumption increases iHg bioaccumulation in both LMA and HMA sponges, it does not explain the extremely high values observed, suggesting a reduced iHg release rate in sponges. We propose that this low Hg release rate is due to sulfated polysaccharides, which are abundant in sponges, especially HMA sponges. Finally, our model suggests that HMA sponges could potentially reduce the MeHg content in benthic fish by up to 45 % when HMA sponges dominate at the base of the food web. While these findings suggest an important role of sponges in Hg cycling and emphasize the need to preserve sponge grounds to mitigate human MeHg exposure through seafood, this should be seen as a hypothesis-generating model result which would require further empirical validation.