| 12 Nov 2025
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.
Competing interests: One of the authors is a member of the editorial board of biogeosciences.
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This study constructed a model to trace the bioaccumulation of MeHg within the Mediterranean Sea food web in the Bay of Villefranche. The model initially enabled assessing the levels of iHg and MeHg accumulating in LMA sponges. The model was also used to assess the differences in MeHg bioaccumulation in fish in a setup with and without sponges to see if sponges influence the MeHg bioaccumulation in fish. Although an advanced model was developed and the design of scenarios was ingenious in this study, the article reads rather obscure. Many paragraphs are suggested to be refined, and the logicality should be simultaneously increased. Major revision is needed and some comments are shown as follows.
(1) Line 85: The sentence “This is proposed in (Amptmeijer et al., 2025b), which is a……” is confused. Please rewrite the sentence.
(2) Section 2.1: A map is needed for the model domain in the main text.
(3) For the subtitles of sections 2.1, 2.2, 2.3, and 2.3.2, dashes are better than semicolons. And capitalize the first letter of “semi-labile” in Section 2.6.
(4) Section 3.4: In my opinion, the observations of Hg and MeHg in the study domain are too few to support the model evaluation. I suggest more observations particularly for various environments and functional groups.
(5) All the figures should be improved due to their rough presentation.
(6) Section 4.2: What’s the underlying mechanisms for low MeHg due to DOM consumption in HMA sponges? I only see the results of model scenarios. However, the mechanistic inference is more meaningful.
(7) For model studies, uncertainty analysis is necessary to explain the credibility of the model results. I recommend the supplement of uncertainty analysis.