A review on mercury biogeochemistry in mangrove sediments: Hotspots of methylmercury production?
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Pei Lei, Huan Zhong, Dandan Duan, Ke Pan


Wetlands are highly productive and biologically diverse environments that provide numerous ecosystem services, but can also be sources of methylmercury (MeHg) production and export. Mangrove wetlands contribute up to 15% of the coastal sediment carbon storage and ~10% of the particulate terrestrial carbon exported to the ocean. Thus, mercury (Hg) methylation in mangrove sediments and subsequent MeHg output to adjacent waters could have a great impact on global Hg cycling. In this review, we provide a comprehensive analysis of the literature on worldwide Hg concentrations in mangrove ecosystems, and the results reveal that a large range of total Hg (THg) and MeHg concentrations is detected in mangrove systems. Then, we discuss the potential roles of organic matter (OM) in controlling the Hg biogeochemistry in mangrove sediments. The intense OM decomposition by anoxic reduction (e.g., sulfate reduction) drastically affects sediment chemistries, such as redox potential, pH, and sulfur speciation, all of which may have a great impact on MeHg production. While the outwelling of carbon from mangroves has been extensively examined, little is known about their roles in exporting MeHg to adjacent waters. Our understanding of Hg biogeochemical processes in mangrove systems is constrained by the limited MeHg data and a lack of in depth studies on the Hg methylation potential in this ecologically important environment. More efforts are needed to gain better insights into the contributions mangrove wetlands to the global Hg cycle.

Main Results and Conclusions:
  • Mangroves play a critical role in both carbon and mercury storage and export.
    • “Mangrove wetlands contribute up to 15% of the coastal sediment carbon storage and ~10% of the particulate terrestrial carbon exported to the ocean. Thus, mercury (Hg) methylation in mangrove sediments and subsequent MeHg output to adjacent waters could have a great impact on global Hg cycling.” (140)    
    • “Wetlands are highly productive and biologically diverse environments that provide numerous ecosystem services, but can also be sources of methylmercury (MeHg) production and export.” (140)
  • Mangroves are one of the most valuable types of wetlands on earth.            
    • “Mangrove forests are ecologically and economically important wetlands that are estimated to cover approximately 75% of the tropical and subtropical coastlines (Duke et al. 2007; Giri et al. 2015). These wetlands are highly productive ecosystems and support a series of ecosystem services, such as protecting coastlines from erosion, providing nursery grounds and habitats for aquatic organisms, and supplying forestry and fishery products for humans (Alongi 2008; Nagelkerken et al. 2008).” (p. 141)                
    • “Occupying only ~0.5% of the total global ocean area, mangroves account for approximately 10%–15% of the total carbon sequestration in the coastal ocean (Alongi 2012; Donato et al. 2011).” (p. 141)                
    • “In fact, many features of the mangrove environment may be ideal for promoting Hg methylation, including anoxic, high temperature, and low pH conditions, high availability of palatable carbon, and abundant substrate to support sulfate-reducing bacteria (SRB), i.e., an important microbial methylator (Correia and Guimarães 2016).” (141)
  • Organic matter influences much of the MeHg production in mangrove sediments.    
    • “...rich OM is believed to have a major impact on the storage, transportation, distribution, and methylation of Hg in mangrove sediments. The abundant reduced sulfur sites on OM molecules provide strong binding sites for both Hg2+ and MeHg (Khwaja et al. 2006; Tai et al. 2014), enabling the burial of Hg in sediments or transport of Hg to adjacent waters.” (144)                
    • “Many studies have suggested that OM substantially reduces MeHg production by decreasing the bioavailability of inorganic Hg (Leclerc et al. 2015; Bouchet et al. 2018). For example, Wu et al. (2011) found a negative relationship between MeHg concentrations and total OM contents in mangrove sediments.” (144)
    • “...OM can affect MeHg production in mangrove wetlands in different ways (Fig. 1). It can act as a source of energy and nutrition that stimulates the growth of Hg methylators such as SRB, a principal group of microorganisms mediating the transformation of inorganic Hg to MeHg (Beckers and Rinklebe 2017; Gilmour et al. 1992). The process of OM decomposition further changes the sediment chemical environment (e.g., redox potential, pH, and sulfur speciation) (Beckers et al. 2019; Frohne et al. 2012), producing profound effects on MeHg produc- tion in mangrove sediments.” (144)    
  • Mangroves produce something called the “outwelling effect” which is a process where mangroves produce an excess amount of CO2 and “outwell” these organic nutrients into the surrounding area, which is important for the surrounding areas.  
    • “Known as the “outwelling effect”, mangrove forests export carbon, nitrogen, and other elements as a subsidy that nourishes the adjacent coastal and estuarine food webs (Alongi 2014; Ray et al. 2018).” (146)                    
    • “The mean rate of DOC export across all mangroves is 26.6 g C m−2 year−1 (Adame and Lovelock, 2011), which is approxi- mately 13% of the mean C exported as litter (202 ± 241 g C m−2 year−1) and 45% of C export as POC (59.1 ± 88 g C m−2 year−1). The POC exported by mangroves accounts for as much as 10% to 11% of the total terrestrial carbon input to the ocean and 12% to 15% of the total car- bon accumulation in shelf margin sediments (Dittmar et al. 2006).” (146)
    • “Bergamaschi et al. (2012) estimated that the yields of filtered THg and MeHg in the mangrove dominated Shark River estuary were 28 ± 4.5 μg m−2 year−1 and 3.1 ± 0.4 μg m−2 year−1, respectively, rates five times greater than typically reported for terrestrial wetlands, suggesting that tidal flushing of mangrove swamps represents a potentially large source of THg and MeHg to estuarine and coastal aquatic ecosystems.” (146)
  • Mangrove ecosystems have quiet complex sediment chemistry which is very important not only for the sediment but also those that dwell in these ecosystems and the surrounding areas.                  
    • “The abundant OM in mangrove sediments may have great influences on Hg biogeochemistry. As one of the largest carbon pools in coastal water, mangrove sediments may act as a major respiratory factor for Hg. OM itself can affect Hg speciation and bioavailability, altering the Hg methylation rates.” (146)        
    • “OM decomposition impacts sediment chemistry, such as pH, Eh, and sulfur speciation, forming an anoxic and acidic environment that may favor MeHg production. The heterogeneous nature of carbon sources further confounds the methylation process in mangrove sediments.” (146) 
  • A specific gene has been isolated called hgcAB that all Hg methylating microbes possess.
    • “Current evidence indicates that all Hg methylating microbes possess the gene pair hgcAB for Hg methylation (Liu et al., 2018; Parks et al. 2013).” (144)
    • “The hgcAB genes have been identified in several anaerobes, including δ-Proteobacteria (e.g., SRB and FeRB) within the phylum Proteobacteria, bacteria within the phylum Firmicutes (Clostridia), and bacteria in Methanomicrobia within the phylum Euryarchaeota (Gilmour et al. 2013; Parks et al., 2013; Regnell and Watras, 2019). However, the roles of these anaerobes in the production MeHg in mangrove sediments have not been understood.” (144) 
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