Caddo Lake in northeastern Texas is a cypress-Spanish moss dominated lake ecosystem. Contamination of mercury (Hg), especially methylmercury (MeHg), was reported in this lake a decade ago. MeHg is a neurotoxicant accumulated in major fish species and reptiles. Due to the biomagnification feature of MeHg transfer, Hg contamination in the fishes of Caddo Lake has caused health concerns for the wildlife and local people. However, the source and synthesis of MeHg in this lake, primarily the microbial Hg methylation mechanisms, have not been investigated. We investigated the lake for the past three years (2016-2018), by taking sediment and plant samples in several locations of the lake wetland habitats which showed high MeHg levels in fish from previous studies. We employed a culture-independent molecular approach to identify the Hg-methylating microbial community present in sediment as well as the sporangia of the invasive species Giant salvania (Salvania molesta). Total organic carbon, total Hg, MeHg, sulfate, iron(III) and other biogeochemical factors were analyzed in the lake ecosystem. We extracted genomic DNA from all samples and detected functioning genes including the Hg methylation genes (hgcAB), methyl-coenzyme M reductase genes (mcrA) as well as 16S rRNA genes. The 16S rRNA genes were characterized by high throughput next generation sequencing on Illumina MiSeq. In lake sediment samples, a total of 6402 OTUs were discovered, dominated with Crenarcheales (9.7%), Bacteroidales (5.2%), Syntrophobacterales (3.1%). Our results indicated that the lake sediment samples contained diverse potential mercury methylators, including Syntrophobacteraceae (1.4%), Geobacter spp. (1.1%), SRB Desulfovibrio-Desulfobulbus-Desulfobacter (0.6%), and methanogenic archaea (0.6%). It seems that microbial MeHg production in this wetland habitat could be influenced by a complex syntropy among Syntrophobacterales, methanogens, and sulfate-reducing bacteria. Results based on the geochemical data and hgcA gene detection and quantification suggest that Johnsons Ranch and Judd Hole are likely the hot spots for MeHg production in this lake ecosystem.
Date of publication
Riqing Yu Ph.D., Lance Williams Ph.D., Anil Somenahally Ph.D., Ali Azghani Ph.D.
Master of Science: Biology
King, Nevada, "MICROBIAL MERCURY METHYLATION AT CADDO LAKE: A MOLECULAR ECOLOGY APPROACH" (2019). Biology Theses. Paper 60.