Assessing the influence of genotypic diversity on sulfur dynamics in the seagrass halodule wrightii using stable isotope analysis

Coastal development and other mounting anthropogenic pressures are threatening valuable seagrass habitats. The greatest risks posed to seagrasses are the effects of coastal eutrophication, which stimulates primary productivity and ultimately supplies abundant organic matter to marine sediments. The decomposition of this material is initially facilitated by aerobic microorganisms, depleting dissolved oxygen and generating anoxic conditions. Under these conditions, anaerobic microorganisms such as sulfate-reducing bacteria begin to dominate the degradation process, which reduce sulfate (SO42-) to sulfide (H2S) for energy production. The accumulation of H2S in marine sediments is problematic for seagrasses, as this molecule can be highly toxic. Yet, seagrasses can withstand relatively high concentrations of H2S in their environments. Stable isotope analyses have been used to investigate sulfide intrusion in seagrass meadows, as the unique isotopic signature of sediment-derived sulfur can be used to trace the uptake of H2S and its distribution throughout the plant. This technique has allowed the study of factors that may influence sulfide production and intrusion, such as reduced light availability, organic matter enrichment, and high temperatures. However, few studies have examined the biological or biochemical features that enable seagrasses to withstand relatively high sedimentary sulfide levels. One biological feature that may help confer resistance is population genetic diversity, which has been identified as an important trait in the survival and performance of seagrass meadows under environmental stress. In general, genetic diversity is thought to play an important role in population resistance to environmental disturbance, as a wider assortment of functional traits encoded at the molecular level results in a variety of phenotypes likely to possess morphological and physiological differences that are complementary. This genotypic complementarity may extend to biochemical strategies associated with tolerance to, or detoxification of, H­2S. The purpose of this study was to determine whether sulfide intrusion differs between genotypes of the seagrass Halodule wrightii, a prominent species in the Gulf of Mexico. Further, as the sulfur isotopic composition of marine sediments and seagrass vegetation is known to exhibit high spatial variability, this study also sought to assess sulfide intrusion between populations from distinct sites along the Texas Gulf Coast. Stable isotope data was used to infer the proportion of sulfur in H. wrightii tissues that was derived from sedimentary sulfide, while total sulfur (TS) data was also considered to understand the extent of sulfur accumulation within the plant. H. wrightii genotypes were determined by screening each sample at a series of microsatellite loci previously identified for this species. Although no difference in sulfide intrusion was observed between genotypes, sulfide uptake and distribution was significantly different between the three study sites. The results offer important insight to the effect of local conditions on sulfide intrusion in seagrass meadows and may guide future investigations concerned with the influence of genotypic diversity on H2S metabolism in seagrasses.