Toxicity of sulfide to early life stages of wild rice (Zizania palustris)

Abstract

The sensitivity of wild rice (Zizania palustris) to sulfide is not well understood. Because sulfate in surface waters is reduced to sulfide by anaerobic bacteria in sediments and historical information indicated that 10 mg/L sulfate in Minnesota (USA) surface water reduced Z. palustris abundance, the Minnesota Pollution Control Agency established 10 mg/L sulfate as a water quality criterion in 1973. A 21-d daily-renewal hydroponic study was conducted to evaluate sulfide toxicity to wild rice and the potential mitigation of sulfide toxicity by iron (Fe). The hydroponic design used hypoxic test media for seed and root exposure and aerobic headspace for the vegetative portion of the plant. Test concentrations were 0.3, 1.6, 3.1, 7.8, and 12.5 mg/L sulfide in test media with 0.8, 2.8, and 10.8 mg/L total Fe used to evaluate the impact of iron on sulfide toxicity. Visual assessments (i.e., no plants harvested) of seed activation, mesocotyl emergence, seedling survival, and phytoxicity were conducted 10 d after dark-phase exposure. Each treatment was also evaluated for time to 30% emergence (ET30), total plant biomass, root and shoot lengths, and signs of phytotoxicity at study conclusion (21 d). The results indicate that exposure of developing wild rice to sulfide at ≥3.1 mg sulfide/L in the presence of 0.8 mg/L Fe reduced mesocotyl emergence. Sulfide toxicity was mitigated by the addition of Fe at 2.8 mg/L and 10.8 mg/L relative to the control value of 0.8 mg Fe/L, demonstrating the importance of iron in mitigating sulfide toxicity to wild rice. Ultimately, determination of site-specific sulfate criteria taking into account factors that alter toxicity, including sediment Fe and organic carbon, are necessary. Environ Toxicol Chem 2017;36:2217-2226. © 2017 SETAC.

Cumulative Sulfate Loads Shift Porewater to Sulfidic Conditions in Freshwater Wetland Sediment

“…These results are consistent with the range of sulfide concentrations at which wild rice seedling growth is stunted in hydroponic experiments with wild rice (Pastor et al 2017) and in the range of those measured for other sensitive freshwater wetland plants (Lamers et al 2013). These concentrations are well below those reported to be toxic by Fort et al (2017) in hydroponic experiments. The seedlings in the Fort et al (2017) experiment (maximum age 21 d) were allowed to emerge into ambient air above the water, whereas the seedlings in the Pastor et al (2017) experiment (maximum age 17 d) were maintained under water.…”

“…The seedlings in the Fort et al () experiment (maximum age 21 d) were allowed to emerge into ambient air above the water, whereas the seedlings in the Pastor et al () experiment (maximum age 17 d) were maintained under water. The greater tolerance of wild rice to sulfide observed by Fort et al () may be the result of the seedlings’ access to atmospheric oxygen, which would allow the internal detoxification of absorbed sulfide by S dioxygenase (SDO; Krüssel et al ). The activity of SDO is limited by oxygen availability.…”

“…The activity of SDO is limited by oxygen availability. In nature, 21-d-old wild rice seedlings have typically not emerged into the atmosphere (Fort et al 2017) but are not uniformly subject to sulfide exposure of all tissue (Pastor et al 2017).…”

Impact of Hydroponic Oxygen Control in Sulfide Toxicity to Early Life Stages of Wild Rice (Zizania palustris)

“…Seeds were stored n the dark at 4 °C. The percentage of emergence at day 21 in preliminary studies was 47.5%, which is typical of Z. palustris germination (Fort et al 2014, 2017).…”

“…Previously, 21‐d hydroponic studies were conducted to determine the toxicity of sulfate (Fort et al 2014) and sulfide (Fort et al 2017) to wild rice seeds and seedlings; these studies suggested that neither toxicant impacted emergence and seedling development of wild rice at concentrations <5000 mg/L and 3.1 mg/L, respectively. However, these studies were performed with atmospheric oxygen levels in the headspace, and thus oxygen could potentially play a role in sulfide detoxification through oxygen transport from the emerged portions of the plant into the rooting zone.…”