ASTROBIOLOGY Volume 8, Number 4, 2008 © Mary Ann Liebert, Inc. DOI: 10.1089/ast.2007.0262 Hypothesis Paper Amphibian Nitrate Stress as an Additional Terrestrial Threat from Astrophysical Ionizing Radiation Events? Brian C. Thomas 1 and Michelle D. Honeyman 2 Abstract Various astrophysical events have been suggested as sources of ionizing radiation that, by way of destruction of the ozone layer and the subsequent increase in UVB and deposition of nitrate, could pose a threat to life on Earth. We have investigated whether the nitrate deposition that follows an ionizing event is sufficient to cause an additional stress beyond that of the heightened UVB previously considered. Our results show that, subse- quent to the most intense ionization event likely to have occurred in the last billion years, the increase in ni- trate concentration in bodies of water would not be sufficient to cause serious additional stress on amphibian populations and may actually provide some benefit by acting as fertilizer. Key Words: Radiation—Ozone—UV radiation. Astrobiology 8, 731–733. 731 1. Introduction I N PREVIOUS WORK (Melott et al., 2005; Thomas et al., 2005, 2007, 2008; Thomas and Melott, 2006; Ejzak et al., 2007), we evaluated the biological impact of astrophysical ionizing ra- diation events, including gamma-ray bursts, supernovae, and solar flares. Hatch and Blaustein (2000, 2003) and Boone et al. (2007) considered effects of heightened solar UVB irra- diation following depletion of stratospheric ozone. Studies of amphibians have shown that the combination of UVB ir- radiation, increased nitrate concentrations, and decreased pH has a greater effect than the individual stressors or even a simple sum of the effects of each separately. In this study, we considered nitrate deposition as an additional stressor to amphibian populations already impacted by heightened UVB levels after an astrophysical ionizing radiation event. We sought to determine whether increased nitrate concen- trations subsequent to an event would be a significant addi- tional stressor to amphibian populations. Atmospheric ionization caused by photons or charged particles leads to the formation of nitrogen oxides, which cat- alytically deplete ozone and are then removed from the atmosphere (over several months’ time) as HNO 3 . This leads to the deposition of NO 3 - (Thomas et al., 2005). Any event that yields significant ionization of the atmosphere will cause both ozone depletion and deposition of NO 3 - . After a large ionization event, ozone depletion lasts for several years, and nitrate deposition is high for a few years. These timescales can be compared to frog breeding fre- quency, which is on the order of once per year, or even to frog generation times, which are typically about 4 years. This indicates that populations are likely to be impacted by events such as gamma-ray bursts and supernovae. 2. Results We considered the maximum possible impact, given a rea- sonably likely event, and therefore used the largest nitrate de- position from a suite of simulation runs reported in Thomas et al. (2005) for a gamma-ray burst delivering a fluence of 100 kJ m -2 . The burst was short (10 s), but rainout of HNO 3 occurred over a few years’ time as the atmosphere recovered. Rainout of HNO 3 is computed in the model as a flux, and the maxi- mum value from the suite noted above is 3  10 -9 g m -2 s -1 . This value corresponds to a burst occurring over the North Pole in September (the autumnal equinox). Maximum deposi- tion occurred over the course of a few months, between 18 and 24 months after the burst, at latitudes between 30 and 50 de- grees north. To arrive at a total deposition, this rate was mul- tiplied by 3.15  10 7 s to convert to a yearly value, which yielded 0.09 g m -2 . Multiplying by the area of deposition yields 6.12  10 12 g, which is about 10 11 moles of NO 3 - . 1 Department of Physics and Astronomy and 2 Department of Biology, Washburn University, Topeka, Kansas.