Edwards Thea M, McCoy Krista A, Barbeau Tamatha, McCoy Michael W, Thro John Matthew, Guillette Louis J
Department of Zoology, University of Florida, Gainesville, FL 32611, USA.
Aquat Toxicol. 2006 Jun 10;78(1):50-8. doi: 10.1016/j.aquatox.2006.02.003. Epub 2006 Mar 6.
Aquatic nitrate contamination has escalated over the past 50 years, primarily due to intensified fertilizer application and sewage production worldwide. Nitrate's role in the decline of amphibian populations remains unclear, although studies suggest that nitrate exposure affects larval development. We exposed Bufo terrestris tadpoles to environmentally relevant nitrate concentrations from Gosner stage 25 through forelimb emergence. Tadpoles were exposed to fluctuating (0-30 mg/L NO(3)-N, alternated three times per week) or constant nitrate concentrations (0, 5, 15, or 30 mg/L NO(3)-N), and effects were compared in two water types: natural spring water and reverse-osmosis filtered water, fortified with electrolytes (RO(e)). We measured growth and developmental rates, survival, time to metamorphosis, metamorph body size, hepatosomatic index, and whole-body thyroxine (T(4)) concentrations at forelimb emergence. Based on our observations, we reached three main conclusions: (1) in constant nitrate, tadpoles in RO(e) water grew faster, and were generally larger with higher or similar T(4) at metamorphosis than tadpoles raised in spring water, irrespective of nitrate concentration, (2) in fluctuating nitrate (flux), there were no differences in time to or size at metamorphosis in either water type relative to controls; however, mean T(4) concentrations in the flux treatment showed a reversed pattern in the two water types (SP>RO(e)) compared to the general pattern observed with constant nitrate exposure (RO(e)>SP), and (3) the effect of nitrate on growth and development depended on water type. In RO(e) water with high nitrate (RO(e)-30 mg/L NO(3)-N), tadpoles metamorphosed an average of 5 days (13%) earlier than control animals, but were similar in size to controls. However, spring water tadpoles reared in high nitrate (30 mg/L) delayed metamorphosis by 7 days (18%) compared to animals reared in SP-0, and 11 days (32%) compared to tadpoles raised in RO(e)-30. This delayed development allowed SP-30 tadpoles to reach a larger size that was more similar to metamorphs raised in RO(e) water. Based on information from other studies, we conclude that, in RO(e) water, tadpoles exhibited an expected stress response to nitrate (e.g. metamorphosed earlier as nitrate concentration increased). However, we suggest that, in spring water, tadpoles were exposed to additional stressors that decreased growth rate and thyroxine concentrations, and that this effect was modified by nitrate.
在过去50年里，水体硝酸盐污染不断加剧，这主要是由于全球范围内化肥施用量增加以及污水排放量增多。硝酸盐在两栖动物种群数量减少中所起的作用仍不明确，不过研究表明，接触硝酸盐会影响幼体发育。我们将北美蟾蜍蝌蚪从戈斯纳25期饲养至前肢长出，使其暴露于环境相关的硝酸盐浓度下。蝌蚪被暴露于波动的（0 - 30毫克/升硝酸根氮，每周交替三次）或恒定的硝酸盐浓度（0、5、15或30毫克/升硝酸根氮）中，并在两种水体类型中比较其影响：天然泉水和添加电解质的反渗透过滤水（RO(e)）。我们测量了生长和发育速率、存活率、变态时间、变态时的身体大小、肝体指数以及前肢长出时的全身甲状腺素（T(4)）浓度。基于我们的观察，我们得出三个主要结论：（1）在恒定硝酸盐环境中，无论硝酸盐浓度如何，RO(e)水中的蝌蚪生长更快，变态时总体上更大，且T(4)浓度更高或与在泉水中饲养的蝌蚪相似；（2）在波动硝酸盐（通量）环境中，与对照组相比，两种水体类型中蝌蚪变态的时间或大小均无差异；然而，通量处理中的平均T(4)浓度在两种水体类型中呈现出与恒定硝酸盐暴露时观察到的一般模式相反的模式（SP>RO(e)），而恒定硝酸盐暴露时的一般模式为（RO(e)>SP）；（3）硝酸盐对生长和发育的影响取决于水体类型。在高硝酸盐（RO(e)-30毫克/升硝酸根氮）的RO(e)水中，蝌蚪变态时间比对照动物平均提前5天（13%），但大小与对照相似。然而，在高硝酸盐（30毫克/升）泉水中饲养的蝌蚪与在SP-0中饲养的动物相比，变态延迟了7天（18%），与在RO(e)-30中饲养的蝌蚪相比延迟了11天（32%）。这种发育延迟使SP-30蝌蚪能够达到更大的尺寸，更类似于在RO(e)水中变态的蝌蚪。根据其他研究的信息，我们得出结论，在RO(e)水中，蝌蚪对硝酸盐表现出预期的应激反应（例如随着硝酸盐浓度增加变态提前）。然而，我们认为，在泉水中，蝌蚪还受到了其他降低生长速率和甲状腺素浓度的应激源影响，并且这种影响因硝酸盐而有所改变。
