What Limits Denitrification and Bacterial Growth in Lake Bonney, Taylor Valley, Antarctica?

Denitrification is the main process by which fixed nitrogen is lost from ecosystems and the regulation of this process may directly affect primary production and carbon cycling over short and long time scales. Previous investigations of the role of bioactive metals in regulating denitrification in bacteria from permanently ice-covered Lake Bonney in the Taylor Valley of East Antarctica indicated that denitrifying bacteria can be negatively affected by metals such as copper, iron, cadmium, lead, chromium, nickel, silver and zinc; and that there is a distinct difference in denitrifying activity between the east and west lobes of the lake.

Low iron concentrations were found to exacerbate the potential toxicity of the other metals, while silver has the potential to specifically inhibit denitrification because of its ability to interfere with copper binding in redox proteins, such as nitrite reductase and nitrous oxide reductase. High silver concentrations might prevent the functioning of nitrous oxide reductase in the same way that simple copper limitation does, thereby causing the buildup of nitrous oxide and resulting in a nonfunctional nitrogen cycle. Other factors, such as oxygen concentration, are likely also to affect bacterial activity in Lake Bonney.

This project will investigate silver toxicity, general metal toxicity and oxygen concentration to determine their effect on denitrification in the lake by using a suite of 'sentinel' strains of denitrifying bacteria (isolated from the lake) incubated in Lake Bonney water and subjected to various treatments. The physiological responses of these strains to changes in metal and oxygen concentration will be quantified by flow cytometric detection of single cell molecular probes whose sensitivity and interpretation have been optimized for the sentinel strains. Understanding the relationships between metals and denitrification is expected to enhance our understanding of not only Lake Bonney's unusual nitrogen cycle, but more generally, of the potential role of metals in the regulation of microbial nitrogen transformations. The broader impacts of this work include not only a better understanding of regional biogeochemistry and global perspectives on these processes; but also the training of graduate students and a substantial outreach effort for school children.

反硝化作用（Denitrification）是生态系统中固定态氮流失的主要过程，该过程的调控可在短、长期尺度上直接影响初级生产与碳循环。此前针对东南极洲（East Antarctica）泰勒谷（Taylor Valley）内永久冰封的邦尼湖（Lake Bonney）水体细菌开展的研究，探究了生物活性金属（bioactive metals）对反硝化细菌（denitrifying bacteria）反硝化作用的调控机制，结果显示铜、铁、镉、铅、铬、镍、银、锌等金属会对反硝化细菌产生抑制性影响，且该湖东西湖盆的反硝化活性存在显著差异。

研究发现，低铁浓度会加剧其余金属的潜在毒性；而银可通过干扰亚硝酸还原酶（nitrite reductase）、一氧化二氮还原酶（nitrous oxide reductase）等氧化还原蛋白（redox proteins）中的铜结合位点，特异性抑制反硝化作用。高浓度银可通过类似铜限制的方式阻断一氧化二氮还原酶的功能，进而导致一氧化二氮（nitrous oxide）积累，使氮循环（nitrogen cycle）失去正常功能。此外，氧浓度等其他因素同样可能影响邦尼湖内的细菌活性。

本项目将采用从邦尼湖分离得到的反硝化细菌哨兵菌株（sentinel strains），将其置于邦尼湖水体中培养并施加不同处理条件，以探究银毒性、一般金属毒性与氧浓度对该湖反硝化作用的影响。研究将通过流式细胞术检测针对哨兵菌株优化了灵敏度与解析方法的单细胞分子探针（single cell molecular probes），定量分析这些菌株对金属与氧浓度变化的生理响应。阐明金属与反硝化作用之间的关联，不仅有助于深入理解邦尼湖独特的氮循环机制，更能从更广泛的视角认识金属在微生物氮转化调控中的潜在作用。本研究的社会与学术影响范畴不仅包括深化对区域生物地球化学（biogeochemistry）过程的理解，拓展相关研究的全球视野，还涵盖研究生（graduate students）培养与面向中小学生的大规模科普推广活动。