10.16
微生物与氮循环
氮循环是一个复杂的生物地球化学过程，对于维持生态系统中含氮化合物的平衡至关重要。该循环包含多个由微生物介导的转化过程，其中氮元素发生氧化态的变化，支持重要的生态功能，并促进植物和微生物的生长。
固氮作用与氨化作用
固氮作用通过将惰性的大气氮气（N₂）转化为生物可利用的氨（NH₃）来启动氮循环，该过程由固…
氮通过固氮、氨化、硝化、反硝化和厌氧氨氧化等过程在不同化学形态和氧化态之间转化。
氮循环始于氮气，而大多数生物无法直接利用氮气。
像 Azotobacter 这样的自由生活的细菌通过固氮作用将氮气转化为氨，并进入土壤。
共生细菌（如 Rhizobium）在豆科植物的根瘤中将氮固定为氨。
在氨化过程中，分解者分解来自死亡生物体和废物的氨基酸和核苷酸，释放出铵离子形式的氮。
在好氧区域，硝化作用分为两个步骤进行。首先，氨氧化菌（如Nitrosomonas）将氨氧化为亚硝酸盐；然后，Nitrobacter将亚硝酸盐转化为硝酸盐。
在低氧区域，假单胞菌属（Pseudomonas）通过反硝化作用将硝酸盐转化为氮气。
而在缺氧区域，Brocadiaceae 科细菌通过厌氧氨氧化过程将氨和亚硝酸盐结合生成氮气。
这种气体随后返回大气中。
View the full transcript and gain access to JoVE Core videos
Q1: How do bacteria convert atmospheric nitrogen into a form plants can use?
Nitrogen fixation converts inert atmospheric nitrogen gas into bioavailable ammonia through nitrogenase enzymes. Free-living bacteria like Azotobacter perform this independently, while symbiotic bacteria such as Rhizobium form associations with legume roots, fixing nitrogen directly in root nodules where plants can access it for growth.
Q2: What happens to nitrogen when organisms die and decompose?
During ammonification, decomposer microbes break down amino acids and nucleotides from dead organisms and waste, releasing nitrogen as ammonium ions. This process recycles nitrogen back into a form suitable for plant uptake or further microbial processing in soil environments. Ammonification is essential for returning organic nitrogen to bioavailable forms.
Q3: How is ammonia converted to nitrate in aerobic soil?
Nitrification occurs in two steps under aerobic conditions. Nitrosomonas bacteria oxidize ammonia to nitrite, then Nitrobacter converts nitrite to nitrate. Recently discovered complete ammonia oxidizers like Nitrospira can perform both steps autonomously, enhancing nitrogen turnover in various environments and simplifying the nitrification pathway.
Q4: Why is denitrification important for closing the nitrogen cycle?
Denitrification reduces excess nitrate back to nitrogen gas under low-oxygen conditions, preventing harmful accumulation in ecosystems. Facultative anaerobes like Pseudomonas facilitate this reduction, returning nitrogen to the atmosphere and completing the cycle's loop. This process is critical for maintaining ecosystem nitrogen balance and preventing eutrophication.
Q5: What is anammox and where does it occur?
Anaerobic ammonium oxidation (anammox) by Brocadiaceae bacteria couples ammonia and nitrite to directly produce nitrogen gas in strictly anoxic environments. This energetically efficient process is vital for nitrogen removal from aquatic ecosystems and complements denitrification in oxygen-depleted zones, contributing significantly to biogeochemical cycling.
Q6: How do different microbial processes work together in the nitrogen cycle?
The nitrogen cycle integrates fixation, ammonification, nitrification, denitrification, and anammox, with each process mediated by specific bacteria under distinct oxygen conditions. Together, these transformations cycle nitrogen through different oxidation states, supporting ecosystem balance, plant growth, and nutrient availability. Understanding these interconnected processes is fundamental to introduction to microbial ecology.
Q7: What role do free-living versus symbiotic bacteria play in nitrogen fixation?
Free-living bacteria like Azotobacter and Clostridium independently fix nitrogen under aerobic and anaerobic conditions respectively. Symbiotic bacteria like Rhizobium form mutualistic associations with legumes, localizing nitrogen fixation in root nodules for direct plant benefit. Both strategies are essential for maintaining bioavailable nitrogen in terrestrial and aquatic ecosystems.