篩選具降解亞硝酸−氮之菌株，並評估其在水產養殖應用之可行性

水產養殖業為台灣重要的糧食產業之一，但礙於土地資源上的劣勢，導致台灣業者須採取集約式養殖以提高經濟效益。在高密度養殖環境下，水中的含氮廢物會因生物體的各種代謝作用而產生，且於環境中大量累積，對生物體的生長及發育造成負面影響，甚至對其生存造成威脅。水產益生菌的發展使現今的養殖業者能透過引進特定的微生物至養殖池內，以達到改善養殖環境之目的，但市面上對於降解亞硝酸−氮的微生物製劑於選擇上較為貧乏，且在使用上效果也不如預期。因此本實驗於鹹水及淡水的養殖環境中共篩選出兩株能夠有效降解亞硝酸−氮的菌株，經微生物鑑定報告顯示，鹹水菌株SW4’-W6及淡水菌株FW4’-T3於分類上皆屬於酵母菌，學名分別為Candida palmioleophila和Pseudozyma churashimaensis，兩菌株的最適生長溫度介於25 °C至35 °C之間，且於鹽度0 ‰ ~ 130 ‰下皆可觀察到生長現象。生物安全性評估顯示兩菌株對水產生物不具有致病性。另外也於不同環境條件下監測菌株對於亞硝酸−氮及氨−氮的降解趨勢及亞硝酸還原酶基因表現之變化。最終結果顯示兩菌株對於碳源的利用較偏好於蔗糖而非葡萄糖。當以亞硝酸−氮作為氮源時最低碳氮比需求應為15，且以蔗糖作為碳源的組別其亞硝酸還原酶基因表現量明顯高於葡萄糖的組別；而當以氨−氮作為氮源時，菌株SW4’-W6的最低碳氮比需求仍為15，但菌株FW4’-T3可下降至10。兩菌株於低鹽度及溫度為30 °C的環境下對亞硝酸−氮及氨−氮皆有較佳的降解效率，且在氨−氮和亞硝酸−氮同時存在的情況下，菌株會將氨−氮作為首選氮源優先利用。

關鍵字

亞硝酸−氮；氨−氮；亞硝酸還原酶；碳氮比；碳源；氮源

並列摘要

Aquaculture is one of Taiwan's most important food industries, but due to the disadvantages of land resources, Taiwanese businesses must adopt intensive culturing to improve economic efficiency. In a high-density culturing environment, nitrogen-containing wastes in the water are produced by various metabolic activities of the organisms and accumulate in the environment, which negatively impacts the growth and development of organisms and even threatens their survival. The development of aquatic probiotics has enabled the current aquaculture trader to introduce specific microorganisms into the culturing ponds to improve the quality of the environment. However, the selection of microbial agents that degrade NO2--N is relatively poor, and the effectiveness is not as expected. Therefore, in this experiment, two strains of bacteria that can effectively degrade NO2--N were isolated from the saltwater and freshwater aquaculture environments. The microbiological identification report showed that the saltwater strain SW4'-W6 and the freshwater strain FW4'-T3 were classified as yeast, namely, Candida palmioleophila and Pseudozyma churashimaensis. The optimal growth temperature of the two strains is between 25°C and 35°C, and the growth phenomenon was observed between salinity 0 ‰ to 130 ‰. The biosafety assessment showed that two strains were not pathogenic to aquatic animals. In addition, the degradation tendency of NO2--N and NH4+-N by strains and the changes of nitrite reductase gene expression were also monitored under different environmental conditions. The final results show that two strains preferred to use sucrose as a carbon source instead of glucose. When NO2--N was used as the nitrogen source, the minimum C/N ratio requirement was 15, and the nitrite reductase gene expression level of the group with sucrose was significantly higher than the group with glucose. Similarly, when NH4+-N was used as the nitrogen source, the minimum C/N ratio requirement of strain SW4'-W6 was 15, but strain FW4'-T3 was decreased to 10. Both strains have better degradation efficiencies for NO2--N and NH4+-N in a low-salinity environment at a temperature of 30°C, and preferential use NH4+-N as the prioritized nitrogen source when NO2--N and NH4+-N appear at the same time.