河口盐度梯度下溶解态核酸的微生物可利用性

核酸物质(DNA和RNA)作为自然水体中重要的溶解有机质组分之一,是水生微生物食物网的重要物质基础,因此核酸的微生物可利用性具有重要研究意义.2012年春季从长江口不同盐度区域采集水样,并分别添加鱼类DNA和酵母RNA进行培养,考察了其微生物可利用性.结果表明,在溶解态核酸添加至不同盐度的培养体系时,添加的DNA大约有20%～50%从溶解态快速转变至颗粒态,转化率随盐度增加而增加,而添加的RNA仅有约10%从溶解态转变至颗粒态,受盐度影响不大.在各培养体系中,溶解态核酸的微生物利用动力学曲线均符合Sigmoid方程,先有30～80 h不等的延滞期,而后进入快速利用期,最后为停止期,其中海水微生物对核酸的最大利用速率比河口/淡水微生物更快.当溶解态核酸进入河口水体后,分为微生物可利用的(自由溶解态/酶可水解的)、胶体结合态和颗粒态,其中RNA中微生物可利用形态百分比(80%～90%)显著高于DNA,且随盐度变化不大,而DNA中微生物可利用形态百分比随盐度升高而从78%逐渐降低至50%.因此,河口盐度梯度下的DNA和RNA赋存形态分布特征和微生物可利用性具有显著差异.

Microbial Bioavailability of Dissolved Nucleic Acids Across the Estuarine Salinity Gradient

As an important component of dissolved organic matter (DOM), nucleic acids (DNA and RNA) are essential nutrient and energy sources in aquatic microbial food web. Therefore, it is important to understand the bioavailability of nucleic acids. The bioavailability of nucleic acids was investigated by a batch of incubation experiments, adding fish DNA and yeast RNA into water samples with different salinity collected from the Yangtze River estuary in the spring of 2012. According to the results, 20%-50% of dissolved DNA was transformed into particulate DNA quickly with the conversion rates increasing with salinity, only 10% dissolved RNA was transformed into particulate RNA and the salinity had no effect on the conversation rates. In each incubation experiment, the microbial utilization kinetic curves of dissolved nucleic acids were fitted to the Sigmoid model. There were lag periods of 30-80 hours followed by the rapid utilization phase and then the stagnation phase. The results also showed that the bacteria in seawater had higher maximum utilization rate than the bacteria in estuarine and fresh water. Dissolved nucleic acids spiked in estuarine water can be bound to colloids and particles at some extent, only those free dissolved or enzymatically hydrolysable forms are bioavailable. The percentage of bioavailable RNA (80%-90%) was significantly higher than that of bioavailable DNA and it did not change significantly with salinity while the percent of bioavailable DNA decreased from 78% to 50% with salinity. Therefore, the speciation and bioavailability are significantly different between DNA and RNA across the estuarine salinity gradient.