Abstract：
East African lakes, especially soda lakes, are home habitats for massive numbers of wildlife such as flamingos, mammals, and fishes. These lakes are known for their high primary production due to local high temperatures, light intensities, and alkalinity (inorganic carbon). However, these lakes, normally within remote areas, receive low nutrient inputs. Ammonium (NH4+) recycling and/or nitrogen fixation can become the major N supply mechanisms for phytoplankton. However, the driving forces on microbial N nutrition in lakes with minimal anthropogenic disturbance remain poorly understood. Using stable isotope tracer techniques, NH4+ recycling rates were measured in 18 lakes and reservoirs in East Africa (Tanzania and Kenya) during the dry season in early 2020. Three functional genes (nifH, gdh, and ureC) relating to microbial N nutrition were also measured. The regeneration of NH4+ supported up to 71 % of the NH4+ uptake. Positive community biological NH4+ demands (CBAD) for all lakes and reservoirs indicate an obvious N demand from microbial community. Our study provides clear evidence that microbial NH4+ uptake rates linked closely to the dissolved organic matter (DOM) properties (e.g., the absorption coefficient at 254 nm, percents of total fluorescence intensity contributed by microbial humic-like and protein-like components) and that water residence time drives microbial NH4+ recycling by regulating the duration of in-lake DOM processing and influencing algal growth. Phytoplankton, especially those of Cyanophyceae, showed maximum biomass and higher NH4+ recycling rates at a certain range of water residence time (e.g., 5-8 years). However, CBAD showed a decreasing trend with longer water residence time, which may be influenced by changes in the algal community composition (e.g., % Cyanophyceae vs. % Bacillariophyceae). These results indicate that DOM dynamics and the water residence time have the potential to facilitate the understanding of microbial nitrogen supply status in East African lakes.
摘要：
东非湖泊,尤其是苏打水湖,是大量野生动物如火烈鸟的栖息地，哺乳动物,和鱼。由于当地高温，这些湖泊以其高初级产量而闻名，光强度,和碱度（无机碳）。然而,这些湖泊,通常在偏远地区，接受低营养输入。铵（NH4）再循环和/或固氮可以成为浮游植物的主要氮供应机制。然而,人为干扰最小的湖泊中微生物氮营养的驱动力仍然知之甚少。使用稳定同位素示踪技术，在2020年初的旱季期间，在东非（坦桑尼亚和肯尼亚）的18个湖泊和水库中测量了NH4回收率。三个功能基因(nifH，gdh,和UreC）也测量了与微生物氮营养有关的信息。NH4+再生支持高达71%的NH4+吸收。所有湖泊和水库的群落生物NH4需求（CBAD）正表明微生物群落对氮的需求明显。我们的研究提供了明确的证据，表明微生物NH4+吸收速率与溶解有机物(DOM)特性密切相关(例如，254nm处的吸收系数，微生物腐殖质样和蛋白质样成分贡献的总荧光强度百分比），并且水的停留时间通过调节湖内DOM处理的持续时间和影响藻类的生长来驱动微生物NH4再循环。浮游植物,尤其是蓝藻科，在一定的水停留时间范围内显示出最大的生物量和更高的NH4+回收率(例如，5-8年）。然而,随着水停留时间的延长，CBAD呈下降趋势，这可能会受到藻类群落组成变化的影响(例如，%蓝藻与%芽孢杆菌科)。这些结果表明，水的停留时间和DOM动力学有可能促进对东非湖泊中微生物氮供应机制的理解。
