Sulphur (S) is a key macronutrient for all organisms, with similar cellular requirements to that of phosphorus (P). Studies of S cycling have often focused on the inorganic fraction, however, there is strong evidence to suggest that freshwater microorganisms may also access dissolved organic S (DOS) compounds (e.g. S-containing amino acids). The aim of this study was to compare the relative concentration and depletion rates of organic 35 S-labelled amino acids (cysteine, methionine) with inorganic S (Na 2 35 SO 4 ) in oligotrophic versus mesotrophic river waters draining from low nutrient input and moderate nutrient input land uses respectively. Our results showed that inorganic SO 4 2− was present in the water column at much higher concentrations than free amino acids. In contrast to SO 4 2− , however, cysteine and methionine were both rapidly depleted from the mesotrophic and oligotrophic waters with a halving time < 1 h. Only a small proportion of the DOS removed from solution was mineralized and excreted as SO 4 2− (< 16% of the total taken up) suggesting that the DOS could be satisfying a demand for carbon (C) and S. In conclusion, even though inorganic S was abundant in freshwater, it appears that the aquatic communities retained the capacity to take up and assimilate DOS.

中文翻译：

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淡水中溶解有机硫 (DOS) 的快速消耗

硫 (S) 是所有生物体的关键常量营养素，其细胞需求与磷 (P) 相似。S 循环的研究通常集中在无机部分，然而，有强有力的证据表明淡水微生物也可以接触溶解的有机 S (DOS) 化合物（例如含 S 的氨基酸）。本研究的目的是比较有机 35 S 标记的氨基酸（半胱氨酸、甲硫氨酸）与无机 S（Na 2 35 SO 4 ）在从低营养输入和中等营养流失的贫营养和中营养河水中的相对浓度和消耗率。养分投入土地利用。我们的结果表明，无机 SO 4 2- 以比游离氨基酸高得多的浓度存在于水体中。然而，与 SO 4 2- 相比，半胱氨酸和甲硫氨酸都从中营养和贫营养水中迅速耗尽，减半时间 < 1 小时。从溶液中去除的 DOS 中只有一小部分被矿化并作为 SO 4 2- 排出（< 总吸收量的 16%），这表明 DOS 可以满足对碳 (C) 和 S 的需求。 总之，即使尽管淡水中无机硫含量丰富，但水生群落似乎保留了吸收和同化 DOS 的能力。