In subterranean estuaries (STE), fresh and saline groundwater introduce dissolved organic matter (DOM) of different origin. This DOM serves as substrate for microorganisms that thrive in the STE. In high-energy beaches with dynamic porewater advection, microbial communities face frequent changes in groundwater composition, even at several meters depth. It is unknown how DOM from deep STE groundwater (> 5 m depth) is transformed by prevailing microbial communities. To address this question, we performed sediment incubations in flow-through reactors (FTRs) with deep (6 m depth) STE groundwater of low (1.6) and high salinity (29.1). FTR setups were sampled daily for quantification of dissolved organic carbon (DOC), and at start and end (day 13) of the incubation for analysis of DOM composition, microbial cell numbers and community composition. Solid-phase extracted DOM was molecularly characterized via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry. Both groundwater types contained mainly reworked DOM. Corroborating its presumed origin, the fresh groundwater had a more terrestrial DOM signature with a higher proportion of aromatic compounds compared to the saline groundwater. Over the course of the incubation, DOC concentrations increased primarily due to leaching of sedimentary organic matter, providing an additional source of DOM. In all setups, the DOM composition changed significantly from start to end, and similarly for fresh and saline groundwater. From the ~2700 molecular formulae (MF) detected on day 0, 34-35% were removed during the incubations, demonstrating the potential of deep STE microbial communities to degrade recalcitrant DOM that is supplied with the advective porewater flow. However, a substantial portion of MF (63-64%) was retained in both groundwater types, indicating that a fraction of deep STE-DOM is resistant to removal. Properties of MF that were newly detected on day 13 (26-28%) were indicative of labile DOM. Some of these newly detected MF were also identified in sediment-leachates, suggesting that beach sediments are a source of fresh DOM for the STE microbial communities. It is likely that due to longer groundwater residence time in the STE, continuous leaching and microbial processing shift the molecular composition of released DOM from more labile to more recalcitrant DOM.

中文翻译：

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不同的溶解有机物来源维持着沙滩地下河口的微生物生命——一项孵化研究


在地下河口 （STE），淡水和含盐地下水会引入不同来源的溶解有机物 （DOM）。该 DOM 作为在 STE 中茁壮成长的微生物的底物。在具有动态孔隙水平流的高能海滩中，微生物群落面临着地下水成分的频繁变化，即使在几米深的地方也是如此。目前尚不清楚来自深部 STE 地下水（x3E 5 m 深）的 DOM 是如何被普遍的微生物群落改变的。为了解决这个问题，我们在流通式反应器 （FTR） 中进行了沉积物孵育，该反应器具有深 （6 m 深） 低盐度 （1.6） 和高盐度 （29.1） 的 STE 地下水。每天对 FTR 设置进行采样以定量溶解有机碳 （DOC），并在孵育开始和结束（第 13 天）采样以分析 DOM 组成、微生物细胞数量和群落组成。通过超高分辨率傅里叶变换离子回旋共振质谱对固相提取的 DOM 进行分子表征。两种地下水类型都主要包含再加工的 DOM。证实其推测的起源，与含盐地下水相比，新鲜地下水具有更陆地的 DOM 特征，芳香族化合物的比例更高。在孵育过程中，DOC 浓度增加主要是由于沉积有机物的浸出，提供了 DOM 的额外来源。在所有设置中，DOM 成分从头到尾都发生了显著变化，淡水和咸水地下水也是如此。从第 0 天检测到的 ~2700 个分子式 （MF） 中，34-35% 在孵育过程中被去除，证明了深层 STE 微生物群落降解由平流孔隙水流供应的顽固 DOM 的潜力。 然而，在两种地下水中都保留了很大一部分 MF （63-64%），这表明一小部分深 STE-DOM 难以去除。第 13 天 （26-28%） 新检测到的 MF 特性表明 DOM 不稳定。其中一些新检测到的 MF 也在沉积物渗滤液中鉴定出来，这表明海滩沉积物是 STE 微生物群落新鲜 DOM 的来源。很可能是由于地下水在 STE 中的停留时间较长，连续浸出和微生物加工使释放的 DOM 的分子组成从更不稳定转变为更顽固的 DOM。