Freshwater sediments in Sea Level Rise Microcosm

As sea level rise impacts coastal wetlands, saltmarsh will overtake freshwater marsh in many areas, but changes in the sediment microbiome in response to saltwater intrusion are difficult to predict. Freshwater marsh sediment was exposed to ambient, brackish, and saline conditions as well as to elevated nitrate and phosphate to model the combined stresses of saltwater intrusion and coastal eutrophication. Initially, sediment bacterial and archaeal composition was similar to prior studies of freshwater marsh but diverged over time proportionally to the increase in salinity. There was no observed effect of nutrient amendment. Although the previously described loss of methanogenic populations and promotion of sulfate reducers in response to saltwater exposure was observed, taxonomic distribution was not similar to typical meso-polyhaline wetlands, representing instead a transitional microbial community. Without colonization by marine taxa, such a community may be short-lived naturally, ultimately equilibrating with more common saltmarsh species. However, such a transitional community may be informative for freshwater marsh not adapted to salinity pulses as marsh migration progresses.

海平面上升对滨海湿地造成影响的背景下，盐沼（saltmarsh）将在诸多区域取代淡水沼泽（freshwater marsh），但沉积物微生物组（sediment microbiome）对海水入侵（saltwater intrusion）的响应变化却难以预测。本研究将淡水沼泽沉积物置于环境（ambient）、半咸水（brackish）与盐水（saline）条件下，同时添加高浓度硝酸盐（nitrate）与磷酸盐（phosphate），以模拟海水入侵与海岸富营养化（coastal eutrophication）的复合胁迫。实验初始阶段，沉积物中的细菌与古菌群落组成与此前淡水沼泽的相关研究结果相似，但随着时间推移，其组成随盐度升高呈比例性分化。未观测到营养物添加（nutrient amendment）带来的显著影响。尽管观测到了此前报道的、响应海水暴露的产甲烷菌种群减少与硫酸盐还原菌（sulfate reducers）增殖现象，但该群落的分类学分布与典型中-多盐度湿地并不一致，而是呈现为过渡型微生物群落。若缺乏海洋类群（marine taxa）的定殖，这类群落自然存续时间可能较短，最终将与更常见的盐沼物种达成动态平衡。然而随着沼泽迁移推进，这类过渡型群落可为尚未适应盐度脉冲（salinity pulses）的淡水沼泽提供参考价值。