Methane production by microphytobenthos and its contribution to the benthic methane flux from the coastal zone of the Baltic Sea

温室气体甲烷(CH4)自然排放量的增加极大地影响了地球气候。在这方面，近岸水域发挥了关键作用，因为与开阔水域相比，这些地区的水柱CH4浓度要高得多。然而，关于沿海浅层地区的甲烷排放者及其对大气甲烷通量的贡献，人们知之甚少。此外，最近的一系列研究表明，微生物产生甲烷并不局限于严格的缺氧条件，可能在含氧水柱中广泛存在。这种氧化甲烷的产生将甲烷源置于接近水面的位置，从而加强了对大气的通量。根据最近的研究和初级生产力与甲烷形成之间的联系，我们假设微型底栖动物(MPB)群落作为许多沿海系统的主要初级生产者，在浅水甲烷动态中起着重要的作用。为了探索MPB相关的CH4产生，我们将通过培养实验和测定丰富的底栖硅藻物种的初级生产和CH4产生的速率来研究这种CH4来源的潜力。在封闭孵化实验中使用一种新的基于腔衰荡光谱的半连续方法，将能够在昼夜循环中以高时间分辨率灵敏地测量CH4的产生。实验将在受控的温度和光照条件下进行，并将使用选定的MPB硅藻的无菌/异种克隆培养，以确定主要的影响因素和具有最高甲烷产生率的物种。将使用13C标记的前体底物进行实验，以识别和表征与MPB相关的CH4产生途径。以前对氧化甲烷生产的研究大多是使用培养样本的机械性实验室研究；测量的甲烷生产速率是否具有自然系统中的代表性尚不确定。在我们的研究中，为了克服这些限制，克隆培养实验将伴随着从AsköIsland(瑞典波罗的海)浅水和Darss-Zingst Bodden链内沿海水域(德国波罗的海)采集的自然MPB群落，从而涵盖不同的沉积物类型、水动力条件和MPB群落。为了使MPB群落的底栖生物CH4通量与整个底栖生物和海-空气CH4通量相一致，我们将测定两个研究区域沉积物、水柱和大气之间的CH4通量。

Increasing natural emissions of the greenhouse gas methane (CH4) substantially impact the Earth climate. In this context, near-shore waters play a key role because water column CH4 concentrations in these areas are much higher when compared with the open waters. However, little is known about CH4 emitters in shallow coastal zones or their contribution to the atmospheric CH4 flux. Moreover, a series of recent studies indicate that microbial CH4 production is not limited to strictly anoxic conditions and might be widespread in the oxic water column. This oxic CH4 production places the CH4 source close to the water surface, thereby intensifying fluxes to the atmosphere. Based on these recent investigations and a proposed linkage between primary production and CH4 formation, we hypothesize that microphytobenthos (MPB) communities, as major primary producer in many coastal systems, play a significant role in shallow-water CH4 dynamics. To explore MPB-associated CH4 production, we will investigate the potential of this CH4 source, by conducting incubation experiments and determining the rates of both primary production and CH4 production by abundant benthic diatom species. The use of a novel cavity ring-down spectroscopy-based semi-continuous method in closed incubation experiments will enable sensitive measurements of CH4 production in high temporal resolution over day/night cycles. The experiments will be undertaken under controlled temperature and light conditions and will use axenic/xenic clonal cultures of selected MPB diatoms to identify the main effectors and those species with the highest CH4 production rates. Experiments using 13C-labeled precursor substrates will be performed to identify and characterize MPB-associated CH4 production pathways. Previous studies of oxic CH4 production were mostly mechanistic laboratory studies that used culture samples; whether the measured CH4 production rates were representative of those in natural systems was not determined. To overcome these limitations in our study, the incubation experiments with clonal cultures will be accompanied by natural MPB communities sampled from the shallow waters at Askö island (Swedish Baltic Sea) and from the inner coastal waters of the Darss-Zingst Bodden Chain (German Baltic Sea), thus covering different sediment types, hydrodynamic conditions, and MPB communities. To align benthic CH4 fluxes from MPB communities with overall benthic and sea-to-air fluxes of CH4, we will determine CH4 fluxes between the sediment, water column, and atmosphere in both study areas.