Table S3 from Diatoms dominate the eukaryotic metatranscriptome during spring in coastal ‘dead zone’ sediments.

An important characteristic of marine sediments is the oxygen concentration that affects many central metabolic processes. There has been a widespread increase in hypoxia in coastal systems (referred to as ‘dead zones’) mainly caused by eutrophication. Hence, it is central to understand the metabolism and ecology of eukaryotic life in sediments during changing oxygen conditions. Therefore, we sampled coastal ‘dead zone’ Baltic Sea sediment during autumn and spring, and analysed the eukaryotic metatranscriptome from field samples and after incubation in the dark under oxic or anoxic conditions. Bacillariophyta (diatoms) dominated the eukaryotic metatranscriptome in spring and were also abundant during autumn. A large fraction of the diatom RNA reads was associated with the photosystems suggesting a constitutive expression in darkness. Microscope observation showed intact diatom cells and these would, if hatched, represent a significant part of the pelagic phytoplankton biomass. Oxygenation did not significantly change the relative proportion of diatoms nor resulted in any major shifts in metabolic ‘signatures’. By contrast, diatoms rapidly responded when exposed to light suggesting that light is limiting diatom development in hypoxic sediments. Hence, it is suggested that diatoms in hypoxic sediments are on ‘standby’ to exploit the environment if they reach suitable habitats.

海洋沉积物的一项关键特征是其氧浓度，该参数可调控诸多核心代谢过程。沿海海域的低氧现象（又称“死亡区”）已呈大范围加剧态势，其主要诱因是水体富营养化。因此，解析氧浓度动态变化条件下沉积物中真核生物的代谢与生态特征，具有核心学术价值。为此，本研究于秋季与春季采集了波罗的海沿海“死亡区”的沉积物样本，并针对野外采集样本，以及在黑暗环境中分别于有氧、无氧条件下培养后的样本，开展了真核生物宏转录组（eukaryotic metatranscriptome）分析。硅藻门（Bacillariophyta，俗称硅藻）在春季的真核生物宏转录组中占据主导地位，秋季其种群丰度同样处于较高水平。硅藻的RNA测序读段中，有较大比例与光系统相关，这表明硅藻在黑暗环境中存在组成型表达模式。显微镜观测显示样本中存在完整的硅藻细胞，若这些细胞得以复苏并增殖，将构成远洋浮游植物生物量的重要组成部分。氧浓度调控并未显著改变硅藻的相对丰度占比，也未使代谢特征谱出现明显偏移。与之形成鲜明对比的是，硅藻在暴露于光照时会快速产生响应，这表明光照是限制低氧沉积物中硅藻生长发育的关键限制因子。因此，本研究推测，低氧沉积物中的硅藻处于“待机”状态，一旦抵达适宜生境即可快速利用环境资源完成种群扩张。