Table_8_Seaweed Loads Cause Stronger Bacterial Community Shifts in Coastal Lagoon Sediments Than Nutrient Loads.XLSX

The input of nutrients from anthropogenic sources is the leading cause of coastal eutrophication and is usually coupled with algal/seaweed blooms. Effects may be magnified in semi-enclosed systems, such as highly productive coastal lagoon ecosystems. Eutrophication and seaweed blooms can lead to ecosystem disruption. Previous studies have considered only one of these factors, disregarding possible interactive effects and the effect of the blooming species’ identity on sediment bacterial communities. We tested the effect of experimental nutrient loading and two common blooming seaweeds (Ulva rigida and Gracilaria vermiculophylla) in coastal lagoon sediments, on the structure of bacterial communities (using 16S rRNA amplicon sequencing) and corresponding putative functional potential (using PiCRUSt). At the Operational Taxonomic Unit (OTU) level, the addition of nutrients reduced bacterial community α-diversity and decreased the abundance of sulfate reducers (Desulfobacterales) compared to sulfur oxidizers/denitrifiers (Chromatiales and Campylobacterales), whereas this was not the case at the order level. Seaweed addition did not change bacterial α-diversity and the effect on community structure depended on the taxonomic level considered. The addition of Gracilaria increased the abundance of orders and OTUs involved in sulfate reduction and organic matter decomposition (Desulfobacterales, Bacteroidales, and Clostridiales, respectively), an effect which was also detected when only Ulva was added. Nutrients and the seaweeds combined only interacted for Ulva and nutrients, which increased known sulfide oxidizers and denitrifiers (order Campylobacterales). Seaweed enrichment affected putative functional profiles; a stronger increase of sulfur cycling KEGG pathways was assigned to nutrient-disturbed sediments, particularly with the seaweeds and especially Ulva. In contrast, nitrogen and sulfur cycle pathways showed a higher abundance of genes related to dissimilatory nitrate reduction to ammonium (DNRA) in Ulva+nutrients treatments. However, the other seaweed treatments increased the nitrogen fixation genes. Thiosulfate reduction, performed by sulfate-reducing bacteria, increased in seaweed treatments except when Ulva was combined with nutrients. In conclusion, the in situ addition of nutrients and the seaweeds to intertidal sediments affected the bacterial communities differently and independently. The predicted functional profile suggests a shift in relative abundances of putative pathways for nitrogen and sulfur cycles, in line with the taxonomic changes of the bacterial communities.

人为来源的营养盐输入是引发近海富营养化的首要诱因，且通常伴随藻类/大型海藻藻华的发生。此类影响在半封闭水体系统中会被放大，例如高生产力的近海潟湖生态系统。富营养化与海藻藻华均可引发生态系统紊乱。既往相关研究仅单独考量了其中一类因素，忽略了二者可能存在的交互效应，以及藻华物种种类对沉积物细菌群落的影响。本研究以近海潟湖沉积物为研究对象，通过实验添加营养盐与两种常见的海藻藻华物种——硬石莼（Ulva rigida）和细基江蓠（Gracilaria vermiculophylla），探究其对细菌群落结构（采用16S rRNA扩增子测序（16S rRNA amplicon sequencing）技术）及相应推定功能潜力（采用PiCRUSt工具）的影响。在操作分类单元（Operational Taxonomic Unit, OTU）水平上，营养盐添加会降低细菌群落的α多样性，且相较于硫氧化菌/反硝化菌（着色菌目（Chromatiales）与弯曲菌目（Campylobacterales）），硫酸盐还原菌（脱硫杆菌目（Desulfobacterales））的丰度有所下降；但在目级分类水平上并未观察到该现象。海藻添加则未对细菌α多样性产生显著影响，其对群落结构的效应取决于所考量的分类学水平。细基江蓠的添加会提升参与硫酸盐还原与有机质分解的目级类群及OTU的丰度（分别对应脱硫杆菌目（Desulfobacterales）、拟杆菌目（Bacteroidales）与梭菌目（Clostridiales）），仅添加硬石莼时也可观测到类似效应。营养盐与海藻的交互效应仅存在于硬石莼与营养盐组合处理组中，该组合提升了已知的硫化物氧化菌与反硝化菌（弯曲菌目（Campylobacterales））的丰度。海藻富集会改变推定功能谱；营养盐扰动的沉积物中硫循环相关KEGG通路（KEGG pathways）的丰度提升更为显著，当同时添加海藻时该效应更强，尤以硬石莼处理组为甚。相较之下，硬石莼+营养盐组合处理组的氮循环与硫循环通路中，与异化硝酸盐还原为铵（dissimilatory nitrate reduction to ammonium, DNRA）相关的基因丰度更高。而其余海藻处理组则提升了固氮相关基因的丰度。由硫酸盐还原菌介导的硫代硫酸盐还原反应，在除硬石莼+营养盐组合外的所有海藻处理组中均有所增强。综上，在潮间带沉积物中原位添加营养盐与海藻，会以独立且各异的方式影响细菌群落。预测得到的功能谱显示，氮循环与硫循环相关推定通路的相对丰度发生了改变，这与细菌群落的分类学变化相一致。