Marine Biological Section, Department of Biology, University of Copenhagen, Helsingør, Denmark.
Center for Volatile Interactions, Department of Biology, University of Copenhagen, Copenhagen, Denmark.
Microb Ecol. 2024 Aug 14;87(1):106. doi: 10.1007/s00248-024-02424-w.
Seagrass meadows play pivotal roles in coastal biochemical cycles, with nitrogen fixation being a well-established process associated with living seagrass. Here, we tested the hypothesis that nitrogen fixation is also associated with seagrass debris in Danish coastal waters. We conducted a 52-day in situ experiment to investigate nitrogen fixation (proxied by acetylene reduction) and dynamics of the microbial community (16S rRNA gene amplicon sequencing) and the nitrogen fixing community (nifH DNA/RNA amplicon sequencing) associated with decomposing Zostera marina leaves. The leaves harboured distinct microbial communities, including distinct nitrogen fixers, relative to the surrounding seawater and sediment throughout the experiment. Nitrogen fixation rates were measurable on most days, but highest on days 3 (dark, 334.8 nmol N g dw h) and 15 (light, 194.6 nmol N g dw h). Nitrogen fixation rates were not correlated with the concentration of inorganic nutrients in the surrounding seawater or with carbon:nitrogen ratios in the leaves. The composition of nitrogen fixers shifted from cyanobacterial Sphaerospermopsis to heterotrophic genera like Desulfopila over the decomposition period. On the days with highest fixation, nifH RNA gene transcripts were mainly accounted for by cyanobacteria, in particular by Sphaerospermopsis and an unknown taxon (order Nostocales), alongside Proteobacteria. Our study shows that seagrass debris in temperate coastal waters harbours substantial nitrogen fixation carried out by cyanobacteria and heterotrophic bacteria that are distinct relative to the surrounding seawater and sediments. This suggests that seagrass debris constitutes a selective environment where degradation is affected by the import of nitrogen via nitrogen fixation.
海草草甸在沿海生化循环中起着关键作用，其中氮固定是与活海草相关的一个成熟过程。在这里，我们检验了这样一个假设，即氮固定也与丹麦沿海水域的海草碎屑有关。我们进行了为期 52 天的原位实验，以研究氮固定（通过乙炔还原来表示）以及与分解的马尾藻叶片相关的微生物群落（16S rRNA 基因扩增子测序）和固氮群落（nifH DNA/RNA 扩增子测序）的动态。与周围海水和沉积物相比，这些叶片在整个实验过程中具有独特的微生物群落，包括独特的固氮生物。在大多数日子里都可以测量到氮固定率，但在第 3 天（黑暗，334.8 nmol N g dw h）和第 15 天（光照，194.6 nmol N g dw h）最高。氮固定率与周围海水中无机养分的浓度或叶片中的碳氮比均无相关性。固氮生物的组成从蓝细菌 Sphaerospermopsis 转变为分解过程中的异养属，如 Desulfopila。在固定率最高的日子里，nifH RNA 基因转录本主要由蓝细菌组成，特别是由 Sphaerospermopsis 和一个未知的分类群（Nostocales 目）以及变形菌组成。我们的研究表明，温带沿海水域的海草碎屑蕴藏着大量由蓝细菌和异养细菌进行的氮固定，与周围的海水和沉积物相比，这些氮固定具有明显的区别。这表明海草碎屑构成了一个选择性环境，其中降解受通过氮固定输入的氮的影响。
