Mesopelagic N2 Fixation Related to Organic Matter Composition in the Solomon and Bismarck Seas (Southwest Pacific)

Dinitrogen (N2) fixation was investigated together with organic matter composition in the mesopelagic zone of the Bismarck (Transect 1) and Solomon (Transect 2) Seas (Southwest Pacific). Transparent exopolymer particles (TEP) and the presence of compounds sharing molecular formulae with saturated fatty acids and sugars, as well as dissolved organic matter (DOM) compounds containing nitrogen (N) and phosphorus (P) were higher on Transect 1 than on Transect 2, while oxygen concentrations showed an opposite pattern. N2 fixation rates (up to ~1 nmol N L-1 d-1) were higher in Transect 1 than in Transect 2, and correlated positively with TEP, suggesting a dependence of diazotroph activity on organic matter. The scores of the multivariate ordination of DOM molecular formulae and their relative abundance correlated negatively with bacterial abundances and positively with N2 fixation rates, suggesting an active bacterial exploitation of DOM and its use to sustain diazotrophic activity. Sequences of the nifH gene clustered with Alpha-, Beta-, Gamma- and Deltaproteobacteria, and included representatives from Clusters I, III and IV. A third of the clone library included sequences close to the potentially anaerobic Cluster III, suggesting that N2 fixation was partially supported by presumably particle-attached diazotrophs. Quantitative polymerase chain reaction (qPCR) primer-probe sets were designed for three phylotypes and showed low abundances, with a phylotype within Cluster III at up to 103nifH gene copies L-1. These results provide new insights into the ecology of non-cyanobacterial diazotrophs and suggest that organic matter sustains their activity in the mesopelagic ocean.

本研究针对西南太平洋俾斯麦海（Transect 1）与所罗门海（Transect 2）的海洋中层带，联合开展了固氮作用（Dinitrogen (N2) fixation）与有机物组成的调查。相较于断面2，断面1的透明胞外聚合物颗粒（Transparent exopolymer particles, TEP）、与饱和脂肪酸和糖类共享分子式的化合物，以及含氮（N）、磷（P）的溶解态有机物（dissolved organic matter, DOM）组分含量均更高，而溶解氧浓度则呈现相反的分布趋势。固氮速率最高可达约1纳摩尔氮每升每天（nmol N L-1 d-1），断面1的固氮速率显著高于断面2，且与TEP含量呈正相关，表明固氮微生物（diazotroph）的活性依赖于有机物底物。DOM分子式的多元排序得分及其相对丰度与细菌丰度呈负相关，与固氮速率呈正相关，提示细菌可主动利用DOM，并以此维持固氮微生物的活性。nifH基因序列聚类结果显示其分属于α-、β-、γ-及δ-变形菌门，涵盖第I、III、IV簇的代表类群。三分之一的克隆文库序列与潜在厌氧的第III簇类群亲缘关系较近，表明固氮作用部分由附着于颗粒的固氮微生物所介导。本研究设计了针对3种系统型的定量聚合酶链式反应（Quantitative polymerase chain reaction, qPCR）引物探针组，检测结果显示其丰度较低，其中第III簇内的某一系统型的nifH基因拷贝数最高可达每升10³个。本研究结果为非蓝细菌固氮微生物的生态学特性提供了新认识，并表明有机物可维持海洋中层带非蓝细菌固氮微生物的活性。