Composition and Activity of N2-Fixing Microorganisms in Mangrove Forest Soils

Abstract

Mangrove forests are considered to be a highly productive ecosystem, but they are also generally nitrogen (N)-limited. Thus, soil N2 fixation can be important for the stability of both mangrove ecosystem functions and upland N supply. This study evaluates the N2 fixation activity and composition of relevant microorganisms in two coastal mangrove forests—the Guandu mangrove in an upstream estuary and the Bali mangrove in a downstream estuary—using the acetylene reduction method, real-time polymerase chain reaction, and next-generation sequencing. The results demonstrated that ambient nitrogenase activity was higher in downstream mangrove forests (13.2–15.6 nmol hr−1 g−1 soil) than in upstream mangrove forests (0.2–1.4 nmol hr−1 g−1 soil). However, both the maximum potential nitrogenase activity and nitrogenase gene (nifH gene) copy number were found to be higher in the upstream than in the downstream mangrove forests, implying that the nitrogenase activity and diazotrophic abundance may not necessarily be positively correlated. In addition, amended MoO4 (which inhibits the activity of sulfate-reducing bacteria in N2-fixation) yielded low nitrogenase activity, and sulfate-reducing bacteria made up 20–50% of the relative diazotrophic abundance in the mangrove forests, indicating that these bacteria might be the major active diazotrophs in this environment.

High-throughput analysis using 16S rRNA sequencing of bacterial communities associated in selected mangrove species from Bayug Island, Iligan City, Philippines

“…Nevertheless, these techniques have limitations, because of the small size of microorganisms and their lack of well-defined taxonomic characteristics, making accurate microscopic identification challenging (Navgire et al 2022). In recent years, Next-Generation Sequencing (NGS) has become a powerful tool for studying soil microorganisms, including diazotrophic communities (Shiau et al 2021). In particular, ribosomal RNA (rRNA) gene sequencing is widely accepted for assessing bacterial diversity (Zhou et al 2017).…”