Mangrove forests are unique wetland ecosystems distributed along tropical and subtropical coastlines. They play a vital role in stabilizing sea levels, reducing coastal erosion, and providing habitats for diverse organisms (Alongi, 2008; Lee et al., 2014; Siddique et al., 2025). The Zhangjiangkou National Mangrove Nature Reserve (Ramsar Site No. 1726), located in Zhangzhou City, Fujian Province, harbors the largest Avicennia marina forest in China (Yuan et al., 2023). This region functions as an important coastal carbon sink, where microorganisms play a crucial role in the cycling of carbon, nitrogen, and phosphorus (Feng et al., 2017; Hou et al., 2024).

Nitrogen and iron cycles are core biogeochemical elements that significantly influence microbial activity and nutrient turnover in mangrove sediments (Ruiz et al., 2024). In particular, nitrogen fixation and iron (Fe) reduction represent two ecologically relevant and interconnected processes, as they provide essential nutrients and electron flow that sustain microbial communities under fluctuating redox conditions (Li et al., 2020; Wang et al., 2024c). Fe-bearing minerals, such as ferrihydrite and magnetite, are widely distributed in mangrove wetlands, especially in Fe-rich depositional zones shaped by tidal flushing and root exudation (Otero et al., 2009; Yang et al., 2020). These redox-active minerals serve as important electron acceptors in microbial respiration and have been shown to modulate microbial community structures and biogeochemical functions (Jin et al., 2019; Wang et al., 2024b; Zhang et al., 2019). Mangrove environments are characterized by abundant Fe inputs, anoxic conditions, and high nitrogen demand. Microbially driven Fe- and nitrogen-related processes therefore play a critical role in shaping sediment ecology (Guan et al., 2018; Wang et al., 2024c). Accordingly, microorganisms capable of both Fe reduction and nitrogen fixation are of particular ecological importance in mangrove ecosystems.

Recent studies employing metagenomic and culture-based techniques have identified Bacteroidetes as a dominant phylum in mangrove sediments, highlighting their involvement in the degradation of complex organic compounds and nutrient turnover (Hu et al., 2024; Li et al., 2021; Meng et al., 2022). Members of the order Bacteroidales are Gram-stain-negative, non-spore-forming bacteria known for their ability to degrade a wide range of biopolymers. According to the List of Prokaryotic Names with Standing in Nomenclature (LPSN; https://lpsn.dsmz.de/order/bacteroidales), the order currently includes 18 families with validly published names. Despite their abundance, a significant proportion of Bacteroidetes in mangrove environments remain uncultured (Rocha et al., 2016; Zhang et al., 2018a). Many family-level lineages are known only from environmental 16S rRNA gene sequences, largely due to the widespread use of culture-independent approaches and aerobic enrichment methods that are not conducive to the growth of certain anaerobic or slow-growing taxa. Consequently, the physiology and ecological functions of these uncultured Bacteroidetes remain poorly understood (Li et al., 2020; Pan et al., 2022).

To address these limitations, we employed an anaerobic enrichment strategy with Fe(III) as the electron acceptor to promote the growth of difficult-to-culture microorganisms (Lentini et al., 2012). This enrichment culture system used mangrove sediments as substrates and enabled the isolation of previously uncharacterized Bacteroidetes. Here, we describe three Fe(III)-reducing and nitrogen-fixing bacteria, FJH62ᵀ, FJH65ᵀ, and FJH54ᵀ, obtained from mangrove sediments in Fujian, China. Phylogenetic analyses showed that these three strains form a deep-branching lineage within the order Bacteroidales, for which a novel family, Quyinboaceae fam. nov., is proposed.