Recent research suggests that nitrogen (N) cycling microbes are important for coral holobiont functioning. In particular, coral holobionts may acquire bioavailable N via prokaryotic dinitrogen (N₂) fixation or remove excess N via denitrification activity. However, our understanding of environmental drivers on these processes in hospite remains limited. Employing the strong seasonality of the central Red Sea, this study assessed the effects of environmental parameters on the proportional abundances of N cycling microbes associated with the hard corals Acropora hemprichii and Stylophora pistillata. Specifically, we quantified changes in the relative ratio between nirS and nifH gene copy numbers, as a proxy for seasonal shifts in denitrification and N₂ fixation potential in corals, respectively. In addition, we assessed coral tissue-associated Symbiodiniaceae cell densities and monitored environmental parameters to provide a holobiont and environmental context, respectively. While ratios of nirS to nifH gene copy numbers varied between seasons, they revealed similar seasonal patterns in both coral species, with ratios closely following patterns in environmental nitrate availability. Symbiodiniaceae cell densities aligned with environmental nitrate availability, suggesting that the seasonal shifts in nirS to nifH gene abundance ratios were probably driven by nitrate availability in the coral holobiont. Thereby, our results suggest that N cycling in coral holobionts probably adjusts to environmental conditions by increasing and/or decreasing denitrification and N₂ fixation potential according to environmental nitrate availability. Microbial N cycling may, thus, extenuate the effects of changes in environmental nitrate availability on coral holobionts to support the maintenance of the coral–Symbiodiniaceae symbiosis.

最近的研究表明，氮 (N) 循环微生物对于珊瑚全生物功能非常重要。特别是，珊瑚全生物可以通过原核二氮（N ₂ ）固定获得生物可利用的氮，或通过反硝化活动去除多余的氮。然而，我们对收容所这些过程的环境驱动因素的了解仍然有限。利用红海中部强烈的季节性，本研究评估了环境参数对与硬珊瑚Acropora hemprichii和Stylophora pistillata 相关的氮循环微生物比例丰度的影响。具体来说，我们量化了nirS和nifH基因拷贝数之间相对比率的变化，分别作为珊瑚反硝化和 N ₂固定潜力季节性变化的代表。此外，我们评估了珊瑚组织相关的共生科细胞密度并监测环境参数，以分别提供全生物和环境背景。虽然nirS与nifH基因拷贝数的比率随季节变化，但它们揭示了两种珊瑚物种相似的季节模式，其比率密切遵循环境硝酸盐可用性的模式。共生藻科细胞密度与环境硝酸盐可用性一致，表明nirS与nifH基因丰度比的季节性变化可能是由珊瑚全生物中硝酸盐可用性驱动的。 因此，我们的结果表明，珊瑚全生物中的氮循环可能根据环境硝酸盐可用性，通过增加和/或减少反硝化和N ₂固定潜力来适应环境条件。因此，微生物氮循环可能减轻环境硝酸盐可用性变化对珊瑚全生物的影响，以支持珊瑚-共生科共生的维持。