Data from: Assessing the effects of iron enrichment across holobiont compartments reveals reduced microbial nitrogen fixation in the Red Sea coral Pocillopora verrucosa

The productivity of coral reefs in oligotrophic tropical waters is sustained by an efficient uptake and recycling of nutrients. In reef-building corals, the engineers of these ecosystems, this nutrient recycling is facilitated by a constant exchange of nutrients between the animal host and endosymbiotic photosynthetic dinoflagellates (zooxanthellae), bacteria, and other microbes. Due to the complex interactions in this so-called coral holobiont, it has proven difficult to understand the environmental limitations of productivity in corals. Among others, the micronutrient iron has been proposed to limit primary productivity due to its essential role in photosynthesis and bacterial processes. Here, we tested the effect of iron enrichment on the physiology of the coral Pocillopora verrucosa from the central Red Sea during a 12-day experiment. Contrary to previous reports, we did not see an increase in zooxanthellae population density or gross photosynthesis. Conversely, respiration rates were significantly increased, and microbial nitrogen fixation was significantly decreased. Taken together, our data suggest that iron is not a limiting factor of primary productivity in Red Sea corals. Rather, increased metabolic demands in response to iron enrichment, as evidenced by increased respiration rates, may reduce carbon (i.e., energy) availability in the coral holobiont, resulting in reduced microbial nitrogen fixation. This decrease in nitrogen supply in turn may exacerbate the limitation of other nutrients, creating a negative feedback loop. Thereby, our results highlight that the effects of iron enrichment appear to be strongly dependent on local environmental conditions and ultimately may depend on the availability of other nutrients.

在贫营养热带水域中，珊瑚礁的生产力依赖于对营养物质的高效摄取与循环利用。作为这类生态系统的构建者，造礁珊瑚的营养循环依赖于动物宿主与内共生光合甲藻（虫黄藻，zooxanthellae）、细菌及其他微生物之间持续的营养交换。由于这一被称为珊瑚共生体（coral holobiont）的复杂相互作用，学界此前难以阐明珊瑚生产力的环境限制因素。其中，微量营养素铁因在光合作用与细菌代谢过程中的关键作用，被认为可能限制珊瑚的初级生产力。本研究通过为期12天的实验，探究了铁富集对红海中部海域疣状鹿角珊瑚（Pocillopora verrucosa）生理状态的影响。与既往研究报道不同，本研究未观测到虫黄藻种群密度或总光合速率出现提升。与之相反，珊瑚的呼吸速率显著升高，而微生物固氮作用则显著降低。综合来看，本研究数据表明，铁并非红海珊瑚初级生产力的限制因子。反而，由呼吸速率升高所印证的、铁富集引发的代谢需求提升，可能会降低珊瑚共生体内部的碳（即能量）可用性，进而导致微生物固氮作用减弱。这种氮供给的减少又会加剧其他营养物质的限制效应，形成负反馈循环。综上，本研究结果表明，铁富集的效应似乎强烈依赖于当地的环境条件，最终可能取决于其他营养物质的可获得性。