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自养/异养的季节性差异对珊瑚骨骼碳同位素的影响
Effects of seasonal variations in autotrophy/heterotrophy on coral skeletal carbon isotopes
珊瑚骨骼稳定碳同位素(δ13Cs)是记录过去环境变化的重要指标,已广泛应用于不同时间尺度的气候环境重建。然而,δ13Cs的主控因素目前仍存争议,特别是珊瑚营养模式与光合、呼吸作用强度的季节差异对δ13Cs的调控机制尚不明确。本研究以2023年10月(秋季)和2024年4月(春季)采自广西涠洲岛的澄黄滨珊瑚(Porites lutea)为研究对象,综合运用同位素地球化学指标并结合生理参数与环境参数开展分析。结果显示,与春季相比,秋季虫黄藻密度、叶绿素a含量、虫黄藻同位素(δ13Cz)、总光合速率/总呼吸速率比值(P/R)、宿主有机质碳同位素(δ13Ch)以及宿主有机质碳同位素与虫黄藻碳同位素的差值(Δ13Ch-z)均相对较低,表明秋季共生虫黄藻密度降低、光合作用减弱,珊瑚异养贡献占比上升;而春季随着虫黄藻密度回升,光合作用增强,异养贡献占比相应降低。珊瑚骨骼δ13Cs值与虫黄藻密度、δ13Cz均呈正相关关系,且其季节性差异与Δ13Ch-z值及P/R的变化一致,表明珊瑚自养/异养比例及P/R比值是驱动δ13Cs季节差异的关键因素。因此,在基于δ13Cs的古环境重建研究中,应考虑珊瑚共生体自身光合作用/呼吸作用变化对δ13Cs的影响。
Coral skeletal carbon isotope composition (δ13Cs) is an important proxy for reconstructing past environmental changes and has been widely applied in paleoclimate and paleoenvironmental studies across various temporal scales. However, the factors controlling skeletal δ13Cs remain controversial, particularly the roles of seasonal variations in coral nutritional strategy and the relative intensities of photosynthesis and respiration. In this study, Porites lutea samples collected from Weizhou Island, Guangxi, China, in October 2023 (autumn) and April 2024 (spring) were analyzed using isotopic geochemical proxies in combination with physiological and environmental parameters. Compared with spring, corals collected in autumn exhibited lower zooxanthellae density (ZD), chlorophyll a (Chl a) content, zooxanthellae carbon isotope composition (δ13Cz), gross photosynthesis-to-gross respiration ratio (P/R), host tissue carbon isotope composition (δ13Ch), and the isotopic difference between host tissue and zooxanthellae (Δ13Ch-z). These results indicate that autumn corals experienced reduced zooxanthellae density and photosynthetic activity, accompanied by an increased contribution of heterotrophy, whereas enhanced zooxanthellae density and photosynthesis in spring resulted in a lower relative contribution of heterotrophy. Coral skeletal δ13Cs was positively correlated with both ZD and δ13Cz, while its seasonal variation was consistent with changes in Δ13Ch-z and P/R, suggesting that shifts in the autotrophic/heterotrophic balance and metabolic activity contribute to seasonal variations in skeletal δ13Cs. Therefore, variations in the photosynthetic and respiratory activities of the coral holobiont should be taken into account when interpreting coral skeletal δ13Cs records for paleoenvironmental reconstructions.
造礁珊瑚 / 骨骼碳同位素 / 分馏效应 / 营养方式 / 虫黄藻密度 / P/R比值 / 季节性变化 / 涠洲岛
scleractinian corals / skeletal carbon isotopes / fractionation effect / trophic status / zooxanthellae density / P/R ratio / seasonal variation / Weizhou Island
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It is well known that coral thermal bleaching is mainly caused by abnormally high temperatures, but the spatial/intergeneric variation in coral‐algal symbiosis and its linkage to coral bleaching susceptibility is not well understood. In this study, 654 coral samples were collected from six coral reef regions (CRRs) in the South China Sea (SCS), between the Nansha Islands (~10°N) and Daya Bay (22°50′N). They were analyzed to explore how geographical/environmental variation affects coral bleaching susceptibility. The results showed significant spatial and intergeneric variation in zooxanthellae density, Chlacontent, and tissue biomass. Massive corals generally showed higher values than branching corals for all three parameters across CRRs. Spatial analysis showed that these parameters were lowest in the Nansha Islands and highest in Daya Bay, indicating increases with latitude. Several factors may have accounted for this trend, with the most important being sea surface temperature and nutrients. Additionally, we observed a relationship between physiological parameters and bleaching susceptibility, which was that branching corals had greater bleaching susceptibility than massive corals. Coral community composition and live coral cover data showing increasing dominance of massive corals in the SCS support the above inference. Relatively high‐latitude areas have been proposed as potential refuges for corals under continued global warming and the physiological parameters in the present study seem to support this. However, in spite of the aforementioned results, the smallest live coral cover was found in the northern CRRs. This suggests that this refuge function is currently compromised, mainly due to increasing destructive anthropogenic actions.
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During the late summer to fall of 1987, Caribbean reef corals experienced an intense and widespread discoloration event described as bleaching. Contrary to initial predictions, most bleached corals did not die. However, energy input from zooxanthellae decreased, as estimated from: (i) delta(13)C values, a measure of the discrimination against (13)C in (12)C/(13)C assimilation, of skeletal aragonite; (ii) in situ photosynthesis-irradiance measurements; (iii) and tissue biomass parameters of Montastraea annularis and Agaricia lamarcki. The delta(18)O signal, a measure of the discrimination against (18)O in (16)O/(18)O assimilation, from M. annularis skeletons demonstrated that this event coincided with abnormally elevated water temperatures.
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Tissue biomass (ash‐free dry weight) and symbiotic dinoflagellates (density, chlorophyll a cell −1 or cm −2 of coral surface area) of five species of reef‐building corals were monitored seasonally for up to 4 yr at three different depths in the Bahamas. The lowest values of all tissue biomass and algal symbiont parameters occurred during the late summer–fall sample periods. In contrast, the highest densities and pigment content of symbionts usually occurred during the winter, whereas tissue biomass peaked most often in the spring, the time lag implying a functional relationship between these variables. Corals living in shallow water often (but not always) had higher levels of all parameters measured compared to deeper corals, except chlorophyll a content, which usually displayed the opposite trend. The results show that corals from all depths exhibited bleaching (loss of symbiotic dinoflagellates and/or their pigments) every year, regardless of whether they appeared white, tan, or mottled to the human eye. We speculate that these patterns are driven by seasonal changes in light and temperature on algal and animal physiology. Furthermore, we hypothesize that all tropical reef‐building corals, world‐wide, exhibit similar predictable cycles in their tissue biomass and symbiotic algae.\n
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Massive coral bleaching events result in extensive coral loss throughout the world. These events are mainly caused by seawater warming, but are exacerbated by the subsequent decrease in nutrient availability in surface waters. It has therefore been shown that nitrogen, phosphorus or iron limitation contribute to the underlying conditions by which thermal stress induces coral bleaching. Generally, information on the trophic ecology of trace elements (micronutrients) in corals, and on how they modulate the coral response to thermal stress is lacking. Here, we demonstrate for the first time that heterotrophic feeding (i.e. the capture of zooplankton prey by the coral host) and thermal stress induce significant changes in micro element concentrations and isotopic signatures of the scleractinian coral Stylophora pistillata. The results obtained first reveal that coral symbionts are the major sink for the heterotrophically acquired micronutrients and accumulate manganese, magnesium and iron from the food. These metals are involved in photosynthesis and antioxidant protection. In addition, we show that fed corals can maintain high micronutrient concentrations in the host tissue during thermal stress and do not bleach, whereas unfed corals experience a significant decrease in copper, zinc, boron, calcium and magnesium in the host tissue and bleach. In addition, the significant increase in δ Cu and δ Zn signature of symbionts and host tissue at high temperature suggests that these isotopic compositions are good proxy for stress in corals. Overall, present findings highlight a new way in which coral heterotrophy and micronutrient availability contribute to coral resistance to global warming and bleaching.© 2018 John Wiley & Sons Ltd.
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