洞穴溶解有机质组分和循环过程的季节变化特征

刘渝港; 贺秋芳; 沈立成; 范佳鑫

doi:10.11932/karst2023y23

洞穴溶解有机质组分和循环过程的季节变化特征

doi: 10.11932/karst2023y23

西南大学地理科学学院, 岩溶环境重庆市重点实验室, 重庆 400715

基金项目: 国家重点研发计划项目(2016YFC0502306)；国家自然科学基金项目(41472321)

详细信息

作者简介:
刘渝港（1997－），男，硕士研究生，主要研究方向为岩溶作用与碳循环。E-mail：1308782888@qq.com

通讯作者:
贺秋芳（1983－），女，高级实验师。E-mail：hqfeddy@swu.edu.cn

中图分类号: X143；Q938.8
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出版历程
- 收稿日期: 2021-12-11
- 网络出版日期: 2023-09-16
- 刊出日期: 2023-06-30

Seasonal variation characteristics of dissolved organic matter composition and cycle process in caves

Chongqing Key Laboratory of Karst Environment, School of Geographical Sciences, Southwest University, Chongqing 400715, China

摘要

摘要: 溶解有机质（DOM）是岩溶碳汇的关键部分和重要的碳源，但是对于DOM在岩溶含水层中的性质和代谢过程的研究仍然有限。本研究以重庆雪玉洞地下河为例，对洞穴有机碳的来源、组成以及微生物作用对季节补给源变化的响应进行探讨，为进一步了解微生物介导的有机碳转化过程提供研究基础。运用三维荧光EEM研究水体有色溶解有机质（CDOM）的性质和组分并反演地下河水中有机质的来源和组成，结合地下河水水化学特征和16S rDNA细菌群落及功能多样性的季节变化特征，以了解季节性补给源的变化对洞穴地下水DOM输入和性质的影响。结果发现，雪玉洞地下河水以微生物内源有机质为主（61%~77%），降雨是引起岩溶给地下河水中CDOM光谱特征变化的最重要因素，雨季外源有机质输入增加，地下河水中外源有机碳组分含量和芳香性、腐殖酸类物质增加，细菌群落多样性和代谢功能基因随之变化，洞穴中向外输出的外源有机碳增加；旱季地下河水滞留时间和蒸发作用增强，因而微生物对有机质的代谢降解过程更加充分，向洞外输出的有机碳以内源为主。本研究有助于增加对岩溶洞穴地下水系统中微生物对有机碳转化过程的理解。
- 有色溶解有机质 /
- 三维荧光光谱 /
- 微生物 /
- 岩溶洞穴 /
- 地下水
Abstract: Dissolved organic matter (DOM) is the key part and important carbon source of karst carbon sink. The migration and degradation of DOM in karst aquifers is an important carrier of material and energy transfer. However, due to the lack of long-term and in-depth research on the content, composition and migration process of DOM in karst groundwater, the transfer and metabolism process of organic carbon in cave water system and its impact on karst carbon sink process are still unclear. Colored dissolved organic matter (CDOM), a colored soluble component in DOM, is often used to characterize DOM in marine and freshwater systems. It is also used to characterize the composition and change process of organic matter in the study of karst groundwater. 16S rDNA high-throughput sequencing, a commonly used method for the detection of non-cultured microorganisms, can comprehensively detect and analyze the bacterial community composition and species abundance in environmental samples. This study collected samples from the underground river in Fengdu Xueyu Cave, Chongqing on a quarterly basis, analyzed their hydrochemical indexes and organic carbon content, and characterized the changes in the composition and structure of organic carbon using CDOM fluorescence and UV-visible spectrum information. Combined with 16S rDNA high-throughput sequencing, this study also analyzed the structural characteristics of bacterial communities in groundwater and the metabolic function genes of microorganisms based on Pecrust2 prediction. According to the metabolism and transformation process of organic carbon in cave water and its response to the external input of organic matter, we can understand the migration and transformation process of organic carbon in karst groundwater, and provide scientific information for the carbon cycle and carbon sink process of karst groundwater system.Xueyu Cave is located in the Longjiang Gorge in Fengdu county, Chongqing, the southwest karst region. The cave opening is exposed at the mid-mountain elevation of 340 meters. Affected by the subtropical monsoon climate, there are relatively obvious dry seasons with low precipitation (from October to April of the next year) and rainy seasons with high precipitation (from May to September). The flow of underground river in the cave is controlled by precipitation which is much higher in the rainy season than in the dry season. This study collected water samples from the deep part of the cave and at the mouth of the cave in the Xueyu Cave underground river on a quarterly basis in 2018 and analyzed their hydrochemical characteristics. It is found that the chemical type of the underground river water is a typical HCO₃-Ca type, with high pH, high DIC and high Ca. PCA analysis shows that the hydrochemical indicators are mainly controlled by precipitation, and present a two-level distribution in the dry season and the rainy season. δD and δ¹⁸O content indicates that the evaporation effect and retention time of groundwater in the dry season are higher than those in the rainy season. Three CDOM components are found in the underground water of Xueyu Cave by CDOM three-dimensional fluorescence EEM parallel factor analysis, C1 is an endogenous tryptophan-like substance of microbial origin; C2 is an endogenous humic acid or fulvic acid of microbial origin; C3 is an exogenous humic acid component. The results of UV-visible absorption spectrum parameters and fluorescence spectrum parameters are similar, both of which indicate that the DOM components in the Xueyu Cave underground water are affected by inputs both from internal and external sources. The internal organic matter is mainly small molecular endogenous organic matter, accounting for 61%-77%. PCA analysis of organic carbon content and CDOM related indicators shows no obvious seasonal changes in terms of DOC and TOC content, while CDOM components, fluorescence spectrum parameters and absorption spectrum parameters demonstrate obvious seasonal differences. The fluorescence intensity and proportion of exogenous C3 in the rainy season are much higher than those in the dry season, while the proportions of endogenous C1 and C2 in the dry season are higher. In the fluorescence parameters, HIX indicates the humification degree of CDOM, with the average value of 1.89 in the dry season, lower than the average value of 2.00 in the rainy season. On the contrary, BIX indicates the contribution of endogenous DOM in the new generation. The mean value in the dry season is 0.88, higher than that in the rainy season. 16Sr DNA analysis of bacterial community diversity shows that the Xueyu Cave underground water is dominated by heterotrophic microorganisms, and there is a large difference in the microbial community structure between the rainy season and the dry season. The gene abundance of metabolic function predicted by Pecrust 2 demonstrates that the functional gene abundance related to the degradation of aromatic substances in the rainy season is 30%-56%, higher than that in the dry season. The above analyses show that the input of exogenous organic carbon introduced by rainfall is the main control factor of organic carbon composition and carbon metabolism process in karst groundwater. During the rainy season from May to September in the karst area of Southwest China, exogenous organic matter enters karst groundwater with rainstorms, and hence its content increases. The proportions of exogenous components in CDOM and the indexes of HIX, FI and other related spectral also increase. The change of organic matter compositions takes place with the change of bacterial community diversity. The abundance of functional genes that metabolize exogenous aromatic substances in underground water increases, so does the content of exogenous organic carbon output from underground water to the outside of the cave. During the dry season from October to April of the next year, the rainfall is about 1/4 of the rainy season. While the input of exogenous organic matter decreases, the retention time of karst groundwater is longer, and the process of microbial degradation and metabolism is more sufficient. Therefore, the CDOM spectrum of groundwater reflects stronger endogenous characteristics of microorganisms with higher proportion of endogenous CDOM components exported by karst groundwater to the outside of the cave. The study results show that the main source of organic carbon in karst groundwater is controlled by rainfall, and the input amount in the rainy season is higher than that in the dry season. The full degradation and metabolism of organic matter in groundwater by microorganisms in the dry season causes the endogenous enhancement in groundwater system, which has an important impact on the process of karst underground carbon sink.
- colored dissolved organic matter /
- three-dimensional fluorescence spectrum /
- microbes /
- karst cave /
- groundwater

HTML

图 1 雪玉洞剖面和区位图及采样点分布图

Figure 1. Profile and location of Xueyu Cave and the distribution of sampling sites

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图 2 丰都县当地大气降水线（LMWL）与雪玉洞干湿季地下河水δ¹⁸O/δD的相关性^[37-38]

Figure 2. Local Meteoric Water Level (LMWL) of Fengdu county and δ¹⁸O/δD correlation of the Xueyu Cave underground river in the dry and wet seasons ^[37-38]

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图 3 雪玉洞岩溶水水化学指标的变化

Figure 3. Temporal variation of hydrochemical parameters in the karst water of Xueyu Cave

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图 4 DOC与a(355)相关性分析

Figure 4. Correlation analysis between DOC and a (355)

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图 5 雪玉洞吸光参数与荧光参数的旱雨季变化特征

Figure 5. Variation characteristics of light absorption parameters and fluorescence parameters of Xueyu Cave in the dry and rainy seasons

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图 6 雪玉洞地下河水CDOM荧光组分三维荧光图及折半检验

Figure 6. CDOM fluorescence components and split-half test in the Xueyu Cave underground river

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图 7 雪玉洞地下河水CDOM荧光组分的荧光强度的季节变化

Figure 7. Seasonal variation of fluorescence intensity of CDOM component in the Xueyu Cave underground river

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图 8 雪玉洞地下河水中属水平的细菌群落的heatmap图

Figure 8. Bacteria abundance heatmap on genus level in the Xueyu Cave underground river

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图 9 雪玉洞地下河水中旱雨季差异显著的KEGG代谢途径丰度对比和增量(P<0.05)

Figure 9. Comparison of abundance in KEGG metabolic pathways with the significant difference between the dry and wet seasonsand the abundance increment in the Xueyu Cave underground river (P<0.05)

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图 10 雪玉洞地下河水中地球化学指标与光谱参数的PCA分析

Figure 10. PCA analysis of geochemical indexes and spectral parameters in the Xueyu Cave underground river

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图 11 雪玉洞地下河水主要细菌属丰度与有机和无机碳及组分含量斯皮尔曼相关性heatmap分析

Figure 11. Spearman correlation heatmap between top 50 bacteria genera and carbon concentrations in the Xueyu Cave underground river

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表 1 紫外-可见光吸收谱和三维荧光光谱参数的指示含义

Table 1. Indicators calculated from ultraviolet-visible absorption spectrum and 3D fluorescence EEMs of DOM

指标	描述	意义
a₃₅₅	355 nm处的吸光值	CDOM的浓度^[29]
S_250~290	250 nm和290 nm之间的吸光度斜率	表征CDOM的分子量和光化学反应能力^[30]
E₂₅₃/E₂₀₃	253 nm和203 nm处吸光值的比值	表征DOM苯环结构上官能团的构成特征^[31]
M	250 nm和265 nm处吸光值的比值^[32]	DOM的分子量^[33]
FI	激发波长为370 nm时，发射波长为470 nm和 520 nm荧光强度的比值	表征类腐殖酸物质和微生物对DOM的降解程度^[34]
HIX	激发波长为254 nm时，发射波长为534~480 nm 和 300~346 nm荧光强度的比值	表征腐殖化程度^[35]
BIX	激发波长为310 nm时，发射波长为380 nm和 430 nm荧光强度的比值	表征内源DOM的贡献^[36]

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表 2 PARAFAC归纳的三个荧光组分特征^a

Table 2. Characteristics of three fluorescent components summarized by PARAFAC

组分	Exmax/Emmax ^b	峰值	备注
C1	≤250,300/360	Peak T: 275/340[82] C1:240(300)/338[83] C4: ≤230 (280)/340[84]	类蛋白类色氨酸组分，内源^[45]
C2	280/446	Peak M: 290~310/370~420[82] peak 3: 310-330/420-455[85] C1:≤230（305）/ 400[86] C1: <250 (310)/ 422[87]	微生物源或海洋中的腐殖酸类组分^[46]
C3	≤250/450	Peak A: 230~260/380~460 [82] C1: 250/452[88] C1: ＜250/448[81] C1:＜250(345)/460[89]	普遍存在的腐殖酸/富里酸类组分^[47-48]
注：a 括号里为第二峰值的最大强度位置;b Exmax/Emmax表示最大发射波长 (nm)/最大激发波长 (nm)。 Note: (a) numeric value in brackets indicates the maximum intensity position of the second peak; (b) Exmax/Emmax indicates maximum emission wavelength (nm)/maximum excitation wavelength (nm).

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