中国科学院海洋研究所机构知识库

海洋有机质作为全球生态系统中重要的有机碳储库，其循环过程对海洋生态环境和大气二氧化碳浓度具有重要影响。边缘海虽然仅占据海洋总面积的不到8%，但却贡献了海洋有机碳埋藏总量的80%。与此同时，边缘海水动力过程复杂多变，加之受到人类活动和气候变化的双重影响，导致这一区域有机质来源及其生物地球化学过程极其复杂。因此，探究边缘海有机质来源、降解转化及其控制机制对于深入理解海洋有机碳关键循环过程以及生态环境变化规律具有重要意义。

本论文选取受人类活动影响严重并具有复杂水动力过程的黄东海区域为研究对象，通过现场大面调查并结合室内培养实验，基于有机分子及其单体稳定碳同位素以及同步获得的物理、水文、生化参数，对黄东海有机质降解转化及其控制机制等关键生物地球化学过程进行了系统研究，并探究了有机质循环过程与低氧、酸化的耦合关系以及潜在的低氧反演指标。主要研究结果如下：

（1）基于氨基糖构成及丰度的结果发现，黄东海颗粒有机质（POM）的生物活性主要受控于其来源，河流输送而来的POM通常相对惰性，而初级生产力较高的区域POM活性整体较高。具有高生物活性的POM可迅速激发微生物代谢活动，从而形成异养转化的热点。有机质的降解转化及其随后的埋藏过程受到水动力过程的塑造，气旋型涡流、冷水团等物理水文条件会促进POM在水柱中的细菌转化，降低POM生物活性，但有利于沉积有机质（SOM）的长久埋藏；相比之下，水柱中新鲜POM的快速沉降和底层强烈的再悬浮过程则会降低SOM埋藏效率。

南黄海35°N断面POM中葡萄糖胺/半乳糖胺（GlcN/GalN）比值与Chl-a浓度具有显著正相关关系，表明初级生产是POM生物活性的重要调控因素。总体而言，南黄海GlcN/GalN比值普遍较低（<3），特别是在黄海冷水团中具有极低的GlcN/GalN比值（~0.7），表明POM已经历了广泛的微生物降解。基于胞壁酸（MurA）的估算表明，南黄海颗粒有机碳（POC）平均~13%来源于细菌贡献。在近岸区由于强烈的水体混合以及冷水团上方气旋性涡流的存在，增加了POM在水体中的停留时间，从而促进了细菌对POM的转化，导致细菌贡献率较高（~25%）。水动力过程导致的水体中广泛的成岩作用降低了生物碳泵效率，但可能通过各种细菌代谢途径助力长期的碳封存。相比之下，长江口主断面POM浓度基本呈现随盐度增大而降低的变化特征。但在中等盐度区，由于较高的初级生产力，导致POM存在高值。长江口POM生物活性随盐度梯度呈现先升高后降低的变化趋势。长江水体POM的生物惰性与高度降解的陆地土壤有机质输入有关。在中等盐度区较高的POM生物活性则与此区域浮游植物生产有关；而在外海区，由于陆源营养盐供给减少，初级生产力下降，导致POM生物活性再次降低。受控于陆源输入和初级生产，长江口区域细菌有机碳贡献（12.35±8.67%）沿盐度梯度呈现V型分布模式。

对黄东海SOM的研究发现，黄东海表层沉积有机碳（SOC）以海源自生为主（~70%），且有机碳经过了深度的异养改造，约25%的SOC来源于细菌贡献。SOC的分布与沉积物粒径显著相关，南黄海泥质区和东海泥质区为两个有机碳埋藏的热点区域，但埋藏模式却截然不同。南黄海泥质区由于受到黄海冷水团（低温、低氧特征）和气旋型涡流的影响，导致这一区域POC沉积速率较慢，且在水柱中经历了较高程度的降解，其所埋藏的有机碳活性较低，倾向于长久保存。而在东海泥质区，沉积速率较快，埋藏的有机碳活性相对较高，但复杂的水动力过程促使的物理再造过程以及激发效应使得SOC的再矿化速率较高。尽管东海泥质区有机碳的埋藏通量是南黄海泥质区的5倍，但水动力过程塑造的有机碳埋藏过程使得两个区域有机碳埋藏效率却相差不大（~35%）。

（2）基于氨基糖单体碳同位素分馏特征的研究发现，异养细菌氨基糖碳同位素特征受控于基底碳源，且细菌在降解过程中优先利用δ¹³C相对偏正的活性物质；而藻类氨基糖的合成相比于其初始碳源CO₂，则相对富集¹²C。GlcN和GalN的δ¹³C在不同异养细菌和藻类中的变化特征显示了两种氨基糖组分在细菌体内具有相似的合成路径，但藻类则具有相对独立和特异的合成途径，同时异养细菌特异性氨基糖MurA的δ¹³C则显著低于其GlcN-δ¹³C和GalN-δ¹³C。基于氨基糖在异养细菌和藻类中的分馏差异建立的FD_AS指数为解析有机质降解特征提供了另一有利工具。细菌再合成、初级生产以及陆源输入的共同作用导致了近海有机质中氨基糖δ¹³C相对离散的变化特征。

室内POM降解实验结果显示，随着浮游生物来源的有机质逐渐向细菌源有机质过渡，GlcN-δ¹³C和GalN-δ¹³C呈现先降低后增大的变化趋势，MurA-δ¹³C则在降解前期快速增大而后相对不变；相比之下，POC-δ¹³C则随POM降解程度的加深逐渐亏损。这一变化特征与细菌在降解过程中优先利用δ¹³C相对偏正的碳水化合物和氨基酸等活性物质有关。室内纯培养实验结果显示藻类来源的GlcN-δ¹³C高于其GalN-δ¹³C且两者差值较大，而异养细菌来源的GlcN-δ¹³C与其GalN-δ¹³C则较为接近，表明了两种氨基糖在藻类中可能具有相异的合成路径，而异细菌则可能具有相似的合成途径。相比之下，异养细菌MurA-δ¹³C普遍低于其GlcN-δ¹³C和GalN-δ¹³C，显示了其合成相对富集¹²C。基于异养细菌与藻类氨基糖δ¹³C分馏模式的差异，建立了氨基糖分馏差异指数FD_AS，发现其值在POM降解实验中随时间增大呈现逐渐降低的变化趋势，显示了其可作为潜在的有机质降解指标。

对长江-长江口-东海连续体中颗粒态氨基糖δ¹³C的研究发现其基本呈现向外海逐渐增大的变化特征。河道内氨基糖δ¹³C明显亏损，这可能与甲烷氧化菌和陆源土壤输入有关。在长江到东海的输运过程中，氨基糖逐渐由陆源有机质中δ¹³C亏损的组分过渡为海源有机质中δ¹³C相对偏正的组分，表明陆源有机质在河口区经历了快速的降解转化和埋藏过程。相比之下，受控于细菌再合成、初级生产以及陆源输入的共同作用，黄东海泥质区及长江口表层沉积物中氨基糖δ¹³C分布则相对离散。

（3）有机质的矿化驱动了黄海冷水团内低氧、酸化及营养盐的累积。冷水团内部具有低DO，低pH，高表观耗氧量（AOU）和高营养盐的特征，且水团内DO与pH之间的显著正相关性、AOU与POP/DIP和PON/DIN的显著负相关性均揭示了有机质的矿化再生伴随低氧和酸化的发生。水体层化为营养盐的再生和累积提供了有利条件，使冷水团成为南黄海重要的营养盐储库。冷水团的季节性消亡特征导致其内部营养盐经历释放-累积的循环过程，这一营养盐循环模式成为冷水团内低氧和酸化频发的重要“幕后推手”。

秋季冷水团主要位于50 m以深的南黄海中部区域，并且冷水团内存在明显的低溶解氧（DO）、低pH，高无机营养盐的特征。冷水团内AOU与硝酸盐和磷酸盐的显著正相关关系表明有机质的矿化是其内部营养盐重要来源且伴随低氧和酸化的发生。富营养化驱动南黄海藻华频发，大量有机质沉降至南黄海表层以下，加之冷水团低温高盐的特征使得水体交换缓慢，从而为有机质耗氧降解提供了良好场所。而冷水团这一相对孤立的特征为无机营养盐的再生及累积提供了有利条件，并随冷水团的逐渐发育不断累积，直至秋季达到峰值。定量估算结果表明，尽管冷水团体积仅占到南黄海的16.4%，但其溶解无机氮、磷酸盐和硅酸盐储量却分别达到整个南黄海的30.8%，52.1%和33.0%，与河流年输入量相当，表明冷水团在南黄海扮演着重要营养盐储库的角色。随着冬季水体混合的加剧，大量储存在冷水团中的营养盐得以释放，初步估算结果显示约12.0，1.0和15.0×10⁹ mol的溶解无机氮、磷酸盐和硅酸盐可进入到南黄海30 m水层以上，进而支撑浮游植物生长，再次导致次年冷水团区的酸化和低氧。

（4）黄东海沉积物中古菌甘油二烷基甘油四醚（GDGTs）构成及丰度指示了底层海水DO的变化。底层DO的降低导致GDGT-0相对丰度的降低并伴随Cren相对丰度的增加，其驱动机制可能与古菌自身适应，群落结构更替以及GDGTs降解有关，但底层DO对古温度代用指标TEX₈₆并无显著影响。沉积物中GDGTs对底层海水DO的快速响应显示其可作为潜在的氧化还原代表指标。

研究结果显示GDGTs两个主要组分的相对丰度（%GDGT-0和%Cren）分别与DO具有显著正相关和负相关关系，这可能是由于低DO条件下厌氧古菌回收利用了更多的GDGT-0。另一可能的原因可能是GDGTs的降解与生成同时存在，并且在高DO条件下Cren的降解速率相对GDGT-0更高。此外，GDGTs的环化指数（RI）与DO具有显著正相关关系，这可能是由于古菌在低氧条件下为降低能量损失，通过增加多环GDGTs来增加细胞膜致密性。尽管GDGTs构成及丰度在不同DO梯度下发生变化，但古温度代用指标TEX₈₆却保持相对恒定，表明TEX₈₆并不受底层DO影响。RI，%GDGT-0和GDGT-0/Cren与DO的强烈关系显示了这些参数可作为底层海水DO含量的潜在代用指标。对黄东海样品的分析发现，上述指标更适合用于表底海水温差大于2℃的相对稳定的水体环境中。由于外海环境中GDGTs影响因素复杂多样，需要进一步的研究来验证RI，%GDGT-0和GDGT-0/Cren对底层水体DO指示的潜力以及适用性。

As an important organic carbon reservoir in the global ecosystem, the cycle of marine organic matter exerts an important impact on the marine ecological environment and atmospheric carbon dioxide (CO₂) concentration. Although marginal seas only occupy less than 8% of the total ocean area, they contribute 80% of the marine organic carbon burial. Meanwhile, the hydrodynamic processes of marginal seas are complex and changeable, coupled with the dual effects of human activities and climate change, resulting in the sources and biogeochemical processes of organic matter in this area are extremely complicated. Therefore, exploring the source, degradation and transformation of organic matter in marginal seas and their control mechanisms is of great significance for better understanding of the key cycle processes of marine organic carbon and the changes of ecological environment.

This dissertation selects the Yellow Sea and East China Sea, which are seriously affected by human activities and have complex hydrodynamic processes, as the research region. The key biogeochemical processes such as the degradation and transformation of organic matter and its control mechanisms were investigated based on the large-scale field investigations combined with laboratory incubation experiments. In addition, organic molecules and their stable carbon isotopes, as well as the simultaneously obtained physical, hydrological and biochemical parameters were also used to systematically decipher the key biogeochemical processes of organic matter, such as the coupling relationship between the organic matter cycle and hypoxia, acidification. Moreover, the potential seawater oxygen proxies were explored. The main findings are as follows:

(1) Based on the results obtained from the composition and abundances of amino sugars, it was found that the bioactivity of particulate organic matter (POM) in the Yellow Sea and East China Sea was mainly controlled by its sources. River-derived POM was generally bio-refractory, while the POM bioactivity was higher in areas with higher primary productivity. Production of bioavailable POM could rapidly stimulate microbial activity, generating hot spots of heterotrophic alteration. The degradation and transformation of organic matter and its subsequent burial are shaped by hydrodynamic processes. Physical and hydrological conditions such as cyclonic eddy and cold water masses can promote the bacterial transformation of POM in the water column and reduce the bioactivity of POM, but are conducive to the long-term burial of sedimentary organic matter (SOM). In contrast, the rapid sedimentation of fresh POM in the water column and the intense resuspension process of the bottom layer reduce the burial efficiency of SOM.

A significant positive correlation between glucosamine/galactosamine (GlcN/GalN) ratio and Chl-a concentration was found in the 35°N section of the South Yellow Sea, indicating that primary production is an important regulator of POM bioreactivity. The GlcN/GalN ratio in the South Yellow Sea is generally low (<3), especially in the Yellow Sea Cold Water (YSCW) mass with an extremely low GlcN/GalN ratio (~0.7), indicating that POM has undergone extensive microbial degradation. Estimates based on muramic acid (MurA) indicated that, on average, ~13% of the particulate organic carbon (POC) in the South Yellow Sea was derived from bacterial contribution. In the nearshore area, due to the strong water mixing and the existence of cyclonic eddies above the cold water mass, the residence time of POM in the water column is increased, thereby promoting the bacterial reworking of POM, resulting in relatively high bacterial contribution (~25%). Extensive diagenesis in water column induced by hydrodynamic processes reduces biological carbon pump efficiency, but may assist the long-term carbon sequestration through various bacterial metabolic pathways. In contrast, POM concentrations in the main section of the Changjiang Estuary basically decreased with the increasing of salinity. However, in the moderate salinity zone, elevated concentrations of POM were observed due to higher primary productivity. The bioreactivity of POM in the Changjiang Estuary showed a trend of increasing first and then decreasing along the salinity gradient. Refractory POM in the Changjiang River is related to the highly degraded terrestrial soil organic matter. The higher bioavailability of POM in the middle salinity area is associated with the phytoplankton production, while in the open sea, the primary productivity declines due to the reduction of terrestrial nutrient supply, resulting in the lower POM reactivity. The average proportion of bacterial organic carbon in POC (12.35±8.67%) in the Changjiang Estuary was controlled by terrigenous input and primary production and showed a V-shaped distribution pattern along the salinity gradient.

Investigations of the surface SOM in the Yellow Sea and East China Sea found that sedimentary organic carbon (SOC) was dominated by indigenous production (~70%), and underwent extensive heterotrophic transformation. Calculations based on MurA showed that about 25% of the SOC in the Yellow Sea and East China Sea was contributed by bacteria. Distributions of SOC were significantly related to sediment grain size. The South Yellow Sea mud area (SYSMA) and East China Sea mud area (ECSMA) both represent hot spots for SOC burial, but the burial patterns are quite different. Due to the influence of the YSCW (low temperature, low oxygen) and cyclonic eddies in the SYSMA, the POC deposition rate in this area is slow, resulting in a high degree of POC degradation in the water column. These processes result in the buried organic carbon in the SYSMA being less reactive and tending to be preserved for a long time. By contrast, the sedimentation rate is relatively fast in the ECSMA, and the reactivity of the buried organic carbon is relatively high. However, the remineralization rate of SOC in the ECSMA is also relatively high due to the physical mobilization and the priming effect caused by the complex hydrodynamic processes. Burial of organic carbon shaped by the hydrodynamic processes resulted in the burial flux of SOC in the ECSMA was five times that of the SYSMA, but the burial efficiency in the two areas was similar (~35%).

(2) Based on the carbon isotope fractionation characteristics of amino sugars, it was found that the carbon isotopes of amino sugars in heterotrophic bacteria were controlled by the substrate carbon source, and bacteria preferentially used labile substances with the relatively positive δ¹³C in the degradation process. Compared with initial carbon source CO₂, the synthesis of algal amino sugars is relatively enriched in ¹²C. The variation characteristics of δ¹³C of GlcN and GalN in different heterotrophic bacteria and algae showed that the two amino sugar components have similar synthetic pathways in bacteria, but algae have relatively independent and specific synthetic pathways. In addition, the δ¹³C of bacteria-specific amino sugar MurA was significantly lower than that of bacterial GlcN and GalN. The FD_AS index, established based on the fractionation differences of amino sugars in heterotrophic bacteria and algae, provides another powerful tool to characterize the degradation of organic matter. The combined effect of bacterial resynthesis, primary production, and terrestrial input resulted in a relatively discrete pattern of the amino sugar δ¹³C in coastal organic matter.

The results of POM degradation experiments showed that with the gradual transformation from plankton-derived organic matter to bacterial organic matter, GlcN-δ¹³C and GalN-δ¹³C exhibited a trend of first decreasing and then increasing, while MurA-δ¹³C increased rapidly in the early stage of degradation and then relatively constant over the next several days. In contrast, POC-δ¹³C was gradually depleted with increasing POM degradation. This variation is related to the preferential utilization of bioactive substances such as carbohydrates and amino acids with relatively positive δ¹³C by bacteria during the degradation process. The results of pure culture experiments on phytoplankton and heterotrophic bacteria showed that the values of GlcN-δ¹³C derived from algae were higher than their GalN-δ¹³C and the difference between GlcN-δ¹³C and GalN-δ¹³C values was larger, while GlcN-δ¹³C values derived from heterotrophic bacteria were closer to their GalN-δ¹³C. This phenomenon indicates that GlcN and GalN may have different synthetic pathways in algae but similar synthetic pathways in bacteria. The MurA-δ¹³C of heterotrophic bacteria was generally lower than its GlcN-δ¹³C and GalN-δ¹³C, showing that the synthesis of MurA is relative enriched in ¹²C. Based on the difference in the fractionation patterns of amino sugar δ¹³C between heterotrophic bacteria and algae, the amino sugar fractionation difference index (FD_AS) was established. The values of FD_AS were found to gradually decrease with progressing POM degradation, indicating that FD_AS can be used as a potential organic matter degradation index.

The study on the δ¹³C values of particulate amino sugar in the Changjiang River-Estuary-East China Sea continuum found that they basically showed a trend of gradually increasing towards the sea. The amino sugar δ¹³C was obviously depleted in the river channel, which may be related to the input of terrigenous soil and production of methanotrophs. During the transportation from the Changjiang River to the East China Sea, the amino sugars gradually transitioned from the depleted δ¹³C component in the terrigenous organic matter to the relatively positive δ¹³C component in the marine organic matter, indicating that the terrigenous organic matter undergo rapid degradation, transformation and burial in the estuary area. In comparison, controlled by the combined effects of bacterial resynthesis, primary production and terrigenous input, distributions of amino sugar δ¹³C in surface sediments of the SYSMA, ECSMA and Changjiang Estuary were relatively discrete.

(3) The mineralization of organic matter drives the hypoxia, acidification and nutrient accumulation in the YSCW. The cold water mass is characterized by low dissolved oxygen (DO) and low pH, but high apparent oxygen utilization (AOU) and high nutrients. In addition, there was a significant positive correlation between DO and pH, and a significant negative correlation between AOU and POP/DIP and PON/DIN, indicating that the mineralization of organic matter in the YSCW was accompanied by the occurrence of hypoxia and acidification. Water stratification provides favorable conditions for the regeneration and accumulation of nutrients, making the YSCW an important nutrient reservoir in the South Yellow Sea. The seasonal demise of the YSCW leads to a nutrient release-accumulation cycle within the cold water mass. This nutrient cycling pattern forms an important driving force behind hypoxia and acidification in the YSCW.

The results showed that the cold water mass in autumn was mainly located in the central area of the South Yellow Sea with a depth of more than 50 m. Low DO and low pH, but high concentrations of inorganic nutrients were found in the cold water mass. AOU in YSCW was significantly positively correlated with nitrate and phosphate, suggesting that mineralization-driven regeneration is an important source of nutrients in YSCW, accompanied by hypoxia and acidification. Eutrophication drives the frequent occurrence of algal blooms in the South Yellow Sea, and a large amount of organic matter settles below the surface layer. In addition, the cold water mass is characterized by low temperature and high salinity, which makes the water exchange slow, thus providing an incubator for the oxygen-consuming degradation of organic matter. The relatively isolated feature of the cold water mass provides favorable conditions for the regeneration and accumulation of inorganic nutrients. The accumulation of nutrients continues with the gradual development of the cold water mass until it reaches the peak in autumn. Quantitative estimation results show that although the volume of cold water mass only accounts for 16.4% of the South Yellow Sea, its dissolved inorganic nitrogen, phosphate and silicate reserves reach 30.8%, 52.1% and 33.0% of the entire South Yellow Sea, respectively, which is comparable to that of the annual river input, indicating that the cold water mass plays an important nutrient storage role in the South Yellow Sea. With the intensification of water mixing in winter, a large amount of nutrients stored in the cold water mass are released. Preliminary estimates show that about 12.0, 1.0 and 15.0 × 10⁹ mol of dissolved inorganic nitrogen, phosphate and silicate can enter the upper layer (~30 m), which in turn supports the growth of phytoplankton, leading to acidification and hypoxia in the cold water mass the following year.

(4) The composition and abundance of archaeal glycerol dialkyl glycerol tetraethers (GDGTs) in the Yellow Sea and East China Sea sediments indicate changes in bottom seawater DO. The decrease of bottom DO leads to the decrease of the relative abundance of GDGT-0 and the increase of the relative abundance of Cren. The driving mechanism may be related to the archaeal adaptation, shift of community structure and the degradation of GDGTs. However, the bottom DO has a limited impact on the paleotemperature proxy TEX₈₆. The rapid response of GDGTs in sediments to bottom seawater DO suggests that they can be used as potential redox proxies.

Significant positive and negative correlations with DO were found for %GDGT-0 and %Cren, respectively. Potential mechanisms for this phenomenon may be that anaerobic archaea recycle more GDGT-0 under low DO condition. Another possible interpretation is that the degradation and production of GDGTs coexist, and the degradation rate of Cren is higher than that of GDGT-0 under high DO conditions. In addition, the ring index (RI) of GDGTs had a significant positive correlation with DO, which may be due to the fact that archaea increase cell membrane density by increasing polycyclic GDGTs in order to reduce energy loss under low DO conditions. Although the composition and abundance of GDGTs changed under different DO gradients, the paleotemperature proxy TEX₈₆ remained relatively constant, indicating that TEX₈₆ was not affected by the bottom DO. The strong relationships of RI, %GDGT-0 and GDGT-0/Cren with DO suggest that these parameters could serve as potential proxies for bottom seawater DO content. Through the analysis of samples from the Yellow Sea and the East China Sea, it is found that the above indicators are more suitable for the relatively stable environment where the temperature difference between the surface and bottom seawater is greater than 2°C. Due to the complex and diverse influencing factors of GDGTs in the open sea environment, further researches are needed to verify the potential and applicability of RI, %GDGT-0 and GDGT-0/Cren for bottom DO proxies.