中国科学院海洋研究所机构知识库

颗粒有机物（Particulate organic matter，POM）在海洋有机质的生物地球化学循环中占有重要地位。在循环过程中，POM被细菌降解，部分被细菌吸收、利用并转化为细菌有机质进入微食物环。随着细菌对POM的降解，颗粒有机质的活性逐渐降低。氨基酸作为重要的有机质储库，其含量、组成和单体同位素的变化可有效指示细菌对有机质的降解以及再合成过程。本研究选取长江口及其邻近海域和西太平洋海山区作为研究海域，分析不同海域颗粒态氨基酸含量、组成及其单体同位素的变化特征，结合水文环境要素、有机质参数以及细菌相关参数，探讨了有机质的降解状态与循环过程，并综合分析不同环境因素对有机质降解的影响。同时，在实验室进行了颗粒物降解的模拟培养实验，与现场调查相结合，系统分析了有机质的降解过程。本研究获得的主要结果与认识如下：

（1）自长江河水端、河口到外海，表层水体颗粒有机质具有不同的降解程度。河水端颗粒有机质经历了显著的细菌降解与再改造，且以细菌和维管植物的有机质来源为主。河口区表层水体具有浮游植物来源的高浓度新鲜有机质，但近口门海域底层水体由于沉积物再悬浮导致有机质呈现难降解特征。氨基酸单体同位素也指示了河口区表层水体中颗粒有机质来源以浮游植物为主，而底层水体中有机质来源于细菌再合成有机质和陆源维管植物。在河口区底层水体中，降解指示因子与表观耗氧量（Apparent oxygen utilization，AOU）具有显著相关关系，表明颗粒物有机质降解消耗了大量溶解氧，加之水体层化从而造成了低氧。

有机质降解指示因子指示了河水端颗粒有机质主要受到陆源难降解有机质输入的影响，活性相对较低。河水端的表层和底层颗粒物具有相对较高的营养级指标（TP_Tr-Sr）和异养细菌再合成指标（ΣV）指示河水端有机质经过了细菌深度降解与再合成。同时，在人类活动以及硝化作用的影响下，河水端具有相对较高的“营养级”氨基酸和“来源型”氨基酸的同位素值（δ¹⁵N_Tr-AA和δ¹⁵N_Sr-AA）。必需氨基酸的单体碳同位素（δ¹³C_EAA）的线性判别分析（Linear Discriminant Analysis，LDA）结果显示河水端颗粒有机质具有细菌和维管植物两种混合有机质来源。由于近口门海域的底层水体中难降解沉积物的再悬浮，导致底层水体具有较高浓度的难降解有机质。

河口区上层水体受到长江冲淡水的影响，较高丰度的浮游植物向长江口表层水体输入了高浓度且高活性的颗粒有机质。河口区与外海表层颗粒物具有相对较低的TP_Tr-Sr和ΣV，且δ¹³C_EAA的LDA结果显示河口与外海颗粒有机质以微藻为主要有机质来源，均表明了在河口与外海水体中新鲜浮游植物是颗粒有机质的重要来源。在河口与外海区，随着深度的增加，颗粒有机质被异养细菌降解，有机质含量降低，降解程度升高。相对表层，由于底层水体中细菌的降解与再合成使颗粒物具有相对较高的TP_Tr-Sr和ΣV，与细菌的密切相关。δ¹³C_EAA的LDA结果同样表明底层水体中颗粒有机质受到细菌有机质和陆源维管植物有机质混合来源的影响。此外，降解指示因子与AOU均呈现显著相关关系，指示了河口区底层水体中有机质降解消耗氧气，有机质活性降低，再悬浮的难降解有机质与现场降解的有机质共同组成了底层水体中的难降解颗粒有机质。底层水体中相对较高的ΣV指示了较强的异养细菌的再合成过程，进一步证实了低氧产生的原因是细菌对有机质的耗氧降解过程。

（2）随着生源颗粒有机质的降解，有机质以及氨基酸的含量迅速降低，有机质活性也随之迅速降低，多种降解指示因子在不同的降解阶段的灵敏度具有差异。降解初期，氨基酸的单体氮同位素（δ¹⁵N_AA）的富集以及ΣV的升高指示了细菌对颗粒有机质的降解与再合成过程。降解后期δ¹⁵N_AA与ΣV的回落与培养体系中细菌对氨基酸的选择性保存以及有限的有机质来源有关。降解初期，异养细菌分泌胞外肽酶促进了有机质的降解，且拟杆菌有机质对总有机质在降解中期具有较高贡献。

颗粒有机质降解培养实验中，颗粒有机质含量迅速降低。随着颗粒有机质的降解，颗粒有机质的活性迅速降低，氨基酸的碳、氮归一化产率（PAA-C%、PAA-N%）和反应活性指数（reactivity index，RI）在降解初期迅速降低。降解因子（degradation index，DI）在降解中期出现较明显的降低趋势。D型氨基酸占比（mol% D-AA）在降解全阶段均有升高趋势。降解初期，异养细菌分泌了大量胞外肽酶降解颗粒有机质，从而导致有机质的含量和活性迅速降低。拟杆菌在降解中期呈现较高的相对丰度，同时有机质活性呈现短时间的升高且细菌贡献率升高，指示了拟杆菌有机质对颗粒有机质的贡献。在降解初期ΣV迅速升高，指示了细菌对颗粒有机质的再合成过程。降解后期δ¹⁵N_AA与ΣV的回落与培养体系中细菌对氨基酸的选择性保存以及有限的有机质来源有关。有机质降解对大部分氨基酸单体碳同位素（δ¹³C_AA）的变化相对较小，δ¹³C_AA不同的变化趋势与异养细菌的选择性改造有关。

（3）长江口及邻近海域水动力条件显著影响了颗粒有机物的活性与降解程度。长江冲淡水营养盐的输入导致了该海域产生较高浓度的新鲜有机质，黄海冷水团（YSCW）以及东海冷涡（CEECS）温跃层上方为高初级生产力区域，POM活性较高，上升流和较弱的水动力过程提供了较长的停留时间和稳定的降解环境，导致温跃层下方POM活性大大降低。台风过境后CJ断面远海端垂直混合均匀并诱发了浮游植物水华，使得有机质含量和其活性升高，而受台风影响更为显著的DH断面尽管垂直混合均匀，但未暴发水华，致使颗粒有机质含量和活性均保持较低水平。

春季和秋季长江口颗粒有机质的含量与分布均呈现不同趋势，其中原位浮游植物产生的新鲜有机质是重要的颗粒有机质来源。春季外海区域受到YSCW和CEECS两个冷涡的影响，上层水体中产生较高浓度的新鲜颗粒有机质，而底层水体中颗粒有机质的降解程度相对较高。在秋季，YSCW和台风“泰利”的影响主要控制了长江口颗粒有机质的循环过程。YSCW以及长江冲淡水促进了3300断面远岸产生较高浓度的新鲜有机质以及营养盐再生过程。台风“泰利”促进了远岸CJ-4站位高活性颗粒有机质含量的升高。然而，台风对DH断面的影响与CJ-4不同，由于台湾暖流的输入，DH断面的营养盐浓度和有机质浓度相对较低，且有机质活性相对较低。虽然台风使DH断面的水体更为均匀，但营养盐的缺乏限制了浮游植物的生长和新鲜有机质的产生，因此台风对DH断面的颗粒有机质循环的影响相对较小。

（4）西太平洋水体中颗粒有机质的含量显著低于长江口水体，但上层水体中仍具有与长江口高生产力海域较为接近的有机质降解状态。上层水体中颗粒有机质含量较高且具有较高活性，随着有机质的降解，有机质含量与活性快速降低。在最小含氧带（Oxygen Minimum zone，OMZ）上部的200 m处颗粒有机物已呈现出明显的细菌降解和再合成过程，而在OMZ内部整体较低且稳定的有机质活性指示了颗粒有机质的降解耗氧可能并非是维持西太平洋OMZ的主要因素。

西太平洋海山区上层水体中，浮游植物产生新鲜的颗粒有机质，随着水深的增加，新鲜有机质贡献降低，异养细菌对有机质进行降解并通过再合成细菌有机质对颗粒有机质产生重要贡献。随着深度的增加，δ¹⁵N_Glu和δ¹⁵N_Phe呈现升高的趋势，但由于有机质来源缺乏以及低δ¹⁵N_AA的细菌有机质的附着在颗粒物上，100 m水层以下颗粒有机质中δ¹⁵N_Glu、δ¹⁵N_Phe和ΣV呈现降低趋势。此外，200 m水层中ΣV高于2，且具有较高的细菌贡献率，共同指示了异养细菌的再合成过程。δ¹³C_EAA的LDA分析结果也指示了大部分颗粒有机质的来源以初级生产者藻类为主，然而200 m和300 m水层中细菌有机质是重要的颗粒有机质来源。OMZ区域中颗粒有机质的活性相对较低且基本保持稳定，指示了颗粒有机质的降解耗氧对OMZ的形成贡献较小。

Particulate organic matter (POM) plays an important role in the biogeochemical cycling of marine carbon. During the cycling of POM, POM is degraded by bacteria, and part of those is absorbed and transformed by bacteria into bacterial organic matter and then into the microbial food loop. Through the degradation of POM via bacteria, the bioavailability of organic matter gradually decreased. The degradation degree of organic matter could be indicated by the changes in the contents, compositions and compound-specific isotopes of amino acid, which is an important reservoir of particulate organic matter. In this study, the Yangtze River estuary and its adjacent area and the seamount area in the Western Pacific Ocean were selected as the study areas. The hydrological and environmental characteristics, organic matter parameters and bacteria-related parameters of the study area were investigated. The particulate amino acids, compound-specific isotopic analysis of amino acids, the degradation state and cycling process of organic matter were explored. And the effects of different environmental factors on organic matter degradation were analyzed comprehensively. Incubation experiment on the degradation of particulate matter was also carried out in the laboratory, which was combined with field investigations to systematically analyze the degradation process of organic matter. The main results and insights obtained from this study are as follows:

(1) From Changjiang River to estuary and offshore water, the POM in the surface waters had significantly different degradation states. The POM in the river was degraded and resynthesized by bacteria, and POM predominantly originated from bacteria and vascular plants. A large amount of fresh POM from phytoplankton was observed in the surface water of the estuary, while POM in the bottom waters of the estuary was highly degraded due to sediment resuspension. The compound-specific isotopes of amino acids also indicated that POM in the surface water of the estuary was dominated by phytoplankton organic matter, while POM in the bottom waters of the estuary was dominated by organic matter from bacterial resynthesis and terrestrial vascular plants. In the bottom water of the estuary, the degradation indicators had significant correlations with apparent oxygen utilization (AOU), which indicated that degradation of POM consumed a large amount of dissolved oxygen, along with the stratification of the water column, resulting in hypoxia.

The organic matter degradation indicators indicated the relatively highly altered POM in the river, which was mainly influenced by the input of refractory organic matter from terrestrial sources. The relatively high trophic level indicator (TP_Tr-Sr) and the heterotrophic bacterial resynthesis indicator (ΣV) in the surface and bottom layers of the river indicated that organic matter in the river underwent deeply bacterial degradation and resynthesis. Meanwhile, the river had relatively high isotopic values of “trophic” and “source” amino acids (δ¹⁵N_Tr-AA and δ¹⁵N_Sr-AA) under the influence of human activities and nitrification. The Linear Discriminant Analysis (LDA) results of the monomeric carbon isotope of the essential amino acids (δ¹³C_EAA) indicated that the POM originated from both bacteria and vascular plants in the river. Particulate organic matters showed high contents in the bottom water and relatively low availabilities due to the resuspension of refractory sediments in the bottom waters near the entrance of the Changjiang River.

The upper water of the estuary was highly influenced by CDW and had high phytoplankton biomass which input a large amount of fresh POM to the surface water of the Changjiang estuary. The relatively low TP_Tr-Sr and ΣV of POM in surface water of estuary and offshore water and LDA results of δ¹³C_EAA indicated that microalgae were the main source of POM in the estuarine and offshore waters, and fresh phytoplankton organic matter was an important source of POM in the estuarine and offshore waters. In the estuarine and offshore water, the POM was degraded by heterotrophic bacteria, contents of POM decreased with increasing depth, and the degree of degradation of POM increased. Compared to the surface layer, the particulate matter had relatively higher TP_Tr-Sr and ΣV in the bottom layer, which was closely related to bacterial degradation and resynthesis. The LDA results for δ¹³C_EAA indicated that POM in the bottom water was influenced by a mixed source of bacterial organic matter and terrestrial vascular plant organic matter. In addition, the degradation indicators all showed significant correlations with AOU, indicating that the degradation of organic matter in the bottom water column of the estuary consumed dissolved oxygen, and the bioavailability of organic matter activity decreased. The resuspended refractory organic matter together with in situ degraded organic matter comprised the refractory POM in the bottom water. The relatively high ΣV in the bottom water column indicated heterotrophic bacterial resynthesis, furtherly confirming that the hypoxia was induced by the oxygen-consuming degradation process of organic matter by bacteria.

(2) With the degradation of the biogenic POM, the concentrations of POM and particulate amino acid (PAA) decreased rapidly, and the bioavailability of POM decreased. The different degradation indicators had different sensitivities at different stages of degradation. In the early stage of degradation, the enrichment of the monomeric nitrogen isotope of amino acids (δ¹⁵N_AA) and the increase of ΣV indicated the degradation and resynthesis of POM by bacteria. The decrease of δ¹⁵N_AA and ΣV in the later stages of degradation was related to the selective conservation of amino acids by bacteria in the culture system and the limited source of organic matter. In the early stages of degradation, heterotrophic bacteria secreted extracellular peptidases to promote organic matter degradation, and organic matter of Bacillus had a high contribution to POM in the middle stages of degradation.

The POM concentrations decreased rapidly in the POM degradation incubation experiment. With the degradation of POM, the reactivity of POM decreased rapidly, and the carbon and nitrogen normalized yields (PAA-C%, PAA-N%) and reactivity index (RI) of amino acids decreased rapidly at the beginning of degradation. The overall change in degradation index (DI) was relatively small, with a decrease in the middle stage of degradation. Mol% D-AA tended to increase throughout all stages of degradation. In the early stages of degradation, heterotrophic bacteria secreted large amounts of extracellular peptidases to degrade the POM, which led to a rapid decrease in organic matter content and bioavailability. The higher relative abundance of Bacillus in mid-degradation was also correlated with a short period of elevated activity of organic matter and an elevated bacterial contribution, indicative that the contribution of Bacillus to the POM. The ΣV increased in the early stages of degradation, indicating the bacterial resynthesis of the POM. The decrease of δ¹⁵N_AA and ΣV in the later stages of degradation was related to the selective conservation of amino acids by bacteria in the culture system and the limited source of organic matter. Organic matter degradation showed relatively little change in the carbon isotope of most amino acid monomers (δ¹³C_AA), and the different trends in δ¹³C_AA were associated with selective modification by heterotrophic bacteria.

(3) Hydrodynamic conditions in the Changjiang Estuary and adjacent waters significantly affect the activity and degradation of POM. The input of nutrients from the CDW led to the production of higher concentrations of fresh organic matter in the area. The thermocline above Yellow Sea Cold Water (YSCW) and Cold eddy in the East China Sea (CEECS) were areas of high primary productivity with high POM activity. The upwelling and weaker hydrodynamic processes provided a longer residence time and a stable degradation environment, resulting in a significant reduction in POM activities below the thermocline. After the typhoon, vertical mixing induced the phytoplankton bloom, resulting in higher organic matter contents and bioavailability in offshore water of section CJ. Conversely, section DH was more significantly affected by the typhoon than section CJ, leading to the uniform vertical mixing. However, bloom did not occur in section DH, resulting in lower POM contents and bioavailability.

The contents and distributions of POM in the Changjiang Estuary showed different distributions in spring and autumn, with fresh organic matter produced by phytoplankton being an important source of organic matter. In spring, the offshore area was influenced by two cold eddies, the YSCW and the CEECS, which produced higher concentrations of fresh POM in the upper water column, while the degradation of POM in the bottom water column was relatively high. In autumn, the influence of the cold eddies of the YSCW and the typhoon “Talim” mainly controlled the circulation of POM in the study area. The YSCW and CDW promoted the phytoplankton bloom in the offshore area of section 3300, and higher concentrations of fresh POM and nutrient regeneration processes were found in offshore water, and there was a clear bacterial response to the concentration and degradation of POM. Typhoon “Talim” promoted phytoplankton blooms at station CJ-4, resulting in higher POM contents and relatively high POM activities. However, the impact of the typhoon on the DH section was different from that of CJ-4. The concentrations of nutrients and POM in the DH section were relatively low and the POM was highly degraded due to the input of the warm Taiwan current. Although the typhoon made the water column in the DH section more homogeneous, the lack of nutrients limited the growth of phytoplankton and the production of fresh organic matter, so the typhoon had a relatively small effect on the POM cycle in the DH section.

(4) The concentrations of POM in the Western Pacific Ocean were significantly lower than those in the Changjiang Estuary, but the degradation states in upper water were similar to the POM in Changjiang Estuary with high production. In upper water, high concentrations of fresh POM were observed, and POM concentrations and bioavailability decreased rapidly with the degradation of organic matter. POM in the 200 m depth of water above oxygen minimum zone (OMZ) had shown significant bacterial degradation and resynthesis processes. While the low concentrations of highly degraded POM in the OMZ suggested that the maintenance of the OMZ in the western Pacific may not be induced by oxygen depletion by particulate organic matter degradation.

In the upper waters of the M5 seamount of the Western Pacific Ocean, phytoplankton produced fresh POM. With increasing water depth, the contribution of fresh organic matter decreased and heterotrophic bacteria degraded organic matter and make an important contribution to POM through the resynthesis of bacterial organic matter. With depth increasing, δ¹⁵N_Glu and δ¹⁵N_Phe showed an increasing trend, but δ¹⁵N_Glu, δ¹⁵N_Phe and ΣV showed lower values in the POM of the 200 m and 300 m depth due to the lack of organic matter sources and the attachment of low δ¹⁵N_AA bacterial organic matter to the particulate matter. In addition, ΣV above 2 in the 200 m water layer indicated a higher bacterial contribution, and also a heterotrophic bacterial resynthesis process. LDA analysis of δ¹³C_EAA results also indicated that the source of most of the POM was dominated by primary producer microalgae, however bacterial organic matter was an important source of POM in the 200 m and 300 m water layers. The relatively low concentrations of highly degraded POM in the OMZ region indicated that oxygen consumption by the degradation of particulate organic matter contributed a small contribution to the maintenance of the OMZ.