Dynamics and characterization of refractory dissolved organic matter produced by a pure bacterial culture in an experimental predator-prey system

We studied the effects of a bacterium (Pseudomonas chlororaphis) and a bactivorous protozoan (Uronema sp.) on transformations of labile dissolved organic carbon (DOC). In 36-day time series experiments, bacteria were grown on glucose both with and without protozoa. We measured bulk organic carbon pools and used electrospray ionization mass spectrometry to characterize dissolved organic matter on a molecular level. Bacteria rapidly utilized glucose, depleting it to nondetectable levels and producing new DOC compounds of higher molecular weight within 2 days. Some of these new compounds, representing 3 to 5% of the initial glucose-C, were refractory and persisted for over a month. Other new compounds were produced and subsequently used by bacteria during the lag and exponential growth phases, pointing to a dynamic cycling of organic compounds. Grazers caused a temporary spike in the DOC concentration consisting of labile compounds subsequently utilized by the bacteria. Grazing did not increase the complexity of the DOC pool already established by the bacteria but did continually decrease the particulate organic carbon pool and expedited the conversion of glucose-C to CO2. After 36 days, 29% of initial glucose-C remained in pure bacteria cultures, while only 6% remained in cultures where a grazer was present. In this study the bacteria were the primary shapers of the complex DOC continuum, suggesting higher trophic levels possibly have less of an impact on the qualitative composition of DOC than previously assumed.     

Bacterial Growth in Mixed Cultures on Dissolved Organic Carbon from Humic and Clear Waters

Interactions between bacterial assemblages and dissolved organic carbon (DOC) from different sources were investigated. Mixed batch cultures were set up with water from a humic and a clear-water lake by a 1:20 dilution of the bacterial assemblage (1.0 μm of prefiltered lake water) with natural medium (sterile filtered lake water) in all four possible combinations of the two waters and their bacterial assemblages. Bacterial numbers and biomass, DOC, thymidine incorporation, ATP, and uptake of glucose and phenol were followed in these cultures. Growth curves and exponential growth rates were similar in all cultures, regardless of inoculum or medium. However, bacterial biomass produced was double in cultures based on water from the humic lake. The fraction of DOC consumed by heterotrophic bacteria during growth was in the same range, 15 to 22% of the total DOC pool, in all cultures. Bacterial growth efficiency, calculated from bacterial biomass produced and DOC consumed, was in the order of 20%. Glucose uptake reached a peak during exponential growth in all cultures. Phenol uptake was insignificant in the cultures based on the clear-water medium, but occurred in humic medium cultures after exponential growth. The similarity in the carbon budgets of all cultures indicated that the source of the bacterial assemblage did not have a significant effect on the overall carbon flux. However, fluxes of specific organic compounds differed, as reflected by glucose and phenol uptake, depending on the nature of the DOC and the bacterial assemblage.     

Dissolved and particulate organic matter in contrasting Zostera marina (eelgrass) sediments

The influence of seagrass Zostera marina on sediment characteristics was examined in two contrasting sediments, one organic-rich and one organic-poor. The presence of plants leads to reduced sediment redox potential in both sediment types compared to bare sediment with the largest effects in the organic-poor sediment. Z. marina stimulated the sulfate reduction rates in organic-poor sediment with ∼50% and higher pools of dissolved organic carbon (DOC) were found. In contrast, sulfate reduction rates were lower in vegetated compared to bare sites in the organic-rich sediment. Despite a low contribution of dissolved carbohydrate (DCHO) to the DOC pool (<5%), the seagrass vegetation was responsible for an increase of ∼50% in DCHO pools with a peak in the root zone suggesting that Z. marina supplied DCHO to the pore waters. The Z. marina meadows also enhanced the contribution of particulate carbohydrate (PCHO) to sedimentary particulate organic carbon (POC) pools by 6-14% compared to bare sediment. Although the PCHO pools were higher in organic-rich than organic-poor sediments, the analyses of carbohydrate composition revealed that three groups of neutral sugars including glucose, galactose and mannose+xylose were the major compounds of PCHO and contributed with >60% to sedimentary carbohydrate pools at both sites. Only glucose showed depletion with depth in the vegetated sediments, whereas the percentage of ribose and rhamnose increased indicating a selective degradation of labile carbohydrates in the meadows. Galactose and mannose+xylose appeared to represent a refractory part of carbohydrate that remained after degradation of the more labile components. The sugar content was rather constant with depth at the bare organic-rich sediment indicating that only recalcitrant carbohydrate pools were buried. There was less difference in the PCHO composition profiles between vegetated and bare organic-poor sediments.     

Bacterial consumption of total and dissolved organic carbon in the Great Barrier Reef

Heterotrophic bacteria typically take up directly dissolved organic matter due to the small molecular size, although both particulate and dissolved organic matter have labile (easily consumed) compounds. Tropical coastal waters are important ecosystems because of their high productivity. However, few studies have determined bacterial cycling (i.e. carbon uptake by bacteria and allocation for bacterial biomass and respiration) of dissolved organic carbon in coastal tropical waters, and none has determined bacterial cycling of total and dissolved organic carbon simultaneously. In this study we followed bacterial biomass and production, and organic carbon changes over short-term (12 days) dark incubations with (total organic carbon, TOC) and without particulate organic carbon additions (dissolved organic carbon, DOC). The study was performed at three sites along the middle stretch of the Great Barrier Reef (GBR) during the dry and wet seasons. Our results show that the bacterial growth efficiency is low (0.1–11.5%) compared to other coastal tropical systems, and there were no differences in the carbon cycling between organic matter sources, seasons or locations. Nonetheless, more carbon was consumed in the TOC compared to the DOC incubations, although the proportion allocated to biomass and respiration was similar. This suggests that having more bioavailable substrate in the particulate form did not benefit bacteria. Overall, our study indicates that when comparing the obtained respiration rates with previously measured primary production rates, the GBR is a heterotrophic system. More detailed studies are required to fully explore the mechanisms used by bacteria to cycle TOC and DOC in tropical coastal waters.

     

Impacts of Labile Organic Carbon Concentration on Organic and Inorganic Nitrogen Utilization by a Stream Biofilm Bacterial Community

In aquatic ecosystems, carbon (C) availability strongly influences nitrogen (N) dynamics. One manifestation of this linkage is the importance in the dissolved organic matter (DOM) pool of dissolved organic nitrogen (DON), which can serve as both a C and an N source, yet our knowledge of how specific properties of DOM influence N dynamics are limited. To empirically examine the impact of labile DOM on the responses of bacteria to DON and dissolved inorganic nitrogen (DIN), bacterial abundance and community composition were examined in controlled laboratory microcosms subjected to various combinations of dissolved organic carbon (DOC), DON, and DIN treatments. Bacterial communities that had colonized glass beads incubated in a stream were treated with various glucose concentrations and combinations of inorganic and organic N (derived from algal exudate, bacterial protein, and humic matter). The results revealed a strong influence of C availability on bacterial utilization of DON and DIN, with preferential uptake of DON under low C concentrations. Bacterial DON uptake was affected by the concentration and by its chemical nature (labile versus recalcitrant). Labile organic N sources (algal exudate and bacterial protein) were utilized equally well as DIN as an N source, but this was not the case for the recalcitrant humic matter DON treatment. Clear differences in bacterial community composition among treatments were observed based on terminal restriction fragment length polymorphisms (T-RFLP) of 16S rRNA genes. C, DIN, and DON treatments likely drove changes in bacterial community composition that in turn affected the rates of DON and DIN utilization under various C concentrations.     

Constant release of photosynthate from marine phytoplankton.

The release rate of dissolved organic carbon (DOC) by unialgal cultures and natural phytoplankton assemblages was constant over a wide range of dissolved inorganic carbon concentrations. DOC release was not proportional to the particulate organic carbon production rate. We postulate that intracellular DOC, fated for release, exists either as a separate pool from that leading to particulate organic carbon production or that there is active metabolic control on one portion of a common pool.     

Compositions isotopiques naturelles des bactéries hétérotrophes et détermination de l'origine du carbone organique dissous biodisponible

Several studies of salt marsh systems have attempted to quantify the flow of organic matter between the land and coastal waters. However, the techniques used could not identify sources of dissolved organic carbon (DOC) rapidly assimilated by heterothrophic bacteria. Recently, the assay of carbon isotope ratios has allowed characterization of the different sources of organic matter in salt marshes. In this study, we wanted to find out if the natural isotopic composition assayed in heterotrophic bacteria distinguished the origin of bioavailable DOC. We determined the δ¹³C values for 1) three bacterial strains and their nucleic acids cultured on glucose and tryptose substrates, respectively, and 2) naturally occurring bacteria recovered from seawater in which salt marsh vegetation had been immersed. First, we showed that the isotopic fractionation was the same for the three bacterial strains cultured on the same synthetic substrate, but could vary depending on the nature of DOC. There was no significant difference between the δ¹³ C values of bacteria and their nucleic acids. Second, natural bacteria were grown in a medium enriched in DOC from halophytic plants. The δ¹³C values of this community were close to those of dissolved organic carbon from plant leachates. The comparison between the isotopic ratios of natural bacteria in Vibrio alginolyticus showed that the heterogeneity of the bacterial community averaged the isotopic fractionation from the preferential assimilation of organic compounds in the medium by each bacterial strain. The δ¹³ C values recorded for the bacterial community in the field and their nucleic acids made it possible to identify the source of organic matter readily accessible to microorganisms in a coastal ecosystem.     

Lability of organic carbon in lakes of different trophic status

1. We used first‐order kinetic parameters of biological oxygen demand (BOD), the constant of aerobic decomposition (k) and the asymptotic value of BOD (BOD_ult), to characterise the lability of organic carbon pools in six lakes of different trophic state: L. Naroch, L. Miastro and L. Batorino (Belarus), L. Kinneret (Israel), L. Ladoga (Russia) and L. Mendota (U.S.A.). The relative contributions of labile and refractory organic carbon fractions to the pool of total organic carbon (TOC) in these lakes were quantified. We also determined the amounts of labile organic carbon within the dissolved and particulate TOC pools in the three Belarus lakes. 2. Mean annual chlorophyll concentrations (used as a proxy for lake trophic state) ranged from 2.3 to 50.6 μg L⁻¹, labile organic carbon (OC_L = 0.3BOD_ult) from 0.75 to 2.95 mg C L⁻¹ and k from 0.044 to 0.14 day⁻¹. 3. Our data showed that there were greater concentrations of OC_L but lower k values in more productive lakes. 4. In all cases, the DOC fraction dominated the TOC pool. OC_L was a minor component of the TOC pool averaging about 20%, irrespective of lake trophic state. 5. In all the lakes, most (c. 85%) of the DOC pool was refractory, corresponding with published data based on measurements of bacterial production and DOC depletion. In contrast, a larger fraction (27–55%) of the particulate organic carbon (POC) pool was labile. The relative amount of POC in the TOC pool tended to increase with increasing lake productivity. 6. Long‐term BOD incubations can be valuable in quantifying the rates of breakdown of the combined particulate and dissolved organic carbon pools and in characterising the relative proportions of the labile and recalcitrant fractions of these pools. If verified from a larger number of lakes our results could have important general implications.     

Hydrologic Drivers and Seasonality of Dissolved Organic Carbon Concentration, Nitrogen Content, Bioavailability, and Export in a Forested New England Stream

We present the results of a full year of high-resolution monitoring of hydrologic event-driven export of stream dissolved organic matter (DOM) from the forested Bigelow Brook watershed in Harvard Forest, Massachusetts, USA. A combination of in situ fluorescent dissolved organic matter (FDOM) measurement, grab samples, and bioassays was utilized. FDOM was identified as a strong indicator of concentration for dissolved organic carbon (DOC, r ² = 0.96), dissolved organic nitrogen (DON, r ² = 0.81), and bioavailable DOC (BDOC, r ² = 0.81). Relationships between FDOM and concentration were utilized to improve characterization of patterns of hydrological event-driven export and the quantification of annual export. This characterization was possible because DOM composition remained relatively consistent seasonally; however, a subtle shift to increased fluorescence per unit absorbance was observed for summer and fall seasons and percent BDOC did increase slightly with increasing concentrations. The majority of export occurred during pulsed hydrological events, so the greatest impact of bioavailable exports may be on downstream aquatic ecosystems. Export from individual events was highly seasonal in nature with the highest flow weighted mean concentrations (DOC_FW) being observed in late summer and fall months, but the highest total export being observed for larger winter storms. Seasonal trends in DOC export coincide with weather driven changes in surface and subsurface flow paths, potential for depletion and rebuilding of a flushable soil organic matter pool, and the availability of terrestrial carbon sources such as leaf litter. Our approach and findings demonstrate the utility of high frequency FDOM measurement to improve estimates of intra-annual temporal trends of DOM export.     

Interactions between aquatic bacteria and an aquatic hyphomycete on decomposing maple leaves

Processing of leaf litter is an important function in many environments and is influenced strongly by microorganisms. We investigated interactions between an aquatic hyphomycete, Tetrachaetum elegans, and two bacteria from the Cytophaga-Flavobacterium-Bacteroides group, that were isolated from decaying leaves in a stream. Laboratory experiments were used to examine interactions, as indicated by growth, between bacteria and fungi on sugar maple (Acer saccharum) leaves. Responses to amendments with labile dissolved organic carbon (DOC) were also examined. Fungal biomass was not affected by glucose amendment or bacterial presence. Likewise, bacterial biomass did not respond consistently to the glucose amendment, nor did the fungus affect bacterial biomass. In general, we found little evidence of resource competition or facilitation, in contrast to other studies. Our experiments suggest that fungal–bacterial interactions are not always significant and may depend on environmental conditions and the types of microorganisms examined.     

Viral lysis of Micromonas pusilla: impacts on dissolved organic matter production and composition

The viral mediated transformation of phytoplankton organic carbon to dissolved forms (“viral shunt”) has been suggested as a major source of dissolved organic carbon (DOC) in marine systems. Despite the potential implications of viral activity on the global carbon fluxes, studies investigating changes in the DOC composition from viral lysis is still lacking. Micromonas pusilla is an ecologically relevant picoeukaryotic phytoplankter, widely distributed in both coastal and oceanic marine waters. Viruses have been found to play a key role in regulating the population dynamics of this species. In this study we used axenic cultures of exponentially growing M. pusilla to determine the impact of viral lysis on the DOC concentration and composition, as estimated from lysate-derived production of transparent exopolymer particles (TEP) and two fractions of fluorescent dissolved organic matter (DOM): aromatic amino acids (excitation/emission; 280/320 nm; F(280/320)) and marine humic-like fluorescent DOM (320/410 nm; F(320/410)). DOC concentration increased 4.5 times faster and reached 2.6 times higher end concentration in the viral infected compared with the non-infected cultures. The production of F(280/320) and F(320/410) were 4.1 and 2.8 times higher in the infected cultures, and the elevated ratio between F(280/320) and F(320/410) in lysates suggested a higher contribution of labile (protein) components in viral produced DOM than in algal exudates. The TEP production was 1.8 times faster and reached a 1.5 times higher level in the viral infected M. pusilla culture compared with the non- infected cultures. The measured increase in both DOC and TEP concentrations suggests that viral lysis has multiple and opposite implications for the production and export processes in the pelagic ocean: (1) by releasing host biomass as DOC it decreases the organic matter sedimentation and promotes respiration and nutrient retention in the photic zone, whereas (2) the observed enhanced TEP production could stimulate particle aggregation and thus carbon export out of the photic zone.     

Microbial decomposition of dissolved organic matter in a hypertrophic pond

Planktonic heterotrophic bacteria in lakes utilize the labile fraction of dissolved organic carbon (DOC), although information about seasonal changes in labile DOC in hypertrophic lakes in terms of absolute amount and relative proportion of the total DOC is still limited. We conducted DOC decomposition experiments using GF/F filtrates in water samples from hypertrophic Furuike Pond, together with monitoring of DOC concentration and bacterial abundance in water samples from the pond, to examine seasonal changes in the amount of labile DOC and growth of bacteria on labile DOC. DOC concentrations fluctuated between 2.7 and 11 mg C l⁻¹, and bacterial abundance fluctuated between 1.5 × 10⁶ and 1.0 × 10⁸ cells ml⁻¹. In the DOC decomposition experiment when grazers of bacteria were removed, small portions of DOC (18% ± 12%) were labile for decomposition by bacteria, and the growth yield of bacteria on labile DOC ranged between 3.3% and 19%. Furthermore, addition of nitrogen to water samples enhanced bacterial growth. Thus, not only labile DOC but also nitrogen limited bacterial growth in the pond. Considering the results in the present study together with those of previous studies, bacterial abundance in Furuike Pond is subjected to bottom-up control, such as by limitation of DOC and nitrogen throughout the year, although top-down control of bacterial abundance such as by grazing is seasonally important. Received: May 1, 2001 / Accepted: July 22, 2001     

Dissolved Organic Carbon: Patterns of Utilization and Turnover in Two Small Lakes

Planktonic bacterial utilization of ¹⁴C-labelled glucose and acetate was monitored by kinetic measurements throughout an annual period in a small lake. Resulting kinetic uptake data have shown that from 1–15% of the total dissolved organic carbon pool was removed chemo-organotrophically per day during the year by bacteria capable of metabolizing these substrates. The kinetic uptake of nine selected organic compounds was measured in a second small lake during summer thermal stratification. Metabolism of glucose, acetate, and glycollate was preferred. Respiration rates of the nine compounds varied generally between 20–60% of the total uptake. The uptake of these compounds accounted for removal of 3–8% of the total dissolved organic carbon pool per day.     

Metabolism of dissolved organic carbon by planktonic bacteria and mixotrophic algae in lake neutralisation experiments

1. Lakes formed in mining pits often contain high concentrations of dissolved ferric iron and sulphate (e.g. 2 and 16 mmol L^?1, respectively) and the pH is buffered between 2.5 and 3.5. Efforts to neutralise their water are based on the stimulation of lake internal, bacterial iron‐ and sulphate reduction. Electron donors may be supplied by organic carbon compounds or indirectly by enhancement of primary production. Here, we investigated the function of mixotrophic algae, which can potentially supplement or deplete the organic carbon pool, in the carbon metabolism and alkalinity budget of an acidic mining lake. 2. Two weeks after organic substrates had been added in a large in situ mesocosm of 30 m diameter, a bloom of Chlamydomonas occurred, reaching a biovolume of 80 mm³ L^?1. Growth experiments using filtered lake water showed that the alga reduced the overall dissolved organic carbon (DOC) concentration despite significant photosynthetic activity. However, when Chlamydomonas were grown together with natural bacterioplankton, net DOC consumption did not increase. 3. Uptake experiments using [¹⁴C]‐glucose indicated that bacteria dominated glucose uptake and remineralisation. Therefore, the DOC leached in the water column was processed mainly by planktonic bacteria. Leached DOC must be regarded as loss, not transferred by larger organisms to the sediment, where reduction processes take place. 4. From phytoplankton biomass and production 2 years after fertilisation we estimated that pelagic photosynthesis does not supply an electron donor capacity capable of reducing more than 2% of actual stock of acidity per year. We estimated that only the benthic primary production was in a range to compensate for ongoing inputs of iron and sulphate.     

Transporter genes expressed by coastal bacterioplankton in response to dissolved organic carbon

Coastal ocean bacterioplankton control the flow of dissolved organic carbon (DOC) from terrestrial and oceanic sources into the marine food web, and regulate the release of inorganic carbon to atmospheric and offshore reservoirs. While the fate of the chemically complex coastal DOC reservoir has long been recognized as a critical feature of the global carbon budget, it has been problematic to identify both the compounds that serve as major conduits for carbon flux and the roles of individual bacterioplankton taxa in mediating that flux. Here we analyse random libraries of expressed genes from a coastal bacterial community to identify sequences representing DOC‐transporting proteins. Predicted substrates of expressed transporter genes indicated that carboxylic acids, compatible solutes, polyamines and lipids may be key components of the biologically labile DOC pool in coastal waters, in addition to canonical bacterial substrates such as amino acids, oligopeptides and carbohydrates. Half of the expressed DOC transporter sequences in this coastal ocean appeared to originate from just eight taxa: Roseobacter, SAR11, Flavobacteriales and five orders of γ‐Proteobacteria. While all major taxa expressed transporter genes for some DOC components (e.g. amino acids), there were indications of specialization within the bacterioplankton community for others (e.g. carbohydrates, carboxylic acids and polyamines). Experimental manipulations of the natural DOC pool that increased the concentration of phytoplankton‐ or vascular plant‐derived compounds invoked a readily measured response in bacterial transporter gene expression. This highly resolved view of the potential for carbon flux into heterotrophic bacterioplankton cells identifies possible bioreactive components of the coastal DOC pool and highlights differing ecological roles in carbon turnover for the resident bacterial taxa.     

胶州湾滨海湿地枯落物分解对土壤活性有机碳含量及其三维荧光特性的影响

通过室内试验模拟胶州湾滨海湿地碱蓬、芦苇、互花米草枯落物的分解过程,测定土壤活性有机碳(可溶性有机碳、微生物生物量碳)含量变化,并利用三维荧光技术对可溶性有机碳(DOC)进行光谱分析.结果表明: 土壤活性有机碳含量呈先增加后降低最终趋于稳定的变化趋势;不同枯落物类型和添加方式对土壤活性有机碳的影响不同,表现为碱蓬＞互花米草＞芦苇,且原状混合＞表面覆盖;光谱分析表明,分解过程各三维荧光光谱的荧光峰数量、荧光中心位置和荧光强度都存在一定差异,添加枯落物后土壤未发现类酪氨酸峰;芳香类蛋白物质占比最高,腐殖质类物质次之.枯落物分解是以微生物分解作用为主导、枯落物性质为本质要素、多重因子综合作用的结果,其对提高土壤活性有机碳含量、增强土壤碳库的稳定性具有促进作用;该过程通过改变DOC的结构和化学组分,提高其在土壤中的迁移转化能力,增强生物可降解性和可利用性,促进微生物内源DOC的产生,加快湿地土壤碳循环.     

Characterisation and reactivity continuum of dissolved organic matter in forested headwater catchments of Andean Patagonia

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Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: Clues from 13C and 14C

Export of dissolved organic carbon (DOC) from forested catchmentsis governed by competing processes of production, decomposition, sorptionand flushing. To examine the sources of DOC, carbon isotopes (¹⁴Cand ¹³C) were analyzed in DOC from surface waters, groundwatersand soils in a small forested catchment on the Canadian Shield in centralOntario. A significant fraction (greater than 50%) of DOCin major inflows to the lake is composed of carbon incorporated into organicmatter, solubilized and flushed into the stream within the last 40 years. Incontrast, ¹⁴C in groundwater DOC was old indicating extensiverecycling of forest floor derived organic carbon in the soil column beforeelution to groundwater in the lower B and C soil horizons. A small uplandbasin had a wide range in ¹⁴C from old groundwater values atbaseflow under dry basin conditions to relatively modern values during highflow or wetter antecedent conditions. Wetlands export mainly recently fixedcarbon with little seasonal range. DOC in streams entering the small lakemay be composed of two pools; an older recalcitrant pool delivered bygroundwater and a young labile pool derived from recent organic matter.The relative proportion of these two pools changes seasonally due thechanges in the water flowpaths and organic carbon dynamics. Althoughchanges in local climate (temperature and/or precipitation) may alterthe relative proportions of the old and young pools, the older pool islikely to be more refractory to sedimentation and decomposition in thelake setting. Delivery of older pool DOC from the catchment andsusceptibility of this older pool to photochemical decomposition mayconsequently be important in governing the minimum DOC concentrationlimit in lakes.     

亚热带米槠林不同更新方式对土壤可溶性有机质降解性的影响

可溶性有机质(DOM)的生物降解性影响着土壤有机质的存留和释放,对深入认识森林土壤养分循环意义重大。为探究森林更新对土壤DOM降解特征的影响,选取亚热带地区米槠天然林(NF)、米槠次生林(SF)和米槠人工促进天然更新林(AR)土壤DOM溶液为研究对象,进行室内降解(42 d)试验。结果表明: 1)3种林分土壤可溶性有机碳(DOC)的降解率和易降解DOC的比例均为SF>AR>NF;可溶性有机氮(DON)和微生物生物量碳(MBC)是显著影响易降解DOC比例的因子;2)难降解组分占3种林分土壤DOC的大部分(72.3%～94.6%),其周转时间长,有利于稳定土壤有机质(SOC)的形成;3)土壤DOM最初的腐殖化指数(HIX_em)会影响易降解DOC的周转时间。DOM光谱结构随降解过程呈现动态变化,说明DOM中易降解组分被消耗完后,微生物会转而降解芳香类和疏水性物质以获取碳源。综上,米槠天然林更新为次生林和人促林后增加了易降解DOC的比例,提高了土壤DOM生物可降解性,不利于SOC的积累。     

Predator‐induced changes in dissolved organic carbon dynamics

The fate of dissolved organic carbon (DOC) is partly determined by its availability to microbial degradation. Organisms at upper trophic levels could influence the bioavailability of DOC via cascading effects on primary producers and bacteria. Here we experimentally tested whether the presence of fish in aquatic food webs can indirectly affect the composition of the DOC pool. We found that fish had strong positive effects on phytoplankton biomass that affected the dynamics of DOC composition. Specifically, fish increased protein‐like, algae‐derived DOC mid‐experiment, concurrent with the strongest fish‐induced increase in phytoplankton biomass. Fish also increased bacterial abundance, altered the community composition and diversity of bacteria, and temporarily increased DOC compounds with fluorescence properties indicative of microbially‐reprocessed organic matter. Overall, our experiment revealed that fish can positively influence the substrate (algae‐produced DOC) and the key players (bacteria) of the microbial carbon pump. Consequently, fish could contribute to carbon sequestration by stimulating both the production of bioavailable DOC and the microbial degradation of bioavailable to persistent DOC. We propose this as a novel mechanism whereby the loss of predators from global ecosystems could alter carbon cycling.