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.     

Composition and variations in the occurrence of dissolved free simple organic compounds of an unproductive lake ecosystem in northern Sweden

Low molecular weight organic carbon compounds are potentially important carbon and energy substrates to heterotrophic production in the aquatic environment. We studied the occurrence of dissolved free amino acids (AA), monosaccharides (CHO), and carboxylic acids (CA) in the subarctic Lake Diktar-Erik. The lake is unproductive with slightly humic water, and receives water via one major inlet stream draining a birch forest environment. The concentration of dissolved organic carbon (DOC) in the inlet stream was strongly correlated with the discharge. This relationship changed from season to season, indicating changes in the sources of the DOC entering the stream. AA and CHO each accounted for an average of less than 0.5% of the DOC. After high discharge events during the ice-free period, AA and CHO occurred in especially high concentrations. CA occurred in higher concentrations during the ice-free period, when it generally accounted for 20–30% of the DOC pool. The CA content relative to the total DOC pool was strongly inversely correlated with overall DOC concentration, and at low DOC levels the relative content of CA was high and vice versa. This followed a seasonal trend, with CA accounting for a smaller proportion of the DOC in winter and a larger part in spring/early summer. A conservative estimate suggested that the studied simple organic carbon compounds potentially could cover 30% of the bacterial gross production in the lake and therefore potentially also was an important source of CO₂ that occur in supersaturated concentrations in the lake.     

Latitudinal gradients in degradation of marine dissolved organic carbon

Heterotrophic microbial communities cycle nearly half of net primary productivity in the ocean, and play a particularly important role in transformations of dissolved organic carbon (DOC). The specific means by which these communities mediate the transformations of organic carbon are largely unknown, since the vast majority of marine bacteria have not been isolated in culture, and most measurements of DOC degradation rates have focused on uptake and metabolism of either bulk DOC or of simple model compounds (e.g. specific amino acids or sugars). Genomic investigations provide information about the potential capabilities of organisms and communities but not the extent to which such potential is expressed. We tested directly the capabilities of heterotrophic microbial communities in surface ocean waters at 32 stations spanning latitudes from 76°S to 79°N to hydrolyze a range of high molecular weight organic substrates and thereby initiate organic matter degradation. These data demonstrate the existence of a latitudinal gradient in the range of complex substrates available to heterotrophic microbial communities, paralleling the global gradient in bacterial species richness. As changing climate increasingly affects the marine environment, changes in the spectrum of substrates accessible by microbial communities may lead to shifts in the location and rate at which marine DOC is respired. Since the inventory of DOC in the ocean is comparable in magnitude to the atmospheric CO(2) reservoir, such a change could profoundly affect the global carbon cycle.     

Dissolved organic matter released by two marine heterotrophic bacterial strains and its bioavailability for natural prokaryotic communities

Marine heterotrophic prokaryotes (HP) play a key role in organic matter processing in the ocean; however, the view of HP as dissolved organic matter (DOM) sources remains underexplored. In this study, we quantified and optically characterized the DOM produced by two single marine bacterial strains. We then tested the availability of these DOM sources to in situ Mediterranean Sea HP communities. Two bacterial strains were used: Photobacterium angustum (a copiotrophic gammaproteobacterium) and Sphingopyxis alaskensis (an oligotrophic alphaproteobacterium). When cultivated on glucose as the sole carbon source, the two strains released from 7% to 23% of initial glucose as bacterial derived DOM (B-DOM), the quality of which (as enrichment in humic or protein-like substances) differed between strains. B-DOM induced significant growth and carbon consumption of natural HP communities, suggesting that it was partly labile. However, B-DOM consistently promoted lower prokaryotic growth efficiencies than in situ DOM. In addition, B-DOM changed HP exoenzymatic activities, enhancing aminopeptidase activity when degrading P. angustum DOM, and alkaline phosphatase activity when using S. alaskensis DOM, and promoted differences in HP diversity and composition. DOM produced by HP affects in situ prokaryotic metabolism and diversity, thus changing the pathways for DOM cycling (e.g. respiration over biomass production) in the ocean.     

Comparative genomics reveals adaptations of a halotolerant thaumarchaeon in the interfaces of brine pools in the Red Sea

The bottom of the Red Sea harbors over 25 deep hypersaline anoxic basins that are geochemically distinct and characterized by vertical gradients of extreme physicochemical conditions. Because of strong changes in density, particulate and microbial debris get entrapped in the brine-seawater interface (BSI), resulting in increased dissolved organic carbon, reduced dissolved oxygen toward the brines and enhanced microbial activities in the BSI. These features coupled with the deep-sea prevalence of ammonia-oxidizing archaea (AOA) in the global ocean make the BSI a suitable environment for studying the osmotic adaptations and ecology of these important players in the marine nitrogen cycle. Using phylogenomic-based approaches, we show that the local archaeal community of five different BSI habitats (with up to 18.2% salinity) is composed mostly of a single, highly abundant Nitrosopumilus-like phylotype that is phylogenetically distinct from the bathypelagic thaumarchaea; ammonia-oxidizing bacteria were absent. The composite genome of this novel Nitrosopumilus-like subpopulation (RSA3) co-assembled from multiple single-cell amplified genomes (SAGs) from one such BSI habitat further revealed that it shares ∼54% of its predicted genomic inventory with sequenced Nitrosopumilus species. RSA3 also carries several, albeit variable gene sets that further illuminate the phylogenetic diversity and metabolic plasticity of this genus. Specifically, it encodes for a putative proline-glutamate ‘switch'' with a potential role in osmotolerance and indirect impact on carbon and energy flows. Metagenomic fragment recruitment analyses against the composite RSA3 genome, Nitrosopumilus maritimus, and SAGs of mesopelagic thaumarchaea also reiterate the divergence of the BSI genotypes from other AOA.     

MIXOTROPHIC ALGAE CONSTRAIN THE LOSS OF ORGANIC CARBON BY EXUDATION1

Algae of various taxonomic groups are capable of assimilating dissolved organic carbon (DOC) from their environments (mixotrophy). Recently, we reported that, with increasing biomass of mixotrophs, heterotrophic bacteria did not increase. We hypothesized that algal uptake of external DOC may outweigh their release of DOC by exudation (H1). Here, we addressed an alternative hypothesis that algae did not assimilate external DOC but constrained the release of DOC (H2). In chemostat experiments, we cultured the mixotrophic Chlamydomonas acidophila Negoro together with heterotrophic bacteria. As external substrates, we used glucose, which was potentially available for both bacteria and algae, or fructose, which was available only for bacteria. We increased the biomass of algae by the stepwise addition of phosphorus. Bacterial biomass did not increase in experiments using glucose or when fructose was offered, suggesting that mechanisms other than algal mixotrophy (H1) kept concentrations of bacteria low. Measured exudation rates (percent extracellular release, PER) of mixotrophic algae (Cd. acidophila, Chlorella protothecoides W. Krüger) were very low and ranged between 1.0% and 3.5% at low and moderately high phosphorus concentrations. In contrast, an obligately phototrophic alga (Chlamydomonas segnis H. Ettl) showed higher exudation rates, particularly under phosphorus limitation (70%). The results support H2. If mixotrophy is considered as a mechanism to recycle organic exudates from near the cell surface, this would explain why algae retained mixotrophic capabilities although they cannot compete with bacteria for external organic carbon.     

Responses of Active Ammonia Oxidizers to Eutrophication and Oxygen Statuses in Taihu Freshwater Sediments

Abstract

Here, we employed DNA-based stable isotope probing (SIP) and molecular biology methods to investigate active ammonia oxidizer communities in suboxic sediments (0 to –2?cm) at the micromolar oxygen level and layers (–2 to –5?cm) at nanomolar oxygen concentrations from meso-eutrophic and light-eutrophic locations in Taihu Lake. The results revealed that ammonia-oxidizing archaea (AOA) were less active in the anoxic layer of meso-eutrophic sites, while ammonia-oxidizing bacteria (AOB) were less active in suboxic sediments of light-eutrophic sites after 8?weeks of incubation. The active AOA in the meso- and light-eutrophic sediments belonged to the Nitrosopumilus, Nitrosotalea, and Nitrososphaera clusters and the Nitrosopumilus and Nitrososphaera clusters, respectively, with Nitrosopumilus cluster as the predominant AOA, which took up a higher ratio in the light-eutrophic and suboxic layers than their counterparts. The advantageous active AOB were numerically predominated by the Nitrosomonas cluster in the suboxic layers, and the Nitrosospira cluster in the anoxic layers, respectively, both of which were distributed in diverse frequencies in different eutrophication statuses. The role and community composition diversities of active ammonia oxidizers in freshwater sediments were attributed to the different eutrophication (including nitrogen and organic carbon content) and oxygen statuses.     

Substrate Use of Pseudovibrio sp. Growing in Ultra-Oligotrophic Seawater

Marine planktonic bacteria often live in habitats with extremely low concentrations of dissolved organic matter (DOM). To study the use of trace amounts of DOM by the facultatively oligotrophic Pseudovibrio sp. FO-BEG1, we investigated the composition of artificial and natural seawater before and after growth. We determined the concentrations of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), free and hydrolysable amino acids, and the molecular composition of DOM by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS). The DOC concentration of the artificial seawater we used for cultivation was 4.4 μmol C L^-1, which was eight times lower compared to the natural oligotrophic seawater we used for parallel experiments (36 μmol C L ^-1). During the three-week duration of the experiment, cell numbers increased from 40 cells mL^-1 to 2x10⁴ cells mL ^-1 in artificial and to 3x10⁵ cells mL ^-1 in natural seawater. No nitrogen fixation and minor CO₂ fixation (< 1% of cellular carbon) was observed. Our data show that in both media, amino acids were not the main substrate for growth. Instead, FT-ICR-MS analysis revealed usage of a variety of different dissolved organic molecules, belonging to a wide range of chemical compound groups, also containing nitrogen. The present study shows that marine heterotrophic bacteria are able to proliferate with even lower DOC concentrations than available in natural ultra-oligotrophic seawater, using unexpected organic compounds to fuel their energy, carbon and nitrogen requirements.     

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.     

Bacterial Contribution to Dissolved Organic Matter in Eutrophic Lake Kasumigaura,Japan

Incubation experiments using filtered waters from Lake Kasumigaura were conducted to examine bacterial contribution to a dissolved organic carbon (DOC) pool. Bacterial abundance, bacterial production, concentrations of DOC, total dissolved amino acids (TDAA), and total dissolved neutral sugars (TDNS) were monitored during the experiments. Bacterial production during the first few days was very high (20 to 35 μg C liter⁻¹ day⁻¹), accounting for 40 to 70% of primary production. The total bacterial production accounted for 34 to 55% of the DOC loss during the experiment, indicating high bacterial activities in Lake Kasumigaura. The DOC degradation was only 12 to 15%, whereas the degradation of TDAA and TDNS ranged from 30 to 50%, suggesting the preferential usage of TDAA and TDNS. The contribution of bacterially derived carbon to a DOC pool in Lake Kasumigaura was estimated using d-amino acids as bacterial biomarkers and accounted for 30 to 50% of the lake DOC. These values were much higher than those estimated for the open ocean (20 to 30%). The ratio of bacterially derived carbon to bulk carbon increased slightly with time, suggesting that the bacterially derived carbon is more resistant to microbial degradation than bulk carbon. This is the first study to estimate the bacterial contribution to a DOC pool in freshwater environments. These results indicate that bacteria play even more important roles in carbon cycles in freshwater environments than in open oceans and also suggests that recent increases in recalcitrant DOC in various lakes could be attributed to bacterially derived carbon. The potential differences in bacterial contributions to dissolved organic matter (DOM) between freshwater and marine environments are discussed.     

In situ release of coral mucus by <Emphasis Type="Italic">Acropora</Emphasis> and its influence on the heterotrophic bacteria

In situ mucus release by Acropora nobilis and degradation of mucus from A. nobilis and Acropora formosa, by heterotrophic bacteria were investigated at Bidong and Tioman Island, Malaysia. Mucus release rate for A. nobilis was on average 38.7 ± 35.2 mg C m⁻² h⁻¹, of which ca. 70% consisted of dissolved organic carbon (DOC) and 30% particulate organic carbon (POC). In the mucus degradation experiment, seawater-mucus mixtures were incubated and compared with control runs for 24 h. Bacterial abundance in the seawater-mucus mixture increased significantly and coincided with a decline in DOC concentration. In controls, bacteria and DOC did not significantly change. The coral mucus had a high content of inorganic phosphate. It is suggested that the coral mucus rich in DOC and phosphate can induce the high bacterial growth.     

Changes in dissolved lignin-derived phenols, neutral sugars, uronic acids, and amino sugars with depth in forested Haplic Arenosols and Rendzic Leptosols

A major part of the dissolved organic matter produced in the organic layers of forest ecosystems and leached into the mineral soil is retained by the upper subsoil horizons. The retention is selective and thus dissolved organic matter in the subsoils has different composition than dissolved organic matter leached from the forest floor. Here we report on changes in the composition of dissolved organic matter with soil depth based on C-to-N ratios, XAD-8 fractionation, wet-chemical analyses (lignin-derived CuO oxidation products, hydrolysable sugars and amino sugars) and liquid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. Dissolved organic matter was sampled directly beneath the forest floor using tension-free lysimeters and at 90cm depth by suction cups in Haplic Arenosols under Scots pine (Pinus sylvestris L.) and Rendzic Leptosols under European beech (Fagus sylvatica L.) forest. At both sites, the concentrations of dissolved organic carbon (DOC) decreased but not as strongly as reported for deeply weathered soils. The decrease in DOC was accompanied by strong changes in the composition of dissolved organic matter. The proportion of the XAD-8-adsorbable (hydrophobic) fraction, carboxyl and aromatic C, and the concentrations of lignin-derived phenols decreased whereas the concentrations of sugars, amino sugars, and nitrogen remained either constant or increased. A general feature of the compositional changes within the tested compound classes was that the ratios of neutral to acidic compounds increased with depth. These results indicate that during the transport of dissolved organic matter through the soils, oxidatively degraded lignin-derived compounds were preferentially retained while potentially labile material high in nitrogen and carbohydrates tended to remain dissolved. Despite the studied soils' small capacity to sorb organic matter, the preferential retention of potentially refractory and acidic compounds suggests sorption by the mineral soil matrix rather than biodegradation to govern the retention of dissolved organic matter even in soils with a low sorption capacity.     

Distribution patterns of ammonia-oxidizing archaea and bacteria in sediments of the eastern China marginal seas

Ammonia-oxidizing archaea (AOA) and bacteria (AOB) vary in their contribution to nitrification in different environments. The eastern China marginal seas (ECMS) are featured by complex river runoffs and ocean currents, forming different sediment patches. Here, via quantitative PCR and clone library analysis of the amoA genes, we showed that AOB were more abundant than AOA in ECMS sediments. The abundance, diversity and richness of AOA, but not AOB, were higher in the East China Sea (ECS) than in the Yellow Sea (YS) and Bohai Sea (BS). Nitrosopumilus (AOA) and Nitrosospira (AOB) were predominant lineages, but their abundances varied significantly between ECS, and BS and YS. This was mainly attributed to salinity and dissolved oxygen of the bottom water. The discovery of a high abundance of Nitrosophaera at estuarine sites suggested strong terrigenous influence exerted on the AOA community. In contrast, variations in ocean conditions played more important roles in structuring the AOB community, which was separated by bottom water dissolved oxygen into two groups: the south YS, and the north YS and BS. This study provides a comprehensive insight into the spatial distribution pattern of ammonia-oxidizing prokaryotes in ECMS sediments, laying a foundation for understanding their relative roles in nitrification.     

Review of ammonia-oxidizing bacteria and archaea in freshwater ponds

Aquaculture ponds are simple and unique ecosystems, which are affected intensively by human activities. In this mini-review, we focus our attention on the distribution and community diversity of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in pond water and sediments, as well as the possible ecological mechanisms involved. Moreover, we discuss the possibility of increasing the activity of ammonia-oxidizing organisms in order to improve the water quality in aquaculture ponds. Compared with eutrophic lakes, the significantly higher ammonia concentration in pond water does not lead to significantly higher AOB levels, and the abundance of AOA is too low to quantify accurately. Similar to eutrophic lakes, high abundances of AOA and AOB are present in the surface sediments at the same time, where the oxidation of ammonia is performed mainly by AOB. AOB and AOA exhibit significant seasonal variations in aquaculture ponds, which are affected by the temperature, pH, and dissolved oxygen. The dominant AOB species are Nitrosomonas and the Nitrosospira lineage in pond environments. Nitrososphaera or members of the Nitrososphaera-like cluster dominate the AOA species in surface sediments, whereas the Nitrosopumilus cluster dominates the deeper sediments. AOB and AOA can be enriched on artificial substrates suspended in the pond water, thereby potentially improving the water quality.

     

Bacterial growth and community composition in fractions of dissolved organic carbon of different molar mass from interstitial water

The ability of different molar-mass fractions of interstitial dissolved organic carbon (DOC) from a small stream to stimulate bacterial growth was determined. Growth was the greatest in the low-molar mass (<1 kDa) (LMM) fraction and the least in the high-molar mass (>10 kDa) (HMM) one. The LMM fraction consistently had a higher ratio of carbon to organic nitrogen than other fractions. The fluorescence in situ hybridization analysis revealed that bacterial populations in interstitial water were dominated by beta-Proteobacteria. gamma-Proteobacteria together with the Cytophaga-Flavobacteria cluster dominated in LMM DOC (66.3%) while the proportions of bacterial groups in HMM and medium-molar mass (MMM) fractions were more equal. The fact that gamma-Proteobacteria strongly dominates in LMM DOC fraction refutes the idea that LMM DOC is readily utilized by all heterotrophic bacteria.     

Concentrations and fluxes of organic carbon substrates in the aquatic environment

Data concerning concentrations and fluxes of dissolved organic compounds (DOC) from marine and lacustrine environments are reviewed and discussed. Dissolved free amino acids and carbohydrates comprised the main fraction in the labile organic carbon pool. Dissolved free amino acids in marine waters varied between 3–1400 nM and those of freshwaters between 2.6–4124 nM. Dissolved free carbohydrates varied between 0.4–5000 nM in marine systems and between 14–1111 nM in freshwaters. The turnover times of both substrate pools varied in marine waters between 1.4 hours and 948 days and in freshwaters between 2 hours and 51 days. Measurements of stable^12/13C-ratio and¹⁴C-isotope dating in ocean deep water samples revealed DOC turnover times between 2000–6000 years. Studies on carbon flows within the aquatic food webs revealed that about 50% of photosynthetically fixed carbon was channelled via DOC to the bacterioplankton. Excreted organic carbon varied between 1–70% of photosynthetically fixed carbon in marine waters and between 1–99% in freshwaters. The labile organic carbon pool represented only 10–30% of the DOC. The majority (70–90%) of the DOC was recalcitrant to microbial assimilation. Only 10–20% of the DOC could be easily chemically identified. Most of the large bulk material represented dissolved humic matter and neither the chemical structure nor the ecological function of the DOC is as yet clearly understood.Abbreviations ATP Adenosine Tri-Phosphate - AMS Accelerated Mass Spectrometry - BSA Bovine Serum Albumin - GlAse GlucosidAse activity - DAA Dissolved Amino Acids - DCAA Dissolved Combined Amino Acids - DFAA Dissolved Free Amino Acids - DTAA Dissolved Total Amino Acids - DCHO Dissolved Carbohydrates - DCCHO Dissolved Combined Carbohydrates - DFCHO Dissolved Free Carbohydrates - DTCHO Dissolved Total Carbohydrates - DLCFaAc Dissolved Long Chain Fatty Acids - DSCFaAc Dissolved Short Chain Fatty Acids - DOC Dissolved Organic Carbon - DOM Dissolved Organic Matter - DHM Dissolved Humic Matter - DTPhOH Dissolved Total Phenolic compounds - DCPhOH Dissolved Combined Phenolic compounds - DFPhOH Dissolved Free Phenolic conpounds - EOC Excreted Organic Carbon - HS Humic Substances - HPLC High Performance Liquid Chromatography - HTCO High-Temperature Catalytic Oxidation - (K_t+S_n) Transport Constant + Natural Substrate from Michaelis Menten Kinetics - LOCP Labile Organic Carbon Pool - OM Organic Matter - MEE Microbial Extracellular Enzymes - PER Percent of Extracellular Release - PhDOC Photosynthetically derived Dissolved Organic Carbon - POC Particulate Organic Carbon - ROCP Recalcitrant Organic Carbon Pool - T_t Turnover time - UDOC Utilizable Dissolved Organic Carbon - V_max Maximum Uptake Velocity - WCO Wet Chemical Oxidation Dedicated to Prof. Drs. J. Overbeck on the occasion of his 70th birthday     

A comparison of phosphorus and DOC leachates from different types of leaf litter in an urban environment

1. The leaching rates of filterable reactive phosphorus (FRP) and dissolved organic carbon (DOC) from five leaf litter types commonly occurring in urban environments in Mediterranean regions of Southern Australia were compared. The relative composition, bioavailability and oxygen demand of this DOC were also assessed. Four tree species were assessed, including the native river red gum (Eucalyptus camaldulensis) and three introduced deciduous species, the English elm (Ulmus procera), London plane (Platanus acerifolia) and white poplar (Populus alba). Grass cuttings (mixed species) were selected as a common garden waste. 2. Except for English elm, the majority of FRP and DOC was released within the first 48 h. Grass cuttings released the highest amount of FRP with white poplar releasing the most DOC. Species that released relatively high amounts of DOC (white poplar, English elm, river red gum) released relatively low amounts of FRP. Conversely, species that released relatively low amounts of DOC (grass cuttings and London plane) tended to release relatively high amounts of FRP. 3. Analysis of DOC composition, combined with the differing oxygen demand and DOC depletion curves, demonstrated that there were substantial differences in the DOC leached from the leaf litter of the different species. Biochemical oxygen demand and the biodegradability of the DOC was positively correlated with the proportion of hydrophilic and hydrophobic acids present in the leachate. 4. These results demonstrate that simple measurements of nutrient release per gram of leaf litter would be insufficient to predict the ecological impact on receiving waters resulting from changes in dominant vegetation. Furthermore, the use of traps to prevent particulate leaf material from entering streams may have limited potential for reducing the load of dissolved nutrients. We conclude that any changes to vegetation type which substantially alter the timing of leaf fall or the composition of leaf leachates should be avoided.     

Resource-Limited Heterotrophic Prokaryote Production and Its Potential Environmental Impact Associated with Mn Nodule Exploitation in the Northeast Equatorial Pacific

Shipboard enrichment incubation experiments were performed to elucidate the limiting resources for heterotrophic prokaryotic production and to discuss the potential impact of bottom water and sediment discharges in relation to manganese (Mn) nodule exploitation on the heterotrophic prokaryotes in the oligotrophic northeast equatorial Pacific. Compared to an unamended control, the production of heterotrophic prokaryotes increased 25-fold in water samples supplemented with amino acids (i.e., organic carbon plus nitrogen), whereas the production increased five and two times, respectively, in samples supplemented with either glucose or ammonium alone. These results indicate that heterotrophic prokaryote production in the northeast equatorial Pacific was co-limited by the availability of dissolved organic carbon and inorganic nitrogen. In samples from the nutrient-depleted surface mixed layer (10-m depth), the addition of a slurry of bottom water and sediment doubled heterotrophic prokaryote production compared to an unamended control, whereas sonicating the slurry prior to addition quadrupled the production rate. However, little difference was observed between an unamended control and slurry-amended samples in the subsurface chlorophyll a (Chl a) maximum (SCM) layer. Thus, the impact of slurry discharge is more significant at the nutrient-depleted surface mixed layer than at the high-nutrient SCM layer. The greatly enhanced prokaryote production resulting from the addition of sonicated slurry further suggests that dissociated organic carbon may directly stimulate heterotrophic prokaryote production in the surface mixed layer. Overall, the results suggest that the surface discharge of bottom water and sediments during manganese nodule exploitation could have a significant environmental impact on the production of heterotrophic prokaryotes that are currently resource limited.     

Sorption of dissolved organic matter by mineral soils of the Siberian forest tundra

Because of low net production in arctic and subarctic surface water, dissolved organic matter (DOM) discharged from terrestrial settings plays an important role for carbon and nitrogen dynamics in arctic aquatic systems. Sorption, typically controlling the export of DOM from soil, may be influenced by the permafrost regime. To confirm the potential sorptive control on the release of DOM from permafrost soils in central northern Siberia, we examined the sorption of DOM by mineral soils of Gelisols and Inceptisols with varying depth of the active layer. Water‐soluble organic matter in the O horizons of the Gelisols was less (338 and 407 mg C kg^?1) and comprised more dissolved organic carbon (DOC) in the hydrophobic fraction (HoDOC) (63% and 70%) than in the O horizons of the Inceptisols (686 and 706 mg C kg^?1, 45% and 48% HoDOC). All A and B horizons from Gelisols sorbed DOC strongly, with a preference for HoDOC. Almost all horizons of the Inceptisols showed a weaker sorption of DOC than those of the Gelisols. The C horizons of the Inceptisols, having a weak overall DOC sorption, sorbed C in the hydrophilic fraction (HiDOC) stronger than HoDOC. The reason for the poor overall sorption and also the preferential sorption of HiDOC is likely the high pH (pH>7.0) of the C horizons and the smaller concentrations of iron oxides. For all soils, the sorption of HoDOC related positively to oxalate‐ and dithionite–citrate‐extractable iron. The A horizons released large amounts of DOC with 46–80% of HiDOC. The released DOC was significantly (r=0.78, P<0.05) correlated with the contents of soil organic carbon. From these results, we assume that large concentrations of DOM comprising large shares of HiDOC can pass mineral soils where the active layer is thin (i.e. in Gelisols), and enter streams. Soils with deep active layer (i.e. Inceptisols), may release little DOM because of more frequent infiltration of DOM into their thick mineral horizons despite their smaller contents of reactive, poorly crystalline minerals. The results obtained for the Inceptisols are in agreement with the situation observed for streams connecting to Yenisei at lower latitudes than 65°50′ with continuous to discontinuous permafrost. The smaller sorption of DOM by the Gelisols is in agreement with the larger DOM concentrations in more northern catchments. However, the Gelisols preferentially retained the HoDOC which dominates the DOC in streams towards north. This discrepancy can be explained by additional seepage water from the organic horizons that is discharged into streams without intensive contact with the mineral soil.