The hyporheic zone as a source of dissolved organic carbon and carbon gases to a temperate forested stream

The objective of this study was to examine chemical changes in porewaters that occur over small scales (cm) as groundwater flows through the hyporheic zone and discharges to a stream in a temperate forest of northern Wisconsin. Hyporheic-zone porewaters were sampled at discrete depths of 2, 10, 15, 61, and 183 cm at three study sites in the study basin. Chemical profiles of dissolved organic carbon (DOC), CO₂, CH₄, and pH show dramatic changes between 61 cm sediment depth and the water-sediment interface. Unless discrete samples at small depth intervals are taken, these chemical profiles are not accounted for. Similar trends were observed at the three study locations, despite each site having very different hydraulic-flow regimes. Increases in DOC concentration by an order of magnitude from 61 to 15 cm depth with a corresponding decrease in pH and rapid decreases in the molecular weight of the DOC suggest that aliphatic compounds (likely organic acids) are being generated in the hyporheic zone. Estimated efflux rates of DOC, CO₂, and CH₄ to the stream are 6.2, 0.79, 0.13 moles m² d^-1, respectively, with the vast majority of these materials produced in the hyporheic zone. Very little of these materials are accounted for by sampling stream water, suggesting rapid uptake and/or volatilization.     

Architecture and spatial organization in a triple-species bacterial biofilm synergistically degrading the phenylurea herbicide linuron

Members of a triple-species 3-(3,4-dichlorophenyl)-1-methoxy-1-methyl urea (linuron)-mineralizing consortium, i.e. the linuron- and 3,4-dichloroaniline-degrading Variovorax sp. WDL1, the 3,4-dichloroaniline-degrading Comamonas testosteroni WDL7 and the N,O-dimethylhydroxylamine-degrading Hyphomicrobium sulfonivorans WDL6, were cultivated as mono- or multi-species biofilms in flow cells irrigated with selective or nonselective media, and examined with confocal laser scanning microscopy. In contrast to mono-species biofilms of Variovorax sp. WDL1, the triple-species consortium biofilm degraded linuron completely through apparent synergistic interactions. The triple-species linuron-fed consortium biofilm displayed a heterogeneous structure with an irregular surface topography that most resembled the topography of linuron-fed mono-species WDL1 biofilms, indicating that WDL1 had a dominating influence on the triple-species biofilm architecture. This architecture was dependent on the carbon source supplied, as the biofilm architecture of WDL1 growing on alternative carbon sources was different from that observed under linuron-fed conditions. Linuron-fed triple-species consortium biofilms consisted of mounds composed of closely associated WDL1, WDL7 and WDL6 cells, while this association was lost when the consortium was grown on a nonselective carbon source. In addition, under linuron-fed conditions, microcolonies displaying associated growth developed rapidly after inoculation. These observations indicate that the spatial organization in the linuron-fed consortium biofilm reflected the metabolic interactions within the consortium.     

Interactive effects of solar radiation and dissolved organic matter on bacterial activity and community structure

We studied the interactive effects of dissolved organic matter (DOM) and solar radiation on the activity and community structure of bacteria from an alpine lake. Activity was assessed both at the community level as leucine incorporation rates and at the single-cell level by microautoradiography. Fluorescent in situ hybridization and signal amplification by catalysed reporter deposition (CARD-FISH) was used to track changes in the bacterial community composition. Bacteria-free filtrates of different DOM sources (lake, algae or soil) were incubated either in the dark or exposed to solar radiation. Afterwards, the natural bacterial assemblage was inoculated and the cultures incubated in the dark for 24-48 h. Bacterial activity was enhanced in the first 24 h in the soil and algal DOM amendments kept in the dark. After 48 h, the enhancement effect was greatly reduced. The initial bacterial community was dominated by Betaproteobacteria followed by Actinobacteria. The relative abundance (expressed as a percentage of DAPI-stained cells) of Betaproteobacteria increased first in dark incubated DOM amendments, but after 48 h no significant differences were detected among treatments. In contrast, the relative abundance of Actinobacteria increased in pre-irradiated DOM treatments. Although Betaproteobacteria dominated at the end of the experiment, the relative abundance of their R-BT subgroup differed among treatments. Changes in bacterial community activity were significantly correlated with those of the relative abundance and activity of Betaproteobacteria, whereas the contribution of Actinobacteria to the bulk activity was very modest. Our results indicate a negative effect of DOM photoalteration on the bulk bacterial activity. The magnitude of this effect was time-dependent and related to rapid changes in the bacterial assemblage composition.     

Chemical and carbon isotopic evidence for the source and fate of dissolved organic matter in the northern Everglades

Surface waters in the Florida Everglades contain high levels ofdissolved organic carbon (DOC) compounds. ¹³C values of DOCsamples collected from the northern Everglades indicate that less than about23%of the DOC was derived from sugarcane (the dominant agricultural crop in thearea), and the amount of DOC from sugarcane was greater during the dry period.Most of the DOC (> 50%) in the northern Everglades was in the low molecularweight (< 1000 Dalton) fraction (LMW-DOC). The relative amount of highmolecular weight DOC (HMW-DOC) was higher in the wet period than in the dryperiod. Radiocarbon ages of the DOC ranged from > modern toabout 2400 years B.P., indicating that DOC was derived from both historic peatdeposits and modern vegetation. At each site, the HMW-DOC had older radiocarbonages than the LMW-DOC, and therefore contained a greater fraction of DOCderivedfrom the historic peat deposits. It appears that at least some of the old DOCcompounds from the historic peat deposits were decomposed during theirresidencein the surface water system in the northern Everglades, and the LMW-DOC wasmoremicrobially labile than the HMW-DOC. Our analysis suggests that accelerateddecomposition of organic matter in the historic peat deposits (due to land-usechange) could be a significant source of DOC and nutrients in the northernEverglades. Our data also suggest that the radiocarbon signature of DOC couldbeused as a sensitive indicator of the overall effectiveness of a wetlandrestoration project.     

Urban infrastructure influences dissolved organic matter quality and bacterial metabolism in an urban stream network

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Influence of cladoceran grazing activity on dissolved organic matter, enzymatic hydrolysis and bacterial growth

To assess the influence of grazing by cladocerans on dissolvedorganic matter (DOM), glycolytic and proteolytic activitiesand bacterial growth were measured by in situ incubation oflake water from the epilimnion of an oligotrophic reservoirin three different treatments: in absence of zooplankton, andin presence of zooplankton (natural abundance and concentratedfour-fold). These experiments were conducted at two periodsin the succession of plankton populations (May and June 1998),that differed in the quality of the prey ingested (Eudorinasp. compared to Cryptomonas sp. and Rhodomonas sp.) and theirgrazing intensity (31.8 ± 2.2 µg C l^–1 day^–1compared to 10.2 ± 0.5 µg C l^–1 day^–1).A systematic increase in bacterial biomass was measured in thetreatments containing the highest zooplankton concentrations.The DOM concentrations produced in situ showed few significantdifferences between the three treatments, but the assimilationof DOM was higher in the presence of zooplankton than in theirabsence. These results show that the influence of cladoceranson the DOM was more of a qualitative than a quantitative nature.The protein compounds derived from the grazing activities ofmetazoans seem to be a major nutrient source for growth forbacteria (r = 0.81, P <0.05). In this study, the highesthydrolytic activities were recorded in the presence of highconcentrations of metazoan zooplankton. However, the processesthat regulated these activities differed between the two experimentaldates (repression compared to enzyme stimulation). Grazing activitiescould lead to an increase in phytoplanktonic excretion duringthe growth phase, and therefore the production of low molecularweight compounds that are easily assimilated by the bacterialplankton.     

Seasonal response of stream biofilm communities to dissolved organic matter and nutrient enrichments

Dissolved organic matter (DOM) and inorganic nutrients may affect microbial communities in streams, but little is known about the impact of these factors on specific taxa within bacterial assemblages in biofilms. In this study, nutrient diffusing artificial substrates were used to examine bacterial responses to DOM (i.e., glucose, leaf leachate, and algal exudates) and inorganic nutrients (nitrate and phosphate singly and in combination). Artificial substrates were deployed for five seasons, from summer 2002 to summer 2003, in a northeastern Ohio stream. Differences were observed in the responses of bacterial taxa examined to various DOM and inorganic nutrient treatments, and the response patterns varied seasonally, indicating that resources that limit the bacterial communities change over time. Overall, the greatest responses were to labile, low-molecular-weight DOM (i.e., glucose) at times when chlorophyll a concentrations were low due to scouring during significant storm events. Different types of DOM and inorganic nutrients induced various responses among bacterial taxa in the biofilms examined, and these responses would not have been apparent if they were examined at the community level or if seasonal changes were not taken into account.     

Regulation of bacterial proteolytic activity at natural concentrations in dissolved organic matter in a maritime pond]

The regulation of the bacterial exoproteolytic activity, at natural substrate concentrations, was studied during the survey of an Atlantic coastal marine pond (France). The regulation of this activity occurs at two different levels: on the one hand, at the cellular level, the ectoenzyme synthesis is regulated by hydrolysis substrates, dissolved combined amino acids (DCAA), and end products, dissolved free amino acids (DFAA), in terms of the relative amounts available to the cell, and on the other hand, at the ecosystem level, i.e. the hydrolytic activity, by the total amounts of DCAA and DFAA in situ. The DFAA acts as an inhibitor in enzymatic synthesis; in contrast, dissolved proteins induce the enzymatic synthesis and the exoproteolytic activity. These results, obtained in natural concentration conditions, confirm the functioning in situ of the ectoenzymatic activity regulation model of Chróst, until now only validated in an enriched experimental medium.     

Vegetation cover regulates the quantity,quality and temporal dynamics of dissolved organic carbon and nitrogen in Antarctic soils

Populations of the two native Antarctic vascular plant species (Deschampsia antarctica and Colobanthus quitensis) have expanded rapidly in recent decades, yet little is known about the effects of these expansions on soil nutrient cycling. We measured the concentrations of dissolved organic carbon (DOC) and nitrogen (DON), amino acids and inorganic N in soils under these two vascular plant species, and under mosses and lichens, over a growing season at Signy Island in the maritime Antarctic. We recorded higher concentrations of nitrate, total dissolved nitrogen, DOC, DON and free amino acids in soil under D. antarctica and C. quitensis than in lichen or moss dominated soils. Each vegetation cover gave a unique profile of individual free amino acids in soil solution. Significant interactions between soil type and time were found for free amino acid concentrations and C/N ratios, indicating that vascular plants significantly change the temporal dynamics of N mineralization and immobilization. We conclude that D. antarctica and C. quitensis exert a significant influence over C and N cycling in the maritime Antarctic, and that their recent population expansion will have led to significant changes in the amount, type and rate of organic C and N cycling in soil.     

Mineralisation of dissolved organic matter by heterotrophic stream biofilm communities in a large boreal catchment

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Influence of dissolved organic matter and inorganic nutrients on the biofilm bacterial community on artificial substrates in a northeastern Ohio, USA, stream

Stream bacteria may be influenced by the composition and availability of dissolved organic matter (DOM) and inorganic nutrients, but knowledge about how individual phylogenetic groups in biofilm are affected is still limited. In this study, the influence of DOM and inorganic nutrients on stream biofilm bacteria was examined. Biofilms were developed on artificial substrates (unglazed ceramic tiles) for 21 days in a northeastern Ohio (USA) stream for five consecutive seasons. Then, the developed biofilm assemblages were exposed, in the laboratory, to DOM (glucose, leaf leachate, and algal exudates) and inorganic nutrients (nitrate, phosphate, and nitrate and phosphate in combination) amendments for 6 days. Bacterial numbers in the biofilms were generally higher in response to the DOM treatments than to the inorganic nutrient treatments. There were also apparent seasonal variations in the response patterns of the individual bacterial taxa to the nutrient treatments; an indication that limiting resources to bacteria in stream biofilms may change over time. Overall, in contrast to the other treatments, bacterial abundance was generally highest in response to the low-molecular-weight DOM (i.e., glucose) treatment. These results further suggest that there are interactions among the different bacterial groups in biofilms that are impacted by the associated nutrient dynamics among seasons in stream ecosystems.     

Allochthonous dissolved organic matter as an energy source for pelagic bacteria and the concept of the microbial loop

Substantial evidence exists that allochthonous dissolved organic matter (DOM) can provide an important carbon source for pelagic bacteria. On the other hand, it is implicit in the concept of the ‘microbial loop’ that the degradation of recalcitrant, allochthonous DOM should be retarded in the pelagic environment, as bacteria able to utilize recalcitrant DOM compounds for slow growth would be outcompeted by faster-growing bacteria utilizing more labile DOM compounds. Several possible solutions of this apparent paradox are suggested in this paper, including formation of labile DOM from recalcitrant DOM by e.g. photochemical reactions, and mechanisms enabling the maintenance of a metabolically diverse bacterioplankton. These mechanisms include an explanation analogous to Hutchinson's classical solution to the ‘paradox of plankton’, and differential mortality of different populations within the bacterioplankton enabled by selective grazing, infections by bacteriophages and predatory bacteria, and spatial micropatchiness.     

Labile dissolved organic carbon and water temperature as regulators of heterotrophic bacterial activity and production in the lakes of Sub-Antarctic Marion Island

Summary The objectives of the 3 year study were to determine the relationship between bacterial numbers and phytoplankton standing crops (chlorophyll a) in sub-antarctic Marion Island lakes (33) and to determine the relative importance of labile dissolved organic carbon and water temperature as regulators of heterotrophic bacterial activity and production. Bacterial activity (the incorporation and respiration rates of ¹⁴C-labelled substrates) and production (the rate of [methyl-³H]thymidine incorporation into DNA) were measured in oligotrophic Lava Lake and Gentoo Lake, an elephant seal wallow. Samples were incubated under ambient conditions as well as at increased temperature and with additions of labile dissolved organic carbon (DOC). Bacterial numbers ranged from 2.13 × 10⁵ cell ml^–1 to 15.17 × 10⁶ cells ml^–1 in the lake survey. The chlorophyll range was 0.18 to >75 g 1^–1. Bacterial numbers were not correlated to chlorophyll concentration in waters where the chlorophyll content was 5 g 1^–1 but were correlated in waters with larger algal contents. Heterotrophic bacterial activity and production, which were similar to rates recorded for equivalent lower latitude systems, were higher in Gentoo Lake than in Lava Lake. As a result of qualitative and quantitative differences in the DOC pools, DOC was the stronger regulator of bacterial activity and production in Lava Lake, while temperature was the stronger factor in Gentoo Lake.     

Limitation of bacterial growth by dissolved organic matter and iron in the Southern ocean

The importance of resource limitation in controlling bacterial growth in the high-nutrient, low-chlorophyll (HNLC) region of the Southern Ocean was experimentally determined during February and March 1998. Organic- and inorganic-nutrient enrichment experiments were performed between 42 degrees S and 55 degrees S along 141 degrees E. Bacterial abundance, mean cell volume, and [(3)H]thymidine and [(3)H]leucine incorporation were measured during 4- to 5-day incubations. Bacterial biomass, production, and rates of growth all responded to organic enrichments in three of the four experiments. These results indicate that bacterial growth was constrained primarily by the availability of dissolved organic matter. Bacterial growth in the subtropical front, subantarctic zone, and subantarctic front responded most favorably to additions of dissolved free amino acids or glucose plus ammonium. Bacterial growth in these regions may be limited by input of both organic matter and reduced nitrogen. Unlike similar experimental results in other HNLC regions (subarctic and equatorial Pacific), growth stimulation of bacteria in the Southern Ocean resulted in significant biomass accumulation, apparently by stimulating bacterial growth in excess of removal processes. Bacterial growth was relatively unchanged by additions of iron alone; however, additions of glucose plus iron resulted in substantial increases in rates of bacterial growth and biomass accumulation. These results imply that bacterial growth efficiency and nitrogen utilization may be partly constrained by iron availability in the HNLC Southern Ocean.     

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.     

Crop residue-derived dissolved organic matter accelerates the decomposition of native soil organic carbon in a temperate agricultural ecosystem

Crop residue-derived dissolved organic matter (DOM) plays an important role in soil carbon (C) cycling. To investigate the effects of maize residue-derived DOM and urea additions on the native soil organic carbon (SOC) decomposition and soil net C balance a pot experiment was carried out during the winter wheat growing season in the North China Plain (NCP). The results showed that adding maize residue-derived DOM alone (RDOM) or together with urea (RDOM?+?N) accelerated the decomposition of native SOC and resulted in a net SOC loss. The net loss of SOC was 3.90?±?0.61 and 3.53?±?0.48?g?C?m^?2 in RDOM and RDOM?+?N treatments, respectively. The stimulatory effect of per unit DOM-C addition on the native SOC decomposition was 0.25?±?0.05 and 0.45?±?0.07 for the RDOM and RDOM?+?N treatments, respectively. Increases in the microbial biomass and the activity of β-glucosidase, invertase and cellobiohydrolase as well as soil mineral N content were responsible for a more intense priming effect in DOM-amended soils. The positive relationship between primed soil C and soil available N (R?=?0.76, P?<?0.05) suggested that the stimulation of decomposition of native SOC by DOM addition would be enhanced by nitrogen fertilizer application.