Adaptations of microbial communities and dissolved organics to seasonal pressures in a mesotrophic coastal Mediterranean lake

In lake ecosystems, changes in eukaryotic and prokaryotic microbes and the concentration and availability of dissolved organic matter (DOM) produced within or supplied to the system by allochthonous sources are components that characterize complex processes in the microbial loop. We address seasonal changes of microbial communities and DOM in the largest Croatian lake, Vrana. This shallow lake is connected to the Adriatic Sea and is impacted by agricultural activity. Microbial community and DOM structure were driven by several environmental stressors, including drought, seawater intrusion and heavy precipitation events. Bacterial composition of different lifestyles (free-living and particle-associated) differed and only a part of the particle-associated bacteria correlated with microbial eukaryotes. Oscillations of cyanobacterial relative abundance along with chlorophyll a revealed a high primary production season characterized by increased levels of autochthonous DOM that promoted bacterial processes of organic matter degradation. From our results, we infer that in coastal freshwater lakes dependent on precipitation-evaporation balance, prolonged dry season coupled with heavy irrigation impact microbial communities at different trophic levels even if salinity increases only slightly and allochthonous DOM inputs decrease. These pressures, if applied more frequently or at higher concentrations, could have the potential to overturn the trophic state of the lake.     

Contrasting effects of solar UV radiation on dissolved organic sources for bacterial growth

The photochemical transformation of dissolved organic matter (DOM) in lakes and oceans has been shown to either reduce or enhance bacterial utilization. We compared the effects of UV radiation on the bacterial use of DOM in a wide range of lakes. Although complex DOM was converted in all irradiated samples into carboxylic acids that are readily utilized by bacteria, irradiation in several lakes resulted in a decreased ability of DOM to support bacterial growth. The effect of irradiation on the ability of DOM to promote bacterial growth was a positive function of the terrestrial humic matter, and a negative function of indigenous algal production. We suggest that the net effect of irradiation is a result of counteracting but concurrent processes rendering DOM either labile or recalcitrant. Humic DOM is predominantly transformed into forms of increased lability, whereas photochemical transformation into compounds of decreased bacterial substrate quality dominates in algal-derived DOM. Hence, solar-induced photochemical reactions interact with microbial degraders in different ways, depending on the origin and nature of the organic matter, affecting the transfer of energy within aquatic food webs, as well as the degradation and preservation of detrital organic matter, in different directions.     

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.     

Spatial and temporal variability of dissolved organic matter quantity and composition in an oligotrophic subtropical coastal wetland

Dissolved organic matter (DOM) is an essential component of the carbon cycle and a critical driver in controlling variety of biogeochemical and ecological processes in wetlands. The quality of this DOM as it relates to composition and reactivity is directly related to its sources and may vary on temporal and spatial scales. However, large scale, long-term studies of DOM dynamics in wetlands are still scarce in the literature. Here we present a multi-year DOM characterization study for monthly surface water samples collected at 14 sampling stations along two transects within the greater Everglades, a subtropical, oligotrophic, coastal freshwater wetland-mangrove-estuarine ecosystem. In an attempt to assess quantitative and qualitative variations of DOM on both spatial and temporal scales, we determined dissolved organic carbon (DOC) values and DOM optical properties, respectively. DOM quality was assessed using, excitation emission matrix (EEM) fluorescence coupled with parallel factor analysis (PARAFAC). Variations of the PARAFAC components abundance and composition were clearly observed on spatial and seasonal scales. Dry versus wet season DOC concentrations were affected by dry-down and re-wetting processes in the freshwater marshes, while DOM compositional features were controlled by soil and higher plant versus periphyton sources respectively. Peat-soil based freshwater marsh sites could be clearly differentiated from marl-soil based sites based on EEM–PARAFAC data. Freshwater marsh DOM was enriched in higher plant and soil-derived humic-like compounds, compared to estuarine sites which were more controlled by algae- and microbial-derived inputs. DOM from fringe mangrove sites could be differentiated between tidally influenced sites and sites exposed to long inundation periods. As such coastal estuarine sites were significantly controlled by hydrology, while DOM dynamics in Florida Bay were seasonally driven by both primary productivity and hydrology. This study exemplifies the application of long term optical properties monitoring as an effective technique to investigate DOM dynamics in aquatic ecosystems. The work presented here also serves as a pre-restoration condition dataset for DOM in the context of the Comprehensive Everglades Restoration Plan (CERP).     

Diversity of catechol 2,3-dioxygenase genes of bacteria responding to dissolved organic matter derived from different sources in a eutrophic lake

Catechol 2,3-dioxygenase (C23O) is an extradiol dioxygenase that plays an important role in degrading aromatic compounds such as those found at polluted sites. However, little is known about the diversity of C23O genes in unpolluted environments. In such environments, various factors, including the quality and quantity of dissolved organic matter (DOM), could influence the composition and behaviour of bacterial community possessing C230 genes. We investigated C23O genes in bacteria responding to DOM from various sources in a eutrophic lake by PCR and cloning. Six microcosms filled with lake water containing indigenous bacteria and DOM from different sources were incubated for 10 days. After 1 or 2 days of incubation, C23O genes were detected in the microcosms enriched with DOM recovered from inflow river water and humus from reed grass. The sequences were very diverse but had features conserved in extradiol dioxygenases. The clone libraries generated on day 2 showed distinctive compositions among microcosms, indicating that bacteria possessing a variety of C23O genes responded differently to DOM from different sources. After 10 days of incubation, C23O genes in a previously unidentified gene cluster, 'Cluster X', became dominant in the libraries.     

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.     

Temperature Dependence of Photodegradation of Dissolved Organic Matter to Dissolved Inorganic Carbon and Particulate Organic Carbon

Photochemical transformation of dissolved organic matter (DOM) has been studied for more than two decades. Usually, laboratory or “in-situ” experiments are used to determine photodegradation variables. A common problem with these experiments is that the photodegradation experiments are done at higher than ambient temperature. Five laboratory experiments were done to determine the effect of temperature on photochemical degradation of DOM. Experimental results showed strong dependence of photodegradation on temperature. Mathematical modeling of processes revealed that two different pathways engaged in photochemical transformation of DOM to dissolved inorganic carbon (DIC) strongly depend on temperature. Direct oxidation of DOM to DIC dominated at low temperatures while conversion of DOM to intermediate particulate organic carbon (POC) prior to oxidation to DIC dominated at high temperatures. It is necessary to consider this strong dependence when the results of laboratory experiments are interpreted in regard to natural processes. Photodegradation experiments done at higher than ambient temperature will necessitate correction of rate constants.     

Carbonate recrystallization in root-free soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by 14C labeling

The photo- and bio-degradation of dissolved organic matter (DOM) in water from the Broad River were investigated in laboratory experiments using a solar simulator to control the intensity and exposure of samples to irradiation. The water samples included a natural assemblage of microorganisms, and the daily exposure of samples to irradiation was varied to distinguish the relative contributions of photochemical and biological degradation. Concentrations of dissolved organic carbon (DOC) and specific components of DOM, including chromophoric DOM (CDOM), lignin phenols and amino acids, were monitored to investigate the reactivity and predominant pathway of degradation of these DOM components. Biodegradation was primarily responsible for the overall remineralization of DOC and losses of the amino acid component of DOM, whereas photodegradation was primarily responsible for losses of the chromophoric and lignin phenol components of DOM. The rates of photodegradation of lignin phenols were strongly influenced by the presence of methoxy groups on the aryl ring. Syringyl (S) phenols have two methoxy substitutions, vanillyl (V) phenols have one methoxy substitution, and p-hydroxy (P) phenols are not substituted with methoxy groups. Photochemical decay constants were highest for S phenols, lowest for P phenols and followed a consistent pattern (S > V > P) in the experiments. The carbon-normalized yields of amino acids and lignin phenols were found to be useful molecular indicators of the highly reactive (i.e. labile) components of biodegradable and photodegradable DOM, respectively.     

Distinct Optical Chemistry of Dissolved Organic Matter in Urban Pond Ecosystems

Urbanization has the potential to dramatically alter the biogeochemistry of receiving freshwater ecosystems. We examined the optical chemistry of dissolved organic matter (DOM) in forty-five urban ponds across southern Ontario, Canada to examine whether optical characteristics in these relatively new ecosystems are distinct from other freshwater systems. Dissolved organic carbon (DOC) concentrations ranged from 2 to 16 mg C L^-1 across the ponds with an average value of 5.3 mg C L^-1. Excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) modelling showed urban pond DOM to be characterized by microbial-like and, less importantly, by terrestrial derived humic-like components. The relatively transparent, non-humic DOM in urban ponds was more similar to that found in open water, lake ecosystems than to rivers or wetlands. After irradiation equivalent to 1.7 days of natural solar radiation, DOC concentrations, on average, decreased by 38% and UV absorbance decreased by 25%. Irradiation decreased the relative abundances of terrestrial humic-like components and increased protein-like aspects of the DOM pool. These findings suggest that high internal production and/or prolonged exposure to sunlight exerts a distinct and significant influence on the chemistry of urban pond DOM, which likely reduces its chemical similarity with upstream sources. These properties of urban pond DOM may alter its biogeochemical role in these relatively novel aquatic ecosystems.     

Complementary pathways of dissolved organic carbon removal pathways in clear-water Amazonian ecosystems: photochemical degradation and bacterial uptake

Dissolved organic carbon (DOC) photochemical reactions establish important links between DOC and planktonic bacteria. We hypothesize that seasonal changes in DOC quality, related to the flood pulse, drive the effects of light-DOC interactions on uptake by planktonic bacteria uptake in clear-water Amazonian ecosystems. Water samples from two ecosystems (one lake and one stream) were incubated in sunlight during different hydrological periods and were then exposed to bacterial degradation. Photochemical and bacterial degradation were driven by seasonal DOC inputs. Bacterial mineralization was the main degradation pathway of autochthonous DOC in the lake, while allochthonous DOC was more available for photochemical oxidation. We suggest that sunlight enhances the bacterial uptake of refractory DOC but does not alter uptake of labile forms. We also observed a positive relationship between sunlight and bacterial degradation of DOC, instead of competition. We conclude that photochemical reactions and bacteria complementarily degrade the different sources of DOC during the flood pulse in Amazonian clear-water aquatic ecosystems.     

Influence of soil temperature and moisture on the dissolved carbon,nitrogen, and phosphorus in organic matter entering lake ecosystems

Concentrations of terrestrially derived dissolved organic matter (DOM) have been increasing in many north temperate and boreal lakes for over two decades. The concentration of DOM in lakes is influenced by a number of environmental factors, but there is still considerable debate about how the availability of terrestrial DOM, and associated dissolved nitrogen and phosphorus, may be affected by drivers of climatic change. Using experimental and observational methods, we considered how changes in soil temperature and moisture affected the composition of carbon, nitrogen, and phosphorus entering freshwater lakes. In our experiment, organic soil cores were collected from the wetland shoreline of a darkly-stained seepage lake in northern Wisconsin, USA and manipulated in laboratory with temperature and moisture treatments. During the 28-day study, soil leachate was sampled and analyzed for optical properties of DOM via UV/Vis absorbance, as well as concentrations of dissolved organic carbon (DOC), total dissolved nitrogen, and total dissolved phosphorus (TDP). DOM optical properties were particularly sensitive to moisture, with drier scenarios resulting in DOM of lower molecular weight and aromaticity. Warmer temperatures led to lower DOC and TDP concentrations. To consider long-term relationships between climate and lake chemical properties, we analyzed long-term water chemistry data from two additional Wisconsin lakes from the long term ecological research (LTER) project in a cross correlation analysis with Palmer drought severity index data. Analysis of the LTER data supported our experimental results that soil moisture has a significant effect on the quality of DOM entering lakes and that climate may significantly affect lake chemical properties. Although unexpected in terms of DOM loading for climate change scenarios, these results are consistent with patterns of decomposition in organic soils and may be attributed to an increase in soil DOM processing.     

High molecular weight dissolved organic matter enrichment selects for methylotrophs in dilution to extinction cultures

The role of bacterioplankton in the cycling of marine dissolved organic matter (DOM) is central to the carbon and energy balance in the ocean, yet there are few model organisms available to investigate the genes, metabolic pathways, and biochemical mechanisms involved in the degradation of this globally important carbon pool. To obtain microbial isolates capable of degrading semi-labile DOM for growth, we conducted dilution to extinction cultivation experiments using seawater enriched with high molecular weight (HMW) DOM. In total, 93 isolates were obtained. Amendments using HMW DOM to increase the dissolved organic carbon concentration 4x (280 μM) or 10x (700 μM) the ocean surface water concentrations yielded positive growth in 4–6% of replicate dilutions, whereas <1% scored positive for growth in non-DOM-amended controls. The majority (71%) of isolates displayed a distinct increase in cell yields when grown in increasing concentrations of HMW DOM. Whole-genome sequencing was used to screen the culture collection for purity and to determine the phylogenetic identity of the isolates. Eleven percent of the isolates belonged to the gammaproteobacteria including Alteromonadales (the SAR92 clade) and Vibrio. Surprisingly, 85% of isolates belonged to the methylotrophic OM43 clade of betaproteobacteria, bacteria thought to metabolically specialize in degrading C1 compounds. Growth of these isolates on methanol confirmed their methylotrophic phenotype. Our results indicate that dilution to extinction cultivation enriched with natural sources of organic substrates has a potential to reveal the previously unsuspected relationships between naturally occurring organic nutrients and the microorganisms that consume them.     

Photo-biochemical transformation of dissolved organic matter on the surface of the coastal East Antarctic ice sheet

Recent studies have highlighted the composition and complexity of dissolved organic matter (DOM) in glacial environments. Climate-induced changes to glacier runoff are projected to be an important source of DOM to coastal ecosystems. Photochemical and microbial (termed photo-biochemical) degradation of DOM would determine its fate on the glacier surface and in recipient coastal ecosystems. In order to understand the molecular imprints of photo-biochemical alteration of DOM, in situ field experiments were conducted over a period of 35 days in a coastal Antarctic site and DOM molecularly characterised using ultrahigh-resolution mass spectrometry. We show that the biogeochemistry of DOM is highly complex and intimately connected with microbial and photochemical processes operating individually or in combination. Photo-biochemical processes resulted in shifts in the nitrogen, sulfur, and phosphorous content of the DOM. These processes are also an important mechanism for transforming refractory DOM, like dissolved black carbon and carboxylic rich alicyclic molecules from the snow surface. This study is unique, as it provides new molecular-level information on compounds that comprise the photo- and bio-labile, photo- and bio-refractory, as well as photo- and bio-produced fractions of the supraglacial DOM pool. These insights into the interactions between microbes, light, and specific components of the DOM pool highlight the need for studies focused on the biogeochemistry of supraglacial carbon and its response to a changing climate.     

Geochemistry of aquatic humic substances in the Lake Fryxell Basin, Antarctica

Dissolved organic carbon (DOC) in Lake Fryxell, 10 streams flowing into the lake, and the moat surrounding the lake was studied to determine the influence of sources and biogeochemical processes on its distribution and chemical nature. Lake Fryxell is an amictic, permanently ice-covered lake in the McMurdo Dry Valleys which contains benthic and planktonic microbial populations, but receives essentially no input of organic material from the ahumic soils of the watershed. Biological activity in the water column does not appear to influence the DOC depth profile, which is similar to the profiles for conservative inorganic constituents. DOC values for the streams varied with biomass in the stream channel, and ranged from 0.2 to 9.7 mg C/L. Fulvic acids in the streams were a lower percentage of the total DOC than in the lake. These samples contain recent carbon and appear to be simpler mixtures of compounds than the lake samples, indicating that they have undergone less humification. The fulvic acids from just above the sediments of the lake have a high sulfur content and are highly aliphatic. The main transformations occurring as these fractions diffuse upward in the water column are 1) loss of sulfur groups through the oxycline and 2) decrease in aliphatic carbon and increase in the heterogeneity of aliphatic moieties. The fraction of modem¹⁴C content of the lake fulvic acids range from a minimum of 0.68 (approximately 3000 years old) at 15m depth to 0.997 (recent material) just under the ice. The major processes controlling the DOC in the lake appear to be: 1) The transport of organic matter by the inflow streams resulting in the addition of recent organic material to the moat and upper waters of the lake; 2) The diffusion of organic matter composed of relict organic material and organic carbon resulting from the degradation of algae and bacteria from the bottom waters or sediments of the lake into overlying glacial melt water, 3) The addition of recent organic matter to the bottom waters of the lake from the moat.     

Substrate incorporation patterns of bacterioplankton populations in stratified and mixed waters of a humic lake

Bacterial incorporation of glucose, leucine, acetate and 4-hydroxybenzoic acid (HBA) was investigated in an artificially divided humic lake (Grosse Fuchskuhle, Germany). Two basins with contrasting influx of allochthonous organic carbon were sampled during late summer stratification (oxic and anoxic layers) and after autumn mixing. High total and cell-specific incorporation rates were observed for glucose and HBA in stratified and mixed waters respectively, but only a small fraction of bacteria visibly incorporated HBA. The oxic layer of the more humic-rich basin featured a significantly lower fraction of glucose incorporating cells and substantially higher proportions of acetate assimilating bacteria. Niche differentiation was observed in two betaproteobacterial populations: cells affiliated with the Polynucleobacter C subcluster efficiently incorporated acetate but little glucose, whereas the opposite was found for members of the R-BT065 clade. By contrast, leucine incorporation was variable in both taxa. Considering the high concentrations and rapid photochemical generation of organic acids in humic waters our results may help to explain the success of the Polynucleobacter C lineage in such habitats. Specific substrate or habitat preferences were also present in three subgroups of the actinobacterial acI lineage: The numerically dominant clade in oxic waters (acI-840-1) was absent in the anoxic zone and did not incorporate acetate. A second group (acI-840-2) was found both in the epi- and hypolimnion, whereas the third one (acI-840-3) only occurred in anoxic waters. Altogether our results suggest a constitutive preference for some substrates versus an adaptive utilization of others in the studied microbial groups.     

Ecological niche separation in the Polynucleobacter subclusters linked to quality of dissolved organic matter: a demonstration using a high sensitivity cultivation-based approach

The free-living, cosmopolitan, freshwater betaproteobacterial bacterioplankton genus Polynucleobacter was detected in different years in 11 lakes of varying types and a river using the size-exclusion assay method (SEAM). Of the 350 strains isolated, 228 (65.1%) were affiliated with the Polynucleobacter subclusters PnecC (30.0%) and PnecD (35.1%). Significant positive correlations between fluorescence in situ hybridization and SEAM data were observed in the relative abundance of PnecC and PnecD bacteria to Polynucleobacter communities (PnecC?+?PnecD). Isolates were mainly PnecC bacteria in the samples with a high specific UV absorbance at 254?nm (SUVA(254) ), and a low total hydrolysable neutral carbohydrate and amino acid (THneutralCH?+?THAA) content of the dissolved organic matter (DOM) fraction, which is known to be correlated with a high humic content. In contrast, the PnecD bacteria were abundant in samples with high chlorophyll a and/or THneutralCH?+?THAA concentrations, indicative of primary productivity. With few exceptions, differences in the relative abundance of PnecC and PnecD in each sample, determined using a high-sensitivity cultivation-based approach, were due to DOM quality. These results suggest that the major DOM component in the field, which is allochthonously or autochthonously derived, is a key factor for ecological niche separation between PnecC and PnecD subclusters.     

Anaerobic microbial associations degrading aromatic amino acids

Anaerobic microbial associations have been isolated that degrade aromatic amino acids to methane and carbon dioxide at high rates. Significant differences between the morphological, cytological, and physiological traits of cultures isolated from samples of adapted and unadapted sludge are shown. The effects of cultivation temperature, illumination, and presence of mineral nitrogen and bicarbonate in the medium upon adaptation of enrichment cultures to substrates and subsequent behavior of the anaerobic associations have been studied. Intermediate and final products of degradation of aminoaromatic compounds and the sequence of their formation in the cultures have been determined. We have also studied the effects of exogenous electron acceptors and additional carbon sources on the degradation of aminoaromatic compounds.     

Colloidal and dissolved organic matter in lake water: Carbohydrate and amino acid composition,and ability to support bacterial growth

Bacterial utilization of dissolved organic matter (DOM) was studied in water from a humic and a clearwater oligotrophic lake. Indigenous bacteria were inoculated into either 0.2 m natural filtered lake water, or lake water enriched fivefold with colloidal DOM >100 kD but below 0.2 m. Consumption of DOM was followed from changes in concentrations of total dissolved organic carbon (DOC), dissolved combined and free carbohydrates and amino acids (DCCHO and DFCHO, and DCAA and DFAA, respectively) and by uptake of monosaccharide and amino acid radioisotopes. DCCHO and DCAA made up 8% (humic lake) to 33–44% (clear-water lake) of the natural DOC pools, while DFCHO and DFAA contributed at most 1.7% to the DOC pools. Addition of >100 kD DOM increased the DOC concentrations by 50% (clearwater lake) to 92% (humic lake), but it only resulted in a higher bacterial production (by 63%) in the humic lake. During the incubations 13 to 37% of the DOC was assimilated by the bacteria, at estimated growth efficiencies of 4–8%. Despite the measured reduction of DOC, statistically significant changes of specific organic compounds, especially of DCCHO and DCAA, generally did not occur. Probably the presence of high molecular weight DOC interfered with the applied analytical procedures. Addition of radiotracers indicated, however, that DFAA sustained 17–58% and 29–100% of the bacterial carbon and nitrogen requirements, respectively, and that glucose met 1–3% of the bacterial carbon requirements. Thus, our experiments indicate that radiotracers, rather than measurements of concentration changes, should be used in studies of bacterial utilization of DOC in freshwaters with a high content of humic or high molecular weight organic matter.     

Gradient-dominated ecosystems: sources and regulatory functions of dissolved organic matter in freshwater ecosystems

The emergent wetland and littoral components of the land-water zone are functionally coupled by the amounts and types of dissolved organic matter that are released, processed, transported to, and then further processed within the recipient waters. Operational couplings and integrations in freshwater ecosystems occur along physical and metabolic gradients of a number of scales from micrometer to kilometer dimensions. The operation and turnover of the microbial communities, largely associated with surfaces, generate the metabolic foundations for material fluxes along larger-scale gradients. Because of the predominance of small, shallow freshwater bodies, most dissolved organic carbon (DOC) of lacustrine and riverine ecosystems is derived from photosynthesis of higher plants and microflora associated with detritus, including sediments, and is only augmented by photosynthesis of phytoplankton. As the dissolved organic compounds generated in the wetland and littoral interface regions move toward the open-water regions of the ecosystems, partial utilization effects a selective increase in organic recalcitrance. Even though DOC from allochthonous and from interface sources is more recalcitrant than that produced by planktonic microflora, decomposition of the much larger interface quantities imported to the pelagic zone dominates ecosystem decomposition. The observed high sustained productivity of the land-water interface zone results from extensive recycling of essential resources (nutrients, inorganic carbon) and conservation mechanisms. On the average in lakes and streams, greater than 90 percent of the decomposition in the ecosystem is by bacteria utilizing DOM from non-pelagic sources of primary productivity. In addition to direct mineralization of DOC from non-pelagic sources, many of the organic compounds function indirectly to influence metabolism. New evidence is presented to demonstrate formation of complexes between humic and fulvic organic acids and extracellular enzymes. These complexes inhibit enzyme activity and can be transported within the ecosystem. The complex can be decoupled by mild ultraviolet photolysis with regeneration of enzyme activity in displaced locations.