Lars J. Tranvik

Freshwater Methane Emissions Offset the Continental Carbon Sink

Inland freshwaters, which include lakes, reservoirs, streams, and rivers, may emit far more methane than previously thought. Inland waters (lakes, reservoirs, streams, and rivers) are often substantial methane (CH4) sources in the terrestrial landscape. They are, however, not yet well integrated in global greenhouse gas (GHG) budgets. Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that freshwaters emit at least 103 teragrams of CH4 year−1, corresponding to 0.65 petagrams of C as carbon dioxide (CO2) equivalents year−1, offsetting 25% of the estimated land carbon sink. Thus, the continental GHG sink may be considerably overestimated, and freshwaters need to be recognized as important in the global carbon cycle.

Availability of dissolved organic carbon for planktonic bacteria in oligotrophic lakes of differing humic content.

Bacterioplankton from 10 oligotrophic lakes, representing a gradient from clearwater to polyhumic, were grown in dilution cultures of sterile filtered lake water. The bacterial biomass achieved in the stationary phase of the dilution cultures was positively correlated with the amount of both humic matter and dissolved organic carbon (DOC) in the lakes. About the same fraction of the total DOC pool was consumed in the dilution cultures of all lakes (average 9.5%, coefficient of variation (CV) 24%), with approximately the same growth efficiency (average 26%, CV 28%). Thus, humic lakes could support a higher bacterial biomass than clearwater lakes due to their larger DOC pools. The relevance of the results to planktonic food webs of humic and clearwater lakes is discussed.

A global inventory of lakes based on high‐resolution satellite imagery

An accurate description of the abundance and size distribution of lakes is critical to quantifying limnetic contributions to the global carbon cycle. However, estimates of global lake abundance are ...

Temperature-controlled organic carbon mineralization in lake sediments

Peatlands, soils and the ocean floor are well-recognized as sites of organic carbon accumulation and represent important global carbon sinks. Although the annual burial of organic carbon in lakes and reservoirs exceeds that of ocean sediments, these inland waters are components of the global carbon cycle that receive only limited attention. Of the organic carbon that is being deposited onto the sediments, a certain proportion will be mineralized and the remainder will be buried over geological timescales. Here we assess the relationship between sediment organic carbon mineralization and temperature in a cross-system survey of boreal lakes in Sweden, and with input from a compilation of published data from a wide range of lakes that differ with respect to climate, productivity and organic carbon source. We find that the mineralization of organic carbon in lake sediments exhibits a strongly positive relationship with temperature, which suggests that warmer water temperatures lead to more mineralization and less organic carbon burial. Assuming that future organic carbon delivery to the lake sediments will be similar to that under present-day conditions, we estimate that temperature increases following the latest scenarios presented by the Intergovernmental Panel on Climate Change could result in a 4–27 per cent (0.9–6.4 Tg C yr−1) decrease in annual organic carbon burial in boreal lakes.

Aquatic Humic Substances

4 Chemical Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 4.1 Elemental Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 4.2 Molecular Size Distribution and Charge . . . . . . . . . . . . . . . . . . . . . . 309 4.3 Spectral Absorbance and Fluorescence . . . . . . . . . . . . . . . . . . . . . . . 311 4.4 Infrared Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 4.5 Mass Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 4.6 NMR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 4.7 Soft Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316

Heterotrophic bacterial growth efficiency and community structure at different natural organic carbon concentrations.

ABSTRACT Batch cultures of aquatic bacteria and dissolved organic matter were used to examine the impact of carbon source concentration on bacterial growth, biomass, growth efficiency, and community composition. An aged concentrate of dissolved organic matter from a humic lake was diluted with organic compound-free artificial lake water to obtain concentrations of dissolved organic carbon (DOC) ranging from 0.04 to 2.53 mM. The bacterial biomass produced in the cultures increased linearly with the DOC concentration, indicating that bacterial biomass production was limited by the supply of carbon. The bacterial growth rate in the exponential growth phase exhibited a hyperbolic response to the DOC concentration, suggesting that the maximum growth rate was constrained by the substrate concentration at low DOC concentrations. Likewise, the bacterial growth efficiency calculated from the production of biomass and CO2 increased asymptotically from 0.4 to 10.4% with increasing DOC concentration. The compositions of the microbial communities that emerged in the cultures were assessed by separation of PCR-amplified 16S rRNA fragments by denaturing gradient gel electrophoresis. Nonmetric multidimensional scaling of the gel profiles showed that there was a gradual change in the community composition along the DOC gradient; members of the β subclass of the class Proteobacteria and members of the Cytophaga-Flavobacterium group were well represented at all concentrations, whereas members of the α subclass of the Proteobacteria were found exclusively at the lowest carbon concentration. The shift in community composition along the DOC gradient was similar to the patterns of growth efficiency and growth rate. The results suggest that the bacterial growth efficiencies, the rates of bacterial growth, and the compositions of bacterial communities are not constrained by substrate concentrations in most natural waters, with the possible exception of the most oligotrophic environments.

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 Hutchinsons 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.

Interactions of bacteria and fungi on decomposing litter: differential extracellular enzyme activities.

Fungi and bacteria are key agents in plant litter decomposition in freshwater ecosystems. However, the specific roles of these two groups and their interactions during the decomposition process are unclear. We compared the growth and patterns of degradative enzymes expressed by communities of bacteria and fungi grown separately and in coexistence on Phragmites leaves. The two groups displayed both synergistic and antagonistic interactions. Bacteria grew better together with fungi than alone. In addition, there was a negative effect of bacteria on fungi, which appeared to be caused by suppression of fungal growth and biomass accrual rather than specifically affecting enzyme activity. Fungi growing alone had a high capacity for the decomposition of plant polymers such as lignin, cellulose, and hemicellulose. In contrast, enzyme activities were in general low when bacteria grew alone, and the activity of key enzymes in the degradation of lignin and cellulose (phenol oxidase and cellobiohydrolase) was undetectable in the bacteria-only treatment. Still, biomass-specific activities of most enzymes were higher in bacteria than in fungi. The low total activity and growth of bacteria in the absence of fungi in spite of apparent high enzymatic efficiency during the degradation of many substrates suggest that fungi provide the bacteria with resources that the bacteria were not able to acquire on their own, most probably intermediate decomposition products released by fungi that could be used by bacteria.

Summer inputs of riverine nutrients to the Baltic Sea: Bioavailability and eutrophication relevance

Most nitrogen and phosphorus transported by world rivers to the oceans is associated with dissolved organic matter. However, organic matter as a potential source of N and P has hitherto been largely neglected in studies of coastal microbial food webs. We examined 50 rivers, draining a major part of the Baltic Sea watershed, with respect to summer concentrations, chemical composition, and biological availability of N and P. The broad spectrum of rivers studied enabled us to assess whether the input of terrigenous organic matter can be an important nutrient source, at various levels of anthropogenic loading of inorganic N and P. Concentrations of total N and P ranged from 9 to 220 mumol/L and from 0.14 to 5.56 mumol/L, respectively, with the highest concentrations in the southern part of the Baltic Sea drainage area and in several rivers on the Finnish western coast. Urea and dissolved combined amino acids (DCAA) each constituted 4-20% of dissolved organic nitrogen (DON), while dissolved free amino acids (DFAA) made up <3% of DON. The contribution of urea and amino acids to the DON pool was inversely correlated with DON concentration. Bacterial regrowth bioassays in selected rivers demonstrated that similar to30% of DON and similar to75% of dissolved organic phosphorus (DOP) was potentially available to the indigenous bacterial assemblage of the Baltic Sea, and hence susceptible to mineralization within the pelagic food web. Our study is among the first to demonstrate that bacterioplankton are able to utilize a major part of DON and DOP from a broad spectrum of natural waters. The C:N ratio, absorbance spectra, and fluorescence properties of the organic matter suggest that the observed high bioavailability of DON and DOP was due to a large contribution of organic matter from riverine primary production compared to the humic matter derived from terrestrial vascular plants. In addition, algal and bacterial cells dominated the transport of particulate organic material, further enhancing productivity of coastal waters. No correlations were found between DON bioavailability and the fraction of DON bound in urea and amino acids, indicating a utilization of other N compounds (e.g., amides) by the bacteria. We estimate that the input of summer riverine N to the Baltic Sea consists of 48% dissolved inorganic N, 41 % DON, and 11 % particulate N. Corresponding values for phosphorus are 46%, 18%, and 36% of dissolved inorganic P, DOP, and particulate P, respectively. During the thermal summer stratification, when freshwater inputs are trapped in the surface layer, rivers contribute similar to30% of N and similar to5% of P needed to support the export production (plankton sedimenting out of the photic layer) in the Baltic Sea. The high availability to bacteria suggests that DOP is a major stimulator of pelagic productivity in the P-limited northern part of the Baltic Sea. Based on reported concentrations in other areas, we suggest that the global contribution of riverine organic N and P to the primary production of coastal waters is comparable to the contribution of inorganic nutrients. (Less)

Antagonism between bacteria and fungi on decomposing aquatic plant litter.

Bacterial and fungal decomposers of aquatic plant litter may exhibit either synergistic or antagonistic interactions, which are likely to influence microbial growth as well as the decomposition of litter and, eventually, the carbon metabolism of aquatic systems. To elucidate such interactions, we inoculated decomposing Phragmites culms in microcosms with fungal isolates and with natural communities of bacteria and fungi in different combinations. The development of fungal and bacterial biomass and the carbon dynamics were studied during several months of degradation. The results show a bilateral antagonistic relationship between bacteria and fungi. After 3 months, fungal biomass accumulation was approximately 12 times higher in the absence than in the presence of bacteria. Bacterial biomass accumulation was about double in the absence of fungi compared to when fungi were present. Similar interactions developed between a natural assemblage of bacteria and five different fungal strains isolated from Phragmites litter (three identified hyphomycetes and two unidentified strains). Despite the great difference in biomass development between the treatments, the carbon metabolism was similar regardless of whether fungi and/or bacteria were present alone or in coexistence. We suggest that the antagonism between bacteria and fungi is an important controlling factor for microbial colonization and growth on aquatic plant litter.