Application of fungal and bacterial production methodologies to decomposing leaves in streams

Periphyton communities associated with submerged plant detritus contain interacting autotrophic and heterotrophic microbes, and are sites of extracellular enzymatic activity. The strength and nature of these interactions might be expected to change over time as microbial communities develop on plant litter. Microbial interactions and enzymatic activity can be altered by nutrient availability, suggesting that litter stoichiometry could also affect these phenomena. We grew wetland plants under ambient and nutrient‐enriched conditions to generate plant litter of differing nutrient content. In two experiments, we investigated: (1) the influence of algal photosynthesis on fungal and bacterial production and the activities of four extracellular enzymes throughout a 54‐day period of microbial colonisation and growth; and (2) the influence of litter stoichiometry on these relationships. Ambient and nutrient‐enriched standing‐dead plant litter was collected and then submerged in wetland pools to allow for natural microbial colonisation and growth. Litter samples were periodically retrieved and transported to the laboratory for experiments manipulating photosynthesis using the photosystem II inhibitor DCMU (which effectively prevents algal photosynthetic activity). Algal (14C‐bicarbonate), bacterial (3H‐leucine), and fungal (14C‐acetate) production, and β‐glucosidase, β‐xylosidase, leucine aminopeptidase, and phosphatase activities (MUF‐ or AMC‐labelled fluorogenic substrates) were measured under conditions of active and inhibited algal photosynthesis. Photosynthesis stimulated overall fungal and bacterial production in both experiments, although the strength of stimulation varied amongst sampling dates. Phosphatase activity was stimulated by photosynthesis during the first, but not the second, experiment. No other enzymatic responses to short‐term photosynthesis manipulations were observed. Microbial communities on high‐nutrient litter occasionally showed increased extracellular enzyme activity, fungal growth rates, and bacterial production compared to communities on non‐enriched litter, but algal and fungal production were not affected. Litter stoichiometry had no effects on fungal, bacterial, or enzymatic responses to photosynthesis, but the mean enzyme vector analysis angle (a measure of P‐ versus N‐acquiring enzyme activity) was positively correlated to litter N:P, suggesting that elevated litter N:P led to an increase in the relative activity of P‐acquiring enzymes. These results supported the hypothesis that algal photosynthesis strongly influences heterotrophic microbial activity on macrophyte leaf litter, especially that of fungi, throughout microbial community development. However, the strength of this photosynthetic stimulation does not generally depend on small differences in litter nutrient content. Stimulation of microbial heterotrophs by algal photosynthesis could drive diurnal shifts in periphyton community and aquatic ecosystem function, as well as linking green (photoautotroph‐based) and brown (detrital‐based) food webs.

We compared the production and biomass of fungi with that of bacteria during the breakdown of yellow poplar leaves (Liriodendron tulipifera) in 2 streams that differed in water chemistry. The hardwater stream contained higher concentrations of nutrients (N and P) than the softwater stream. Fungal biomass (determined from ergosterol concentrations), production (determined from rates of [¹⁴C]acetate incorporation into ergosterol), and sporulation rates associated with leaves were greater in the hardwater stream than in the softwater stream. Bacterial biomass (determined from direct counts and cell volume estimates) was similar on leaves in both streams, but bacterial production (determined from rates of [³H]leucine incorporation into protein) was greater on leaves in the hardwater stream than in the softwater stream. Fungal biomass associated with leaves was always much greater than bacterial biomass (385-1236× in the hardwater stream, 32-185× in the softwater stream) during leaf breakdown. Fungal production reached maximum values within the first 2 wk after leaves were submerged in the hardwater stream. In the softwater stream, fungal production was low and remained relatively constant throughout the study with a minimum occurring after 28 d. In both streams, bacterial production increased throughout leaf breakdown. Even so, with the exception of 1 date, production of fungi was greater (2-108× in the hardwater stream and 0.9-35× in the softwater stream) than bacterial production during leaf breakdown. On the basis of both biomass and production, fungi played a greater role than bacteria in the breakdown of yellow poplar leaves in these streams.

1. Fungi are thought to be important mediators of energy flow in the detritus‐based food webs of woodland streams. However, until recently, quantitative methods to assess their contribution have been lacking. Growth rates of leaf‐decaying fungi can be estimated from rates of acetate incorporation into ergosterol which, together with estimates of fungal biomass from ergosterol concentrations, enables calculation of fungal production. In this study, I used this method to estimate total production of leaf‐decaying fungi over an annual cycle in a small woodland stream, Walker Branch, Tennessee, U.S.A. To calculate fungal biomass and production on an areal basis, I determined the amount of leaf litter occurring in the stream by sampling transects randomly selected in each of ten 10‐m sections every 20–50 days. Subsamples of leaves chosen from five of the transects were used to determine ergosterol concentrations and in situ rates of acetate incorporation into ergosterol.2. Leaf litter, fungal biomass m–2, and fungal production m–2 were highly seasonal. Leaf litter ranged from 249 g m–2 in November to less than 5 g m–2 during the summer. Fungal biomass as percentage of leaf litter ranged from 4.4 to 8.8% during the year, but on an areal basis ranged from 11 to 13 g m–2 during November to January to 0.25 g m–2 in June, primarily due to the seasonal variation in amount of leaf litter present. Fungal growth rates averaged 2.6% day–1 (0.9–7.0% day–1) during the year. Daily production of leaf‐decaying fungi ranged from 0.49 g m–2 in November, when the amount of leaf litter was at its maximum, to 0.006 g m–2 during the summer when the amount of leaf litter was low. Annual production of leaf‐decaying fungi was 34 g m–2, with an annual production to biomass ratio (P/B) of 8.2.3. Fungal spore concentrations in the stream were also seasonal and were correlated with amount of leaf litter m–2 and fungal biomass m–2. Spore concentrations varied between one and four spores ml–1 throughout most of the year, but increased to eighteen spores ml–1 shortly after the greatest amount of leaf litter was present in the stream during November.

Bacteria play an extraordinarily important role in carbon transformations. It is therefore crucial to accurately measure bacterial production. One of the most widely used methods is the leucine method. From rates of leucine incorporation bacterial carbon production can be derived by empirical or theoretical conversion factors (CFs). However, only few empirical CFs have been established, and no estimation of the theoretical conversion factor for freshwater systems exists until today. Therefore, we determined both, the empirical and the theoretical conversion factor, of sediment bacteria from a headwater stream. The empirical conversion factor determined from exponentially growing bacteria was 1.445 kg C mol−1. The theoretical conversion factor derived from the determination of the molar fraction of leucine in bacterial protein (0.081 ± 0.001) was 1.442 kg C mol−1. Both conversion factors are close to each other and similar to conversion factors established for marine bacterioplankton. Therefore, results of the present study indicate that high values of bacterial production determined in freshwater sediments by the leucine method in several studies were not overestimates caused by inappropriate use of CFs from marine systems but represent true high bacterial production in these environments. For studies that apply the leucine technique in freshwaters, we recommend using the theoretical conversion factor for calculation of bacterial carbon production: BCP (kg) = 1.44 × Leuinc (Leuinc = leucine incorporation in mol).

Fungi are key components in the decomposition of plant litter in aquatic environments. Therefore, estimates of rates of fungal growth and biomass production associated with decaying litter are important in assessing the importance of fungi in ecosystems. This chapter describes a method for estimating rates of fungal growth and production. Pieces of plant litter undergoing decomposition are incubated with radiolabelled (14C) acetate for a given time, typically 4 h. Ergosterol, a biomarker largely specific to fungi, is extracted from the litter samples and its concentration determined with HPLC to estimate fungal biomass. The ergosterol fraction eluting from the HPLC column is collected and its radioactivity determined in a scintillation counter to estimate rates of acetate incorporation into ergosterol. Fungal growth rates are directly proportional to acetate incorporation rates, and fungal production is calculated as the product of fungal growth rate and biomass. Production can be expressed per gram of litter or, if the amount of litter is known, per square metre of the environment such as a stream. Application of the method has shown that fungal growth and production can be substantial in streams and other environments, depending on factors such as litter type, nutrient concentrations and temperature.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 157:1-12 (1997) - doi:10.3354/meps157001 Significance of bacteria in the flux of organic matter in the tidal creeks of the mangrove ecosystem of the Indus River delta, Pakistan Nasreen Bano1,*, Mehr-Un Nisa1, Nuzhat Khan1, Monawwar Saleem1, Paul J. Harrison2, Saiyed I. Ahmed3, Farooq Azam4 1National Institute of Oceanography, S.T. 47, Block 1, Clifton, Karachi, Pakistan 2Department of Earth & Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T IZ4 3School of Oceanography, University of Washington, Seattle, Washington 98195, USA 4Scripps Institution of Oceanography, UCSD, La Jolla, California 92093, USA *Present address: Department of Marine Sciences, University of Georgia, Athens, Georgia 30602-2206, USA. E-mail: nasreen@uga.cc.uga.edu We studied bacterial biomass and production in 3 tidal creeks (Isaro, Gharo and Phitti Creeks) in the mangrove forests in the Indus River delta, Pakistan, to assess the significance of bacteria-mediated carbon fluxes in the creek ecosystem. Bacterial biomass, bacterial carbon production (BCP) and primary productivity (PP) were measured periodically for over a year during 1991-92. BCP was high, generally 50 to 300 µg C l-1 d-1. Despite such high BCP, bacterial abundance remained between 1 x 106 ml-1 and 4 x 106 ml-1 (20 to 80 µg C l-1) indicating tight coupling between bacterial production and removal. Specific growth rates generally ranged from 1 to 7 d-1 but the rate reached 24 d-1 during a phytoplankton bloom, apparently a red tide, and this was an unprecedented growth rate for a natural assemblage. The abundance of attached bacteria exhibited a large variation, ranging from 4 to 92% (mean 35 ± 21%, n = 41) in Isaro Creek and from 14 to 84% (mean 37 ± 28%, n = 10) in Gharo Creek. Bacterial production due to attached bacteria was 73 to 96% of the total. Thus, a major fraction of BCP may have been directly available to metazoan grazers. BCP was generally much higher than net PP; the yearly integrated average BCP/PP for all sites was 2.0. Thus, the growth of bacteria, attached and free, probably represented the major pathway of the production of high quality (low C:N) biomass potentially available to the grazers. Average yearly integrated bacterial carbon demand (BCD), estimated conservatively by assuming a 30% growth efficiency for all sites, was 6.9 times net PP. Thus, the creek water columns were strongly and persistently net heterotrophic. Data integrated over the entire study period show that even if all phytoplankton production was utilized by bacteria it would satisfy only 7 to 20% of the BCD; the remaining 80 to 93% of BCD would be met by reduced carbon from other sources. Phytoplankton production was light limited due to high turbidity and, apparently, the majority of BCP could be supported by the input of mangrove detritus. Estimates of utilizable dissolved organic carbon (UDOC) in selected samples were 97 to 656 µg C l-1, indicating that in order to sustain the measured BCD (643 ± 671 µg C l-1 d-1) the UDOC pool would turnover in <1 d to a few days. Limited data suggest that bacterial production was carbon rather than N or P limited, consistent with sustained high levels of inorganic N and P in the surface water. Since mangrove detritus provides most of the energy for bacterial production, which in turn is a significant source of high quality food for grazers, particularly via ingestion of attached bacteria, we predict that the ongoing destruction of mangrove forests in the Indus delta and elsewhere could have a major impact on mangrove ecosystem structure and functioning and the production of economically important fish and shrimp in mangrove creeks. Bacteria · Organic matter · Bacterial production · Mangroves · Tidal creeks · Indus River delta Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 157. Publication date: October 16, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

The relative contributions of fungi and bacteria to carbon flow from submerged decaying plant litter at different levels of inorganic nutrients (N and P) were studied. We estimated leaf mass loss, fungal and bacterial biomass and production, and microbial respiration and constructed partial carbon budgets for red maple leaf disks precolonized in a stream and then incubated in laboratory microcosms at two levels of nutrients. Patterns of carbon flow for leaf disks colonized with the full microbial assemblage were compared with those colonized by bacteria but in which fungi were greatly reduced by placing leaf disks in colonization chambers sealed with membrane filters to exclude aquatic hyphomycete conidia but not bacterial cells. On leaves colonized by the full microbial assemblage, elevated nutrient concentrations stimulated fungi and bacteria to a similar degree. Peak fungal and bacterial biomass increased by factors of 3.9 and 4.0; cumulative production was 3.9 and 5.1 times higher in the high nutrient in comparison with the low nutrient treatment, respectively. Fungi dominated the total microbial biomass (98.4 to 99.8%) and cumulative production (97.3 and 96.5%), and the fungal yield coefficient exceeded that of bacteria by a factor of 36 and 27 in low- and high-nutrient treatments, respectively. Consequently, the dominant role of fungi in leaf decomposition did not change as a result of nutrient manipulation. Carbon budgets indicated that 8% of leaf carbon loss in the low-nutrient treatment and 17% in the high-nutrient treatment were channeled to microbial (essentially fungal) production. Nutrient enrichment had a positive effect on rate of leaf decomposition only in microcosms with full microbial assemblages. In treatments where fungal colonization was reduced, cumulative bacterial production did not change significantly at either nutrient level and leaf decomposition rate was negatively affected (high nutrients), suggesting that bacterial participation in carbon flow from decaying leaf litter is low regardless of the presence of fungi and nutrient availability. Moreover, 1.5 and 2.3 times higher yield coefficients of bacteria in the reduced fungal treatments at low and high nutrients, respectively (percentage of leaf carbon loss channeled to bacterial production), suggest that bacteria are subjected to strong competition with fungi for resources available in leaf litter.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 153:59-66 (1997) - doi:10.3354/meps153059 Bacterial carbon production in the northern Baltic: a comparison of thymidine incorporation and FDC based methods Tuomi P Bacterial production in the open Gulf of Finland was estimated by 3H-thymidine incorporation rate (TTI) and by the frequency of dividing cells (FDC), counted by transmission electron microscopy (TEM). To relate TTI and FDC to bacterial growth rate, batch cultures with the natural bacterial community from the study area were used. Conversion factors between 0.2 and 7.2 x 1018 cells mol-1 TdR, or 0.3 and 24.3 x 1017 µm3 mol-1 TdR, were obtained for TTI, depending on the season and calculation model used (integrative, cumulative, or modified derivative). From FDC and growth rate (µ) based on bacterial cell numbers, the following equation was derived: µ = 0.002FDC - 0.001. Bacterial carbon production in the study area, estimated using the FDC method, was 20 to 80% higher than TTI-based estimates. Virus abundances were followed in the batch cultures and in the field study. Viral lysis may have been a significant cause of bacterial mortality in the field, but it remains to be shown whether viral-induced mortality was already included in the empirical conversion factor. Bacteria · 3H-thymidine incorporation rate · Frequency of dividing cells · Baltic Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 153. Publication date: July 10, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Bacterial carbon production is an important parameter in understanding the flows of carbon and energy in aquatic ecosystems, but has been difficult to measure. Present methods are based on measuring the rate of cell production, and thus require a knowledge of cellular carbon content of the growing bacteria to convert cell production into carbon production. We have examined the possibility that protein synthesis rate of pelagic bacteria might serve as the basis for directly estimating bacterial carbon production. We measured bacterial protein content and protein production of pelagic bacteria. Bacterial protein content was measured as amino acids by high performance liquid chromatography of cell hydrolysates of bacterial assemblages of mean diameters from 0.026 to 0.4 km. Cellular protein:volume (w/v) in the largest bacteria was 15.2 '10 (similar to cultured Escherichia coli] but increased with decreasing cell size to 46.5 % in 0.026 pm bacteria. Protein per bacterium was correlated with cell volume by the power function y = 8 8 . ~ 2 ~ ' (r2 = 0.67; p C 0.01; n = 25) . An inventory of major bacterial macromolecular pools revealed that cell protein:dry weight and cell protein:carbon were essentially constant (63 % and 54 %. respectively) for the entire cell size range although cell protein:volume increased with decreasing cell size. Thus, the smaller cells in the size range were rich in carbon and dry weight and poor in water compared with larger cells. We established the experimental conditions for estimating protein synthesis on the basis of 3H leucine incorporation by bacteria, and determined the necessary parameters (including the intracellular isotope dilution by HPLC) for converting 3~ leucine incorporation into protein synthesis rate. In samples from Scripps Institution of Oceanography pier the intracellular isotope dilution was only 2-fold. In a field study in Southern California Bight bacterial protein production and %I-thymidine incorporation methods yielded comparable rates of bacterial production. Bacterial protein production method was an order of magnitude more sensitive and yielded bacterial carbon production directly without the need to know the cell size of the part of the assemblage in growth state.

Data from hypertrophic Humboldt Lake (Zmax = 6 m), Saskatchewan, support published studies indicating that bacterial numbers and production do not increase proportionally with chlorophyll concentration and primary production. There was no compensation for these relationships with increased bacterial production per cell, but our data showed an increase in production per unit bacterial biomass (273 fmol TdR∙μg C−1∙h−1). Bacterial production (19.8–422 mg C∙m−2∙d−1) was correlated with primary production (r = 0.76), and maximum bacterial production coincided with summer cyanobacterial blooms. Water temperature was a dominant factor correlated with bacterial production (r = 0.85) and growth (r = 0.92). Depending upon the factors used to convert the rate of thymidine incorporation to gross carbon production, heterotrophic bacterial production was able to consume an average of 42% (408 mg C∙m−2∙d−1) to 67% (653 mg C∙m−2∙d−1) of plankton primary productivity. Based on these calculations, hypertrophic prairie lakes might accumulate autochthonously produced organic carbon, but this conclusion takes no account of benthic bacterial production which could be high in shallow lakes.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 138:265-273 (1996) - doi:10.3354/meps138265 Particle-attached bacteria and heterotrophic plankton associated with the Columbia River estuarine turbidity maxima Crump BC, Baross JA A significant fraction of the particulate organic material entering the Columbia River estuary (USA) is metabolized or altered before it is carried out to the ocean. Estuarine turbidity maxima are part of the particle-trapping mechanism that lengthens the residence time of river-borne organic material in an estuary, increasing that material's availability to estuarine bacteria and the estuarine food web. In May and June, 1992, water samples in and around the Columbia River estuarine turbidity maxima were analyzed to determine rates of bacterial carbon production, and bacterial and putative bacterivore abundances. Salinity, turbidity, and tidal data were used to interpret bacterial activity patterns, and to identify distributions of bacterial predators. Bacterial carbon production, based on the rate of 3H-thymidine uptake, correlated with turbidity, and varied from 0.3 to 5.6 ug l-1 h-1. Sharp peaks in bacterial production were found in the estuarine turbidity maxima, and were determined to be due to particle-attached bacteria by measuring bacterial production directly on particles. Variation in bacterial production outside the estuarine turbidity maxima seemed to be related to the tidal cycle, supporting hypotheses on particle cycling in the estuary. Nanoflagellates, small 'oligotrich' ciliates and rotifers were the most numerous grazers in the estuary. Correlation analysis between grazer and bacterial abundances and production suggested that rotifers and small ciliates may be the primary consumers of bacteria outside the estuarine turbidity maxima. Rotifers were enhanced in the estuarine turbidity maxima and therefore may be key consumers of particle-attached bacteria. Estuarine turbidity maximum . Bacteria . Protozoa . Rotifers . Bacterial production . Particle-attached bacteria . Columbia River Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 138. Publication date: July 25, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

To assess the variability and predictability of the empirical conversion factor (CF) for converting the uptake of 3 H-thymidine (TDR) to bacterial carbon production rates in eutrophic environments, we performed 11 dilution culture experiments and results were related to a set of important concomitantly recorded environmental variables. TDR incorporation data from a field study were then converted to carbon production with the established empirical CF and a theoretical CF and compared to production estimates derived from leucine incorporation. Mean empirical CF varied between 0.5 and 7.0 x 10 6 cells (pmol TDR) -1 over the year and showed highly significant negative correlation with TDR incorporation rate and a highly significant positive correlation to temperature. Values of carbon production derived with the variable empirical CF were lower than the values obtained by the use of the theoretical CF of 1 × 10 6 cells (pmol TDR) -1 . They showed less seasonal variation than values obtained by leucine incorporation, and periods of uncoupling were observed. However, when the empirical CF was calculated from the multiple regression equation including TDR incorporation and temperature, the resulting carbon production rates showed a high correspondence with the leucine-derived production rates. Results of the analyses were interpreted as an indication that under favourable, conditions (resulting in high TDR incorporation) bacteria may be able to optimize DNA duplication over protein synthesis as a possible strategy to persist and to maintain their potential to divide under limiting conditions (e.g. decrease in temperature and substrate availability). In unfavourable conditions (resulting in low incorporation rates), bacteria may then use already produced DNA copies for rapid growth, when the environmental conditions turn favourable again. Thus, an experimental set-up causing nutrient enrichment of the sampled water by autoclaving and filtering, as generally used for dilution culture experiments, does not always reflect in situ situations, especially during periods of low nutrient concentrations.

Millions of productive wetlands dot the North American Great Plains. Although these wetlands are key ecological features of the Canadian prairies, their microbial food webs remain relatively unstudied. Over 3 yr, pelagic primary (PP) and bacterial production (BP) and biomass were monitored in 2 wetlands in Central Saskatchewan, Canada. PP ranged from 8.8 to 3911.0 mg C m -3 d -1 (x = 612.0 mg C m -3 d -1 ), while rates of BP ranged from 8.2 to 678.0 mg C m -3 d -1 (x = 140.0 mg C m -3 d -1 ). Nutrients, light and temperature did not appear as major factors influencing these rates. Seasonal mean ratios of PP:BP revealed that both wetlands were net autotrophic (x PP:BP = 7.4; range 0.08 to 42.6), which was not surprising given their eutrophic status. On a smaller temporal scale, these wetlands were, on average, net heterotrophic (PP:BP < 1) on 33% of the sampling dates. High pelagic bacterial carbon demand (greater than twice the phytoplankton carbon production) indicated that bacterial metabolism was not dependent on autochthonous carbon sources. Biological availability of dissolved organic carbon (DOC) was low in both wetlands (x = 3.8%). But this percentage of high ambient concentrations was enough to satisfy bacterial carbon demand. Based on seasonal averages, BP and bacterial numbers decreased across increasing trophic gradients (as chl a or total phospho- rus). This phenomenon could also be observed across a seasonal gradient of changing chl a concen- trations. Prairie wetlands not only experience periods of net heterotrophy, but bacterial carbon demand and production per unit biomass are high, suggesting an important role for bacteria in the metabolism of these eutrophic ecosystems.

Bacterial production is an integral recycling mechanism that facilitates carbon flow through aquatic food webs. Factors influencing bacterial activity therefore impact carbon flow. Although ecologists consider grazing and dissolved organic carbon flux to be the major regulators of bacterial activity, we explored two other important pressures. Virus-like particle abundance ranged from 3.7 × 1010to 37.9 × 1010·L-1in samples collected during August 1997 and July 1998. Bacterial abundance during these periods ranged from 1.8 × 109to 4.6 × 109·L-1. Based on electron microscopic analysis, viruses in Lake Erie would have been responsible for 12.1-23.4% of bacterial mortality and, in quasi-steady-state conditions, a comparable loss of bacterial productivity. In the central basin, solar radiation was also demonstrated to regulate bacterial productivity. Ultraviolet radiation (295-400 nm) was shown to inhibit bacterial productivity according to a cumulative exposure kinetic model, and biological weighting functions were derived to enable calculation of time- and depth-integrated photoinhibition. The daytime photoinhibitory loss of bacterial carbon production was estimated to be 14-30% over the upper 5 m, primarily due to ultraviolet radiation &gt;320 nm. Viruses and sunlight are therefore of comparable importance as regulators of bacterial activity in this system.

Heterotrophic bacteria and fungi are widely recognized as crucial mediators of carbon, nutrient, and energy flow in ecosystems, yet information on their total annual production in benthic habitats is lacking. To assess the significance of annual microbial production in a structurally complex system, we measured production rates of bacteria and fungi over an annual cycle in four aerobic habitats of a littoral freshwater marsh. Production rates of fungi in plant litter were substantial (0.2 to 2.4 mg C g(-1) C) but were clearly outweighed by those of bacteria (2.6 to 18.8 mg C g(-1) C) throughout the year. This indicates that bacteria represent the most actively growing microorganisms on marsh plant litter in submerged conditions, a finding that contrasts strikingly with results from both standing dead shoots of marsh plants and submerged plant litter decaying in streams. Concomitant measurements of microbial respiration (1.5 to 15.3 mg C-CO2 g(-1) of plant litter C day(-1)) point to high microbial growth efficiencies on the plant litter, averaging 45.5%. The submerged plant litter layer together with the thin aerobic sediment layer underneath (average depth of 5 mm) contributed the bulk of microbial production per square meter of marsh surface (99%), whereas bacterial production in the marsh water column and epiphytic biofilms was negligible. The magnitude of the combined production in these compartments (approximately 1,490 g C m(-2) year(-1)) highlights the importance of carbon flows through microbial biomass, to the extent that even massive primary productivity of the marsh plants (603 g C m(-2) year(-1)) and subsidiary carbon sources (approximately 330 g C m(-2) year(-1)) were insufficient to meet the microbial carbon demand. These findings suggest that littoral freshwater marshes are genuine hot spots of aerobic microbial carbon transformations, which may act as net organic carbon importers from adjacent systems and, in turn, emit large amounts of CO2 (here, approximately 870 g C m(-2) year(-1)) into the atmosphere.