Seasonal variation in plankton, submicrometre particles and size-fractionated dissolved organic carbon in Antarctic coastal waters

Concentrations of plankton, suspended particles 0.74–87 μm equivalent spherical diameter and dissolved organic carbon (DOC) were measured from May to February at an Antarctic coastal site. Bacteria-sized particles 0.74–1 μm diameter, and bacterial cells and heterotrophic protists all exhibited a seasonal minimum during winter and maxima in summer. Bacteria composed <10% of the bacteria-sized particles. Release of autotrophic protists from the ice caused water column biomass of autotrophs to reach maximum concentrations in October and November, but maximum cell concentration in the water column was reached in January. Microheterotroph biomass weakly reflected the release of the ice algal community but reached maximum concentration during the water column bloom in January. Total DOC concentrations varied from 0.36 mg C l⁻¹ in July to 3.10 mg C l⁻¹ in October, with a yearly average of 1.51 mg C l⁻¹. Ultrafiltration of DOC revealed that the molecular weight composition of the DOC differed greatly through the year. DOC <5 kDa molecular weight reached a maximum of 1.25 mg C l⁻¹ in October and accounted for up to 60% of total DOC in July. Concentrations of high molecular weight DOC (>100 kDa) were highest in July and November, with the DOC (100 kDa–0.5 μm) fraction reaching a maximum of 1.22 mg C l⁻¹ in November and composing 82% of the total DOC in January. Wet chemical oxidation and high-temperature catalytic oxidation organic carbon analyses were compared. Good correlation was observed between methods during summer but no significant correlation existed in winter, indicating that winter DOC may be refractory. Accepted: 21 March 2000     

Chemical Characteristics and Origin of Dissolved Organic Matter in the Yukon River

Monthly (or bi-weekly) water samples were collected from the Yukon River, one of the largest rivers in North America, at a station near the US Geological Survey Stevens Village hydrological station, Alaska from May to September 2002, to examine the quantity and quality of dissolved organic matter (DOM) and its seasonal variations. DOM was further size fractionated into high molecular weight (HMW or colloidal, 1 kDa–0.45 μm) and low molecular weight (LMW, <1 kDa) fractions. Dissolved organic carbon (DOC), colored dissolved organic matter (C-DOM) and total dissolved carbohydrate (TCHO) species were measured in the size fractionated DOM samples. Concentrations of DOC were as high as 2830 μmol-C l⁻¹ during the spring breakup in May and decreased significantly to 508–558 μmol-C l⁻¹ during open-water season (June–September). Within the DOC pool, up to 85% was in the colloidal fraction (1 kDa–0.45 μm) in early May. As DOC concentration decreased, this colloidal portion remained high (70–85% of the bulk DOC) throughout the sampling season. Concentrations of TCHO, including monosaccharides (MCHO) and polysaccharides (PCHO), varied from 722 μmol-C l⁻¹ in May to 129 μmol-C l⁻¹ in September, which comprised a fairly constant portion of bulk DOC (24±2%). Within the TCHO pool, the MCHO/TCHO ratio consistently increased from May to September. The C-DOM/DOM ratio and the size fractionated DOM increased from May to September, indicating that DOM draining into the Yukon River contained increased amounts of humified materials, likely related to a greater soil leaching efficiency in summer. The average composition of DOM was 76% pedogenic humic matter and 24% aquagenic CHO. Characteristics of soil-derived humic substances and low chlorophyll-a concentrations support a dominance of terrestrial DOM in Yukon River waters.     

Distribution and characteristics of molecular size fractions of freshwater-dissolved organic matter in watershed environments: its implication to degradation

Distributions of molecular size and fluorescence properties of dissolved organic matter (DOM) in the Lake Biwa and Lake Baikal watersheds were investigated using the cross-flow ultrafiltration technique and three-dimensional fluorescence measurements. From the fluorescence properties, protein-like substances were usually found in the 0.1 μm-GF/F fraction (the Durapore membrane retentate of the GF/F filtrate) of the lake DOM. The results indicated autochthonous production of protein-like organic-matters in the lake environment. Fulvic acid (FA)-like components were composed of two fractions with respect to fluorescence properties and molecular size. Two FA-like fluorescence peaks, which showed different fluorescence peak positions in the excitation-emission matrix (EEM), were partly fractionated by the molecular size of 5000 daltons (5 kDa). The FA-like fluorescence peak position of the <5-kDa fraction was observed at the shorter wavelength region compared with that of the fraction between 5 kDa and 0.1 μm (5 kDa20.1 μm fraction). A blue shift of the FA-like fluorescence peak position as well as a decrease in the molecular size of the DOM was observed in lake samples. The relative contribution of the <5 kDa fraction to the DOC concentration was high in lake waters (68%–79%) compared with river waters (44%–68%), suggesting characteristic changes in molecular size between riverine and lacustrine DOM. DOM of the 5 kDa–0.1 μm fraction was relatively higher in river waters than in lake waters. These findings coincided with in situ distributions of the fluorescence properties and molecular size of DOM found in both stream and lake environments. These results indicate that FA-like substances from forested watersheds are decomposed qualitatively and quantitatively in the river-lake environment by photochemical and biological processes.     

Characterization of methylmalonyl-CoA mutase involved in the propionate photoassimilation of Euglena gracilis Z

Significant accumulation of the methylmalonyl-CoA mutase apoenzyme was observed in the photosynthetic flagellate Euglena gracilis Z at the end of the logarithmic growth phase. The apoenzyme was converted to a holoenzyme by incubation for 4 h at 4°C with 10 μM 5′-deoxyadenosylcobalamin, and then, the holoenzyme was purified to homogeneity and characterized. The apparent molecular mass of the enzyme was calculated to be 149.0 kDa ± 5.0 kDa using Superdex 200 gel filtration. SDS–polyacrylamide gel electrophoresis of the purified enzyme yielded a single protein band with an apparent molecular mass of 75.0 kDa ± 3.0 kDa, indicating that the Euglena enzyme is composed of two identical subunits. The purified enzyme contained one mole of prosthetic 5′-deoxyadenosylcobalamin per mole of the enzyme subunit. Moreover, we cloned the full-length cDNA of the Euglena enzyme. The cDNA clone contained an open reading frame encoding a protein of 717 amino acids with a calculated molecular mass of 78.3 kDa, preceded by a putative mitochondrial targeting signal consisting of nine amino acid residues. Furthermore, we studied some properties and physiological function of the Euglena enzyme.     

Effects of the Dry–Wet Hydrological Shift on Dissolved Organic Carbon Dynamics and Fate Across Stream–Riparian Interface in a Mediterranean Catchment

Abstract The stream–riparian interface, characterized by a dynamic and complex hydrology, is an important control point for nutrient fluxes and processing between terrestrial and aquatic systems. Predicted alterations in the discharge regime in Mediterranean climate regions make it necessary to understand the effects of abrupt hydrological transition between dry and wet conditions on the transport and fate of dissolved organic carbon (DOC) across the stream–riparian interface. In this study, the concentrations and fate of total DOC (TDOC) and a subset of four molecular weight fractions (<1 kDa, 1–10 kDa, 10–100 kDa, >100 kDa) were investigated in stream water and riparian groundwater during autumn of 2003 and 2004. The two study periods were characterized by contrasting antecedent hydrological conditions: the streamflow was interrupted in summer 2003 but was permanent in summer 2004. Comparison of the two study periods indicates that an abrupt dry–wet hydrological transition amplifies the water exchange across the stream–riparian interface and favors retention of up to 57% of the TDOC that flows across the interface. Furthermore, the efficiency of DOC retention across the stream-riparian interface also varies greatly depending on DOC molecular size. More than 70% of DOC fractions higher than 10 kDa were retained, whereas the smaller fraction (less than 1 kDa) was nearly conserved. Consequently, our study helps to clarify the effects of extreme hydrological events on DOC transport in running waters in Mediterranean regions.     

Enzyme-linked immunosorbent assay of chlorampenicol in foodstuff

A test-system based on enzyme-linked immunosorbent assay (ELISA) for the quantitative detection of chloramphenicol (CAP) in foodstuff has been developed. The detection limit of the method was 0.05 μg/l. The procedures for milk samples preparation of various fat content and chicken muscles were optimized. Before the analysis milk was diluted 5-fold with a buffer. The detection limit for milk was 0.3 μg/l; recoveries varied from 74 to 118%. Two protocols for chicken muscles preparation were elaborated; extraction with buffer (the express method) and extraction with acetonitrile. The detection limits of CAP in chicken muscles were 0.5 and 0.3 μg/kg, respectively; recovery values were 71–107% and 95–115%, respectively. The results of residual amounts of CAP detection in foodstuff by ELISA and HPLC-MS were in good correlation.     

Physical and chemical limnology of alpine lakes and pools in the Rwenzori Mountains (Uganda–DR Congo)

This study describes the physical and chemical properties of 17 Afroalpine lakes (>2 m deep) and 11 pools (<2 m deep) in the Rwenzori mountains, Uganda-DR Congo, with the aim to establish the baseline conditions against which to evaluate future environmental and biological changes in these unique tropical ecosystems, and to provide the foundation for lake-based paleoenvironmental studies. Most Rwenzori lakes are located above 3,500 m elevation, and dilute (5–52 μS/cm specific conductance at 25°C) open systems with surface in- and outflow. Multivariate ordination and pairwise correlations between environmental variables mainly differentiate between (1) lakes located near or above 4,000 m (3,890–4,487 m), with at least some direct input of glacial meltwater and surrounded by rocky catchments or alpine vegetation; and (2) lakes located mostly below 4,000 m (2,990–4,054 m), remote from glaciers and surrounded by Ericaceous vegetation and/or bogs. The former group are mildly acidic to neutral clear-water lakes (surface pH: 5.80–7.82; Secchi depth: 120–280 cm) with often above-average dissolved ion concentrations (18–52 μS/cm). These lakes are (ultra-) oligotrophic to mesotrophic (TP: 3.1–12.4 μg/l; Chl-a: 0.3–10.9 μg/l) and phosphorus-limited (mass TN/TP: 22.9–81.4). The latter group are mildly to strongly acidic (pH: 4.30–6.69) waters stained by dissolved organic carbon (DOC: 6.8–13.6 mg/l) and more modest transparency (Secchi-disk depth: 60–132 cm). Ratios of particulate carbon, particulate nitrogen and chlorophyll a in these lakes indicate that organic matter in suspension is primarily derived from the lakes’ catchments rather than aquatic primary productivity. Since key features in the Rwenzori lakes’ abiotic environment are strongly tied to temperature and catchment hydrology, these Afroalpine lake ecosystems can be expected to respond sensitively to climate change and glacier melting. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Detection of distinct endocytotic and phagocytotic activities in epithelial cells (pinacocytes) of freshwater sponges (Porifera, Spongillidae)

 Light and electron microscopical investigations using externally applied fluorescent and gold-labeled markers have revealed the existence of distinct endocytotic and phagocytotic activities in basal epithelial cells (pinacocytes) of the freshwater sponges Spongilla lacustris and Ephydatia es) of the f. The total rate of endocytotic membrane uptake, ascertained by the application of the cationic lipid probe TMA-DPH, was found to be 3.2% of the cell surface area/h. A typical fluid-phase endocytosis, demonstrated by the use of the water-soluble membrane-impermeable tracers BCECF-dextran and LY-CH, participates in endocytotic activity at a rate of 0.7% of the cell surface area/h and results in the formation of endosomes measuring 0.8–1 μm in diameter. Moreover, the application of labeled BSA succeeded in the detection of a receptor-mediated endocytosis amounting to a concentration-dependent uptake of 2.3–2.8% of the cell surface area/h. Coated pits and coated vesicles conveying the adsorbed BSA measure 0.3 μm in diameter and are covered on the cytoplasmic face with a clathrin-like protein (HC, 180 kDa; LC, 30 kDa). To facilitate phagocytotic activities, a series of fluorescent–labeled and chemically treated particles such as bacteria or latex beads have been successfully employed. Accordingly, the measured values of phagocytic membrane uptake between 1 and 8% of the cell surface area/h depend on the variety of size as well as the chemical nature of the different bioparticles and clearly point to phagocytosis as a key mechanism for providing freshwater sponges with nourishment. Accepted: 3 April 1997     

Immunological Identification of a Putative Precursor of the Insoluble Glycoprotein Framework of the Chlamydomonas Cell Wall

To identify precursors of the insoluble glycoprotein frameworkof the Chlamydomonas cell wall, a polyclonal antibody was raisedagainst the mixture of polypeptides released from the insolublewall fraction by chemical deglycosylation. This antibody preferentiallycross-reacted with a ‘150 kDa’ salt-soluble cellwall glycoprotein. The conclusion that this ‘150 kDa’glycoprotein is a putative precursor of the insoluble cell wallfraction was corroborated by the results of pulse-chase experimentsand by experiments with antibodies raised against the ‘150kDa’ salt-soluble glycoprotein and against its 100 kDadeglycosylation product, respectively. Whereas the antibodyagainst the ‘150 kDa’ glycoprotein preferentiallyrecognized carbohydrate side chains, the antibody against its100 kDa deglycosylation product was found to have essentiallythe same specificity towards glycosylated and deglycosylatedcell wall components as the antibody against the deglycosylationproducts of the insoluble wall fraction. Furthermore, the antibodyagainst the deglycosylated, insoluble wall fraction recognizedalmost the same set of peptide fragments derived by V8 proteasetreatment from the ‘150 kDa’ salt-soluble cell wallglycoprotein and its 100 kDa deglycosylation product, respectively,as the antibody against the 100 kDa deglycosylated cell wallpolypeptide. (Received April 22, 1994; Accepted November 21, 1995)     

Pigmented Nanoflagellates Grazing on <Emphasis Type="Italic">Synechococcus</Emphasis>: Seasonal Variations and Effect of Flagellate Size in the Coastal Ecosystem of Subtropical Western Pacific

We investigated seasonal variation of grazing impact of the pigmented nanoflagellates (PNF) with different sizes upon Synechococcus in the subtropical western Pacific coastal waters using grazing experiments with fluorescently labeled Synechococcus (FLS). For total PNF, conspicuous seasonal variations of ingestion rates on Synechococcus were found, and a functional response was observed. To further investigate the impact of different size groups, we separated the PNF into four categories (<3, 3–5, 5–10, and >10 μm). Our results indicated that the smallest PNF (<3 μm PNF) did not ingest FLS and was considered autotrophic. PNF of 3–5 μm in size made up most of the PNF community; however, their ingestion on Synechococcus was too low (0.1–1.9 Syn PNF⁻¹ h⁻¹) to support their growth, and they had to depend on other prey or photosynthesis to survive. The ingestion rate of the 3–5 μm group exhibited no significant seasonal variation; by contrast, the ingestion rates of 5–10 and >10 μm PNFs showed significant seasonal variation. During the warm season, 3–5 μm PNF were responsible for the grazing of 12% of Synechococcus production, 5–10 μm PNF for 48%, and >10 μm PNF for 2%. Taken together, our results demonstrate that the PNF of 3–10 μm consumed most Synechococcus during the warm season and exhibited a significant functional response to the increase in prey concentration.     

Distribution and characterization of dissolved and particulate organic matter in Antarctic pack ice

Distribution and composition of organic matter were investigated in Antarctic pack ice in early spring and summer. Accumulation of organic compounds was observed with dissolved organic carbon (DOC) and particulate organic carbon (POC) reaching 717 and 470 μM C, respectively and transparent exopolymeric particles (TEP) up to 3,071 μg Xanthan gum equivalent l⁻¹. POC and TEP seemed to be influenced mainly by algae. Particulate saccharides accounted for 0.2–24.1% (mean, 7.8%) of POC. Dissolved total saccharides represented 0.4–29.6% (mean, 9.7%) of DOC, while dissolved free amino acids (DFAA) accounted for only 1% of DOC. Concentrations of TEP were positively correlated with those of saccharides. Monosaccharides (d-MCHO) dominated during winter–early spring, whereas dissolved polysaccharides did in spring–summer. DFAA were strongly correlated with d-MCHO, suggesting a similar pathway of production. The accumulation of monomers in winter is thought to result from limitation of bacterial activities rather than from the nature of the substrates.     

Partitioning of the food ration of marine ciliates between pico- and nanoplankton

Food size-range for 13 species of Tintinnina and 18 species of Oligotrichina were studied using electronic particle counting and in situ observation of food vacuole contents. Tintinnids consume nanoplankton in the size range 2–20 μm. Oligotrichous naked ciliates consume particles in the size range 0.5–10 μm. Ciliates smaller than 30 μm take 72% picoplankton and 28% nanoplankton. For ciliates between 30 μm and 50 μm the proportions are reversed (30% pico- and 70% nanoplankton), while the larger ciliates (> 50 μm) take nanoplankton almost exclusively (95% nano- and 5% picoplankton). A seasonal study of total Oligotrichida grazing showed that natural particles were consumed at rates that varied from 1 to 20 μg C 1⁻¹ day⁻¹. This included between 1 and 38% of the bacterioplankton production and 9 to 52% of the nanoplankton production. In the N-W Mediterranean the total ciliate production varied from 0.4 to 8.2 μg C 1⁻¹ day⁻¹. Research supported by CNRS-PIROCEAN AIP-RTM-953146-GRECO P4 and by CNRS-UA 716 (FR), and by the University of Nice and by the CNRS-GRECO 88 (MLP)     

Ammonification in soil aggregates: Effect of some physical,physico-chemical and chemical properties

Aggregate diameter affected significantly the intensity of ammonification in chernozemic rendzina but not in lessivē soil. In the latter the process was influenced significantly by the number of microorganisms able to grow on asparagine agar. A high correlation, though not significant at the level of 0.05, was found between the ammonification intensity and the content of pores of radius: 3–1.5, 7.5–5.0, 0.5–0.25 and 0.01–0.005 μm in chernozemic rendzina and those measuring 1.5–0.5, 0.025–0.01, 0.01–0.005 and >7.5 μm in lessivē soil aggregates as well as the percentage of soil particles of 100–50 μm in chernozemic rendzina aggregates and the internal surface area and organic C in aggregates of lessivē soil.     

Three new <Emphasis Type="Italic">Tomentella</Emphasis> species from West Africa identified by anatomical and molecular data

We used a combination of molecular-phylogenetic inference of 82 ITS rDNA sequences and anatomical approach to describe three new west African thelephoroid species, namely Tomentella afrostuposa, T. guineensis and T. guinkoi. Anatomically, T. afrostuposa is reminiscent of T. stuposa with globose to broadly ellipsoid large basidiospores of 8–14 μm, long aculei of up to 3 μm and prominent apiculi of 2 μm width. Molecular-phylogenetically, it falls within the T. stuposa complex. However, T. afrostuposa deviates by at least 7.80–10.74% from T. stuposa in regard with the ITS rDNA sequences. Tomentella guineensis is characterised by long (up to 85 μm) utriform basidia, the presence of reniform basidiospores in lateral view (up to 9 μm) with aculei not exceeding 1 μm and a strong cyanescent reaction of the subhymenial hyphae and basidia in 2.5% KOH. It forms a sister species of the newly described species Tomentella maroana; however, deviating from the last species by at least 9.75–10.04%. The very short, inflated (up to 14 μm) and thick-walled septate (septa up to 1.5 μm) subhymenial hyphae combined with ellipsoid basidiospores (up to 8 μm) and short aculei not exceeding 0.5 μm characterise Tomentella guinkoi. Anatomically, T. guinkoi recalls T. ellisii. Genetic distance between both species ranges from 12.67 to 13.73% according to ITS rDNA sequences analyses. Tomentella guinkoi forms a sister species of the group composed of T. ellisii, T. hjortstamiana and T. pisoniae. Detailed anatomical comparisons between the newly described species and their close relatives are given.     

Distribution of dissolved organic carbon and dissolved fulvic acid in mesotrophic Lake Biwa, Japan

The dissolved organic carbon (DOC) concentrations in mesotrophic Lake Biwa were determined by a total organic carbon (TOC) analyzer, and DOC molecular size distributions were determined by size exclusion chromatography (SEC) using a fluorescence detector at excitation/emission (Ex/Em) levels of 300/425 nm with the eluent at pH 9.7. The fluorescence wavelengths for detection were chosen from the result of excitation–emission matrix spectrometry (EEM) analysis for dissolved fulvic acid (DFA) extracted from Ado River (peak A, Ex/Em = 260–270/430–440 nm; peak B, Ex/Em = 300–310/420–430 nm). Ado River DFA was eluted with a retention time (RT) of 7.4–8.9 min and the apparent molecular weight was estimated at 22–87 kDa based on the elution curve for the spherical protein molecular weight standard. A DFA peak eluted at the same retention time as Ado River DFA also appeared in all the samples of Lake Biwa water. From the linear relationship between the peak areas with an RT of 7.4–8.9 min by SEC analysis and DOC values of DFA by TOC analysis of a series of DFA samples (r² = 0.9995), the concentrations of DFA in the lake water were roughly calculated. DFA was distributed within the range 0.25–0.43 mg C l⁻¹ and accounted for 15%–41% of DOC, with the highest ratios observed at a depth of 70 m in August and the lowest at 2.5 m in May.     

Distribution of dissolved organic carbon in Lake Baikal and its watershed

 Concentrations of dissolved organic carbon (DOC) in Lake Baikal ranged from 90 to 110 μM C, considerably higher than those in oceanic environments. The DOC concentrations in the epilimnion were higher than those in the hypolimnion. Since particulate organic carbon (POC) concentrations in the pelagic waters of Lake Baikal were <10–40 μM C in the epilimnion and 2–5 μM C in the hypolimnion, DOC constitutes a major component of the organic carbon pool in Lake Baikal, especially in the deep layers. The DOC concentrations downstream of the Barguzin and Selenga Rivers were quite high (400–500 μM C). Probably because of the high concentrations of DOC in these rivers, the DOC levels in Barguzin Bay and offshore at the mouth of the Selenga River were higher than those in the pelagic regions of the central and south basins of Lake Baikal. The relationship between DOC and electric conductivity revealed the transport of DOC from rivers to the pelagic area in Lake Baikal. The spatial distribution of DOC suggested that a major part of DOC in the lake was allochthonous (land-derived). Received: July 26, 2002 / Accepted: September 16, 2002 Present address:Research Institute for Humanity and Nature, 335 Takashima-cho, Marutamachi Kawaramachi, Kamigyo-ku, Kyoto 602-0878, Japan Tel. +81-75-229-6167; Fax +81-75-229-6150 e-mail: yos@chikyu.ac.jp Acknowldgments The authors wish to thank Director Prof. M. Grachev and Dr. O. Timoshkin of the Limnological Institute, Siberian Branch, Academy of Science, Russia, for arranging the cruise on Lake Baikal. We are also indebted to Drs. V. Sinyukovich, I. Khanaev, and A. Zhdanov for their kind assistance during the expeditions. We wish to thank Ms. Y. Ito for measuring the DOC concentrations. This work was supported and financed by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 09041159) and for Scientific Research of Priority Area B (No. 11213101) and by the International Geosphere-Biosphere Programme (IGBP) at Nagoya University. Correspondence to:T. Yoshioka     

Isolation and Characterization of Acetate-Utilizing Anaerobes from a Freshwater Sediment

Acetate-degrading anaerobic microorganisms in freshwater sediment were quantified by the most probable number technique. From the highest dilutions a methanogenic, a sulfate-reducing, and a nitrate-reducing microorganism were isolated with acetate as substrate. The methanogen (culture AMPB-Zg) was non-motile and rod-shaped with blunted ends (0.5–1 μm × 3–4 μm long). Doubling times with acetate at 30–35°C were 5.6–8.1 days. The methanogen grew only on acetate. Analysis of the 16S rRNA sequence showed that AMPB-Zg is closely related toMethanosaeta concilii. The isolated sulfate-reducing bacterium (strain ASRB-Zg) was rod-shaped with pointed ends (0.5–0.7 μm × 1.5–3.5 μm long), weakly motile, spore forming, and gram positive. At the optimum growth temperature of 30°C the doubling times with acetate were 3.9–5.3 days. The bacterium grew on a range of organic acids, such as acetate, butyrate, fumarate, and benzoate, but did not grow autotrophically with H₂, CO₂, and sulfate. The closest relative of strain ASRB-Zg isDesulfotomaculum acetoxidans. The nitrate-reducing bacterium (strain ANRB-Zg) was rod-shaped (0.5–0.7 μm × 0.7–1 μm long), weakly motile, and gram negative. Optimum growth with acetate occurred at 20–25°C. The bacterium grew on a range of organic substrates, such as acetate, butyrate, lactate, and glucose, and did grow autotrophically with H₂, CO₂, and oxygen but not with nitrate. In the presence of acetate and nitrate, thiosulfate was oxidized to sulfate. Phylogenetically, the closest relative of strain ANRB-Zg isVariovorax paradoxus.     

Urea decomposing activity of fractionated brackish phytoplankton in Lake Nakaumi

The influence of brackish phytoplankton cell classes upon the response of urea decomposition was investigated in Lake Nakaumi. The urea decomposition rate was 5 to 350 μmol urea m⁻³ h⁻¹ in the light and 3 to 137 μmol urea m⁻³ h⁻¹ in the dark. The urea decomposition rates in the light were obviously higher than in the dark. An extremely high rate (350 μmol urea m⁻³ h⁻¹) was observed in Yonago Bay. The rate in the smaller fraction (<5 μm) exceeded that in the middle (5–25 μm) and larger fractions (>25 μm). The chlorophyll- and photosynthesis-specific rates for urea decomposition in the light were 0.5 to 3.9 μmol urea mg chl.a ⁻¹ h⁻¹ and 0.3 to 1.3 μmol urea mg photo.C⁻¹. The specific urea decomposing activities were higher in the smaller fraction than in the other two fractions. The present results suggest that in brackish waters urea decomposition occurred with coupling to the standing crop and photosynthetic activity of phytoplankton. Received: May 22, 1999 / Accepted: August 15, 1999     

Microbial Availability and Size Fractionation of Dissolved Organic Carbon After Drought in an Intermittent Stream: Biogeochemical Link Across the Stream–Riparian Interface

The evolution of dissolved organic carbon (DOC) molecular-weight fractions, DOC biodegradability (BDOC), DOC origin [fluorescence index (FI)], and enzyme activities between the stream waters (main and ephemeral channel) and ground waters (riparian and hillslope) were analyzed during the transition from drought to precipitation in a forested Mediterranean stream. After the first rains, DOC content in stream water reached its maximum value (10–18 mg L⁻¹), being explained by the leaching of deciduous leaves accumulated on the stream bed during drought. During this period, the largest molecules (>10 kDa), were the most biodegradable, as indicated by high BDOC values measured during storm events and high enzymatic activities (especially for leucine-aminopeptidase). DOC >100 kDa was strongly immobilized (78%) at the stream–riparian interface, whereas the smallest molecules (<1 kDa) were highly mobile and accumulated in ground waters, indicating their greater recalcitrance. Differential enzymatic patterns between compartments showed a fast utilization of polysaccharides in the flowing water but a major protein utilization in the ground water. The results of the FI indicated a more terrestrial origin of the larger molecules in the flowing water, also suggesting that transformation of material occurs through the stream–riparian interface. Microbial immobilization and fast utilization of the most biodegradable fraction at the stream–riparian interface is suggested as a relevant DOC retention mechanism just after initial recharging of the ground water compartment. Large and rapid DOC inputs entering the intermittent river system during the transition from drought to precipitation provide available N and C sources for the heterotrophs. Heterotrophs efficiently utilize these resources that were in limited supply during the period of drought. Such changes in C cycling may highlight possible changes in organic matter dynamics under the prediction of extended drying periods in aquatic ecosystems.     

Abundance of picophytoplankton in the halocline of a meromictic lake, Lake Suigetsu, Japan

Numerous (0.5 to 4.8 × 10⁵ cells/ml), small phytoplankton (smaller than 0.5–1 × 1–2 μm in cell size, picophytoplankton) were distributed in the halocline (depth 2–12 m, 4–14 practical salinity units) of the saline meromictic lake, Lake Suigetsu (35°35′ N, 135°52′ E), located in the central part of the coast of Wakasa Bay along the Japan Sea in Fukui Prefecture, Japan. Vertical distribution of phytoplankton revealed that the maximum number of picophytoplankton was always observed near or a little deeper than the oxic-anoxic boundary layer (depth 5–6 m); they were dominant phytoplankton in the water layer deeper than the oxic-anoxic boundary from July to late September 2005. Spectral analysis of autofluorescence emitted from the particle fractions smaller than 5 μm measured with a spectrofluorometer and from individual cells measured with a microscope photodiode array detector revealed that the major component of picophytoplankton was phycoerythrin-rich, unicellular cyanobacteria (picocyanobacteria). Eukaryotic phytoplankton about 2.5 μm in diameter were also found, but the numbers were low. Fluorescence intensity of chlorophyll a at 685 nm (room temperature) emitted from the particle fractions smaller than 5 μm was increased by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These observations indicated that at least some picophytoplankton had a functional photosystem II in the halocline where sulfide, the potential inhibitor of oxygenic photosynthesis, was always present. The large abundance together with their physiological potency suggest that picophytoplankton are one of the important primary producers in the halocline of Lake Suigetsu. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.