In situ measurements confirm the seasonal dominance of benthic algae over phytoplankton in nearshore primary production of a large lake

1. Despite the recognition of its importance, benthic primary production is seldom reported, especially for large lakes. We measured in situ benthic net primary production by monitoring flux in dissolved inorganic carbon (DIC) concentration in benthic incubation chambers, based on continuous measurements of CO_2(aq) flux, alkalinity, and the temperature‐dependent dissociation constants of carbonic acid (K₁ and K₂). This methodology has the advantages of monitoring net primary production directly as change in carbon, maintaining continuous water recirculation, and having sufficient precision to detect change in DIC over short (i.e. 15 min) incubations, even in alkaline waters. 2. Benthic primary production on Cladophora‐dominated rocky substrata in western Lake Ontario was measured biweekly. Maximum biomass‐specific net photosynthetic rates were highest in the spring (2.39 mgC g Dry Mass^?1 h^?1), decreased to negative rates by early summer (?0.76 mgC g DM^?1 h^?1), and exhibited a regrowth in late summer (1.98 mgC g DM^?1 h^?1). 3. A Cladophora growth model (CGM), previously validated to predict Cladophora biomass accrual in Lake Ontario, successfully simulated the seasonality and magnitude of biomass‐specific primary production during the first cohort of Cladophora growth. Averaged over this growing season (May–Aug), mean areal net benthic production at the estimated depth of peak biomass (2 m) was 405 mg C m^?2 d^?1. 4. We measured planktonic primary production in proximity to the benthic study and constructed a depth‐resolved model of planktonic production. Using the CGM, benthic primary production was compared with planktonic primary production for the period May–Aug. Net benthic production from the shoreline to the 12 m contour (1–2 km offshore) equalled planktonic production. Closer to shore, benthic primary production exceeded planktonic primary production. Failure to account for benthic primary production, at least during abundant Cladophora growth, will lead to large underestimates in carbon and nutrient flows in the nearshore zone of this Great Lake.     

NITROGEN FIXATION,HYDROGEN CYCLING,AND ELECTRON TRANSPORT KINETICS IN TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA) STRAIN IMS1011

This study describes the relationships between dinitrogen (N₂) fixation, dihydrogen (H₂) production, and electron transport associated with photosynthesis and respiration in the marine cyanobacterium Trichodesmium erythraeum Ehrenb. strain IMS101. The ratio of H₂ produced:N₂ fixed (H₂:N₂) was controlled by the light intensity and by the light spectral composition and was affected by the growth irradiance level. For Trichodesmium cells grown at 50 μmol photons · m^?2 · s^?1, the rate of N₂ fixation, as measured by acetylene reduction, saturated at light intensities of 200 μmol photons · m^?2 · s^?1. In contrast, net H₂ production continued to increase with light levels up to 1,000 μmol photons · m^?2 · s^?1. The H₂:N₂ ratios increased monotonically with irradiance, and the variable fluorescence measured using a fast repetition rate fluorometer (FRRF) revealed that this increase was accompanied by a progressive reduction of the plastoquinone (PQ) pool. Additions of 2,5‐dibromo‐3‐methyl‐6‐isopropyl‐p‐benzoquinone (DBMIB), an inhibitor of electron transport from PQ pool to PSI, diminished both N₂ fixation and net H₂ production, while the H₂:N₂ ratio increased with increasing level of PQ pool reduction. In the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), nitrogenase activity declined but could be prolonged by increasing the light intensity and by removing the oxygen supply. These results on the coupling of N₂ fixation and H₂ cycling in Trichodesmium indicate how light intensity and light spectral quality of the open ocean can influence the H₂:N₂ ratio and modulate net H₂ production.     

Factors controlling bacterial production in marine and freshwater sediments

We collected benthic bacterial production data measured by ³H thymidine incorporation (TTI) (25 studies), frequency of dividing cells (FDC) (3 studies), dark-C0₂ assimilation (1 study) and ³H-adenine uptake (2 studies) from the literature, which included 18 marine, 6 river, and 2 lake studies. In all of the studies that used the TTI method, ³H-DNA was isolated and incubations were carried out at in situ temperatures. Most of the researchers also determined ³H-DNA extraction efficiencies and isotope dilution, thus interpretable estimates of bacterial production were used in the analysis. In marine sediments, bacterial production rates were linked to bacterial biomass, bacterial abundance, sediment organic matter, temperature, and sediment chlorophyll a, with these variables explaining between 40% and 68% of the variation in production rates. Simple relationships between production and bacterial biomass or bacterial abundance, or between production and sediment organic matter, were improved by also including temperature in the analysis of marine sediments. Sediment organic matter explained an appreciable fraction (58%) of the observed production in freshwater sediments. Temperature was the most powerful predictor of the observed variability in specific growth rates (r ² = 0.48 and r ² = 0.58) in marine and freshwater sediments, respectively. Thus, bacterial production and specific growth rates are most closely linked to substrate supply and temperature in marine and freshwater sediments. Offprint requests to: B. C. Sander.     

Bacterioplankton production in freshwater Antarctic lakes

1. Bacterioplankton production was measured in the water columns of two ultra‐oligotrophic, freshwater Antarctic lakes (Crooked Lake and Lake Druzhby) during an annual cycle. In both lakes bacterial production, measured by the incorporation of [³H] thymidine, continued in winter and showed a cycle over the year. The range of production was between 0 and 479 ng C L^?1 h^?1 in Crooked Lake and 0–354 ng L^?1 h^?1 in Lake Druzhby. 2. Abundance and mean cell volume both varied, producing marked changes in biomass during the year, with highest biomass occurring in the winter and early spring. Biomass showed similar seasonal trends in both lakes. 3. For most of the year inorganic forms of nitrogen and phosphorus were detectable in the water columns of the lakes and were unlikely to have limited bacterial production. Dissolved organic carbon (DOC) was below 3000 μg L^?1. Dissolved amino acids and carbohydrates contributed 5–25% of the DOC pool in Crooked Lake and 5–64% in Lake Druzhby. Dissolved carbohydrates were consistently low, suggesting that this may have been the preferred carbon substrate for bacterioplankton. 4. Aggregate associated bacteria had higher mean cell volume, abundances and production than freely suspended bacteria in Lake Druzhby, while in Crooked Lake aggregate associated bacteria consistently had higher mean cell volumes than free bacteria, but abundance and production were on occasion higher in free bacteria compared with aggregate associated communities. 5. The data indicated that production is limited by continuous low temperatures and the limited availability of suitable DOC substrate. However, the bacterioplankton functions year round, responding to factors other than temperature.     

Specific growth rates of natural microbial communities measured by adenine nucleotide pool turnover

A novel experimental approach has been development to estimate the mean specific growth rates of natural populations of microorganisms. The method is based on the direct measurement of the turnover rate of the total adenine nucleotide (AN)pool using either [³H]adenine or ³²PO₄ radiotraces. In laboratory cultures of bacteria and unicellular algae, the AN pool turnover rate is highly correlated with specific growth rate. This empirically derived relationship is then used to extrapolate AN pool turnover rates measured in the field to population-specific growth rates. The method is technically simple, straightforward and potentially capable of measuring a wide range of growth rates. Microbial community growth rate estimates measured using this new method ranged from 0.04 to 2.45 per day for samples collected from a variety of aquatic habitats. Growth rate estimates derived from data on AN pooll turnover compared favorably with estimates derived from measurements of microbial biomass and production. The AN pool turnover methos has numerous potential applications in applied and environmental microbiology.     

Use of membrane filters for soil algal bioassays

A culture technique for eucaryotic and procaryotic soil algae involving the use of membrane filters superimposed on the surface of inorganic nutrient agar is described. Nostoc commune grew exponentially when cultured in this manner. No significant differences in biomass production or nitrogenase activity were detected among culture subsets within replicate experiments run under standard conditions. Estimates of daily growth rates (0.340), culture doubling time (48.9 h), and nitrogenase activity (14.54 nM C₂H₂ reduced μg⁻¹ chl a h⁻¹) were consistent with laboratory and field estimates reported for several planktonic species of Anabaena and strains of Nostoc commune isolated from diverse terrestrial habitats. Therefore, the filter-culture technique is an alternative which may be superior to traditional liquid culture methodology for studies involving certain soil procaryotic and eucaryotic algae.     

Microbial community in a geothermal aquifer associated with the subsurface of the Great Artesian Basin, Australia

To investigate the biomass and phylogenetic diversity of the microbial community inhabiting the deep aquifer of the Great Artesian Basin (GAB), geothermal groundwater gushing out from the aquifer was sampled and analyzed. Microbial cells in the groundwater were stained with acridine orange and directly counted by epifluorescence microscopy. Microbial cells were present at a density of 10⁸–10⁹ cells per liter of groundwater. Archaeal and bacterial small-subunit rRNA genes (rDNAs) were amplified by PCR with Archaea- and Bacteria-specific primer sets, and clone libraries were constructed separately. A total of 59 clones were analyzed in archaeal and bacterial 16S rDNA libraries, respectively. The archaeal 16S rDNA clones were divided into nine operated taxonomic units (OTUs) by restriction fragment length polymorphism. These OTUs were closely related to the methanogenic genera Methanospirillum and Methanosaeta, the heterotrophic genus Thermoplasma, or miscellaneous crenarchaeota group. More than one-half of the archaeal clones (59% of total 59 clones) were placed beside phylogenetic clusters of methanogens. The majority of the methanogen-related clones (83%) was closely related to a group of hydrogenotrophic methanogens (genus Methanospirillum). The bacterial OTUs branched into seven phylogenetic clusters related to hydrogen-oxidizing thermophiles in the genera Hydrogenobacter and Hydrogenophilus, a sulfate-reducing thermophile in the genus Thermodesulfovibrio, chemoheterotropic bacteria in the genera Thermus and Aquaspirillum, or the candidate division OP10. Clones closely related to the thermophilic hydrogen-oxidizers in the genera Hydrogenobacter and Hydrogenophilus were dominant in the bacterial clone library (37% of a total of 59 clones). The dominancy of hydrogen-users strongly suggested that H₂ plays an important role as a primary substrate in the microbial ecosystem of this deep geothermal aquifer.     

Using CO2 to enhance carbon capture and biomass applications of freshwater macroalgae

A major limiting factor in the development of algae as a feedstock for the bioenergy industry is the consistent production and supply of biomass. This study is the first to access the suitability of the freshwater macroalgal genus Oedogonium to supply biomass for bioenergy applications. Specifically, we quantified the effect of CO₂ supplementation on the rate of biomass production, carbon capture, and feedstock quality of Oedogonium when cultured in large‐scale outdoor tanks. Oedogonium cultures maintained at a pH of 7.5 through the addition of CO₂ resulted in biomass productivities of 8.33 (±0.51) g DW m^?2 day^?1, which was 2.5 times higher than controls which had an average productivity of 3.37 (±0.75) g DW m^?2 day^?1. Under these productivities, Oedogonium had a carbon content of 41–45% and a higher heating value of 18.5 MJ kg^?1, making it an ideal biomass energy feedstock. The rate of carbon fixation was 1380 g C m^?2 yr^?1 and 1073.1 g C m^?2 yr^?1 for cultures maintained at a pH of 7.5 and 8.5, and 481 g C m^?2 yr^?1 for cultures not supplemented with CO₂. This study highlights the potential of integrating the large‐scale culture of freshwater macroalgae with existing carbon waste streams, for example coal‐fired power stations, both as a tool for carbon sequestration and as an enhanced and sustainable source of bioenergy.     

Abundance and biomass of heterotrophic microorganisms in Lake Tanganyika

1. This study focused on heterotrophic microorganisms in the two main basins (north and south) of Lake Tanganyika during dry and wet seasons in 2002. Bacteria (81% cocci) were abundant (2.28–5.30 × 10⁶ cells mL^?1). During the dry season, in the south basin, bacterial biomass reached a maximum of 2.27 g C m^?2 and phytoplankton biomass was 3.75 g C m^?2 (integrated over a water column of 100 m). 2. Protozoan abundance was constituted of 99% of heterotrophic nanoflagellates (HNF). Communities of flagellates and bacteria consisted of very small but numerous cells. Flagellates were often the main planktonic compartment, with a biomass of 3.42–4.43 g C m^?2. Flagellate biomass was in the same range and often higher than the total autotrophic biomass (1.60–4.72 g C m^?2). 3. Total autotrophic carbon was partly sustained by the endosymbiotic zoochlorellae Strombidium. These ciliates were present only in the euphotic zone and usually contributed most of the biomass of ciliates. 4. Total heterotrophic ciliate biomass ranged between 0.35 and 0.44 g C m^?2. In 2002, heterotrophic microorganisms consisting of bacteria, flagellates and ciliates represented a large fraction of plankton. These results support the hypothesis that the microbial food web contributes to the high productivity of Lake Tanganyika. 5. As the sole source of carbon in the pelagic zone of this large lake is phytoplankton production, planktonic heterotrophs ultimately depend on autochthonous organic carbon, most probably dissolved organic carbon (DOC) from algal excretion.     

Ubiquity of Polynucleobacter necessarius ssp. asymbioticus in lentic freshwater habitats of a heterogenous 2000 km2 area

We present a survey on the distribution and habitat range of Polynucleobacter necessarius ssp. asymbioticus (PnecC), a numerically and functionally important taxon in the plankton of freshwater systems. We systematically sampled stagnant freshwater habitats in a heterogeneous 2000 km² area, together with ecologically different habitats outside this area. In total, 137 lakes, ponds and puddles were investigated, which represent an enormous diversity of habitats differing, e.g. in depth (< 10 cm – 171 m) and pH (3.9–8.5). PnecC bacteria were detected by cultivation‐independent methods in all investigated habitats, and their presence was confirmed by cultivation of strains from selected habitats representing the whole studied ecological range. The determined relative abundance of the subspecies ranged from values close to the detection limit of FISH (0.2%) to 67% (average 14.5%), and the highest observed absolute abundance was 5.3 × 10⁶ cells ml^?1. Statistical analyses revealed that the abundance of PnecC bacteria was partially controlled by factors linked to concentrations of humic substances, which support the hypothesis that these bacteria utilize photodegradation products of humic substances. Based on the revealed statistical relationships, an average relative abundance of this subspecies of 20% in global freshwater habitats was extrapolated. Our study provides important implications for the current debate on ubiquity and biogeography in microorganisms.     

Thermophilic Hydrogen Production from Renewable Resources: Current Status and Future Perspectives

Hydrogen (H₂) is considered an alternative fuel of the future due to its high energy density and non-polluting nature. H₂ energy provides many advantages over fossil fuels in that it is renewable, eco-friendly, and efficient. The global demand for H₂ is increasing significantly; however, matching the supply of cost-competitive H₂ to meet the current demand is a major technological barrier. H₂ can be produced from lignocellulosic biomass and serve as a raw material for the synthesis of many industrially important chemicals. The use of thermophilic bacteria for biological production of H₂ appears to be a promising alternative route to the current H₂ production technologies. However, the carbon and H₂ production metabolisms in most thermophilic bacteria have not yet been completely understood. This paper summarizes the recent research progress made toward understanding the carbon utilization for H₂ production and developing gene manipulation techniques to enhance the H₂ production capabilities in thermophilic bacteria. It reviews the current status, future directions and opportunities that thermophiles can offer to enable a cost-competitive and environmentally benign H₂ production bioprocess.     

Influences of salinity on the physiology and distribution of the Arctic coralline algae,Lithothamnion glaciale (Corallinales,Rhodophyta)

In Greenland, free‐living red coralline algae contribute to and dominate marine habitats along the coastline. Lithothamnion glaciale dominates coralline algae beds in many regions of the Arctic, but never in Godthåbsfjord, Greenland, where Clathromorphum sp. is dominant. To investigate environmental impacts on coralline algae distribution, calcification and primary productivity were measured in situ during summers of 2015 and 2016, and annual patterns of productivity in L. glaciale were monitored in laboratory‐based mesocosm experiments where temperature and salinity were manipulated to mimic high glacial melt. The results of field and cold‐room measurements indicate that both L. glaciale and Clathromorphum sp. had low calcification and photosynthetic rates during the Greenland summer (2015 and 2016), with maximum of 1.225 ± 0.17 or 0.002 ± 0.023 μmol CaCO ₃ · g^?1 · h^?1 and ?0.007 ±0.003 or ?0.004 ± 0.001 mg O₂ · L^?1 · h^?1 in each species respectively. Mesocosm experiments indicate L. glaciale is a seasonal responder; photosynthetic and calcification rates increase with annual light cycles. Furthermore, metabolic processes in L. glaciale were negatively influenced by low salinity; positive growth rates only occurred in marine treatments where individuals accumulated an average of 1.85 ± 1.73 mg · d^?1 of biomass through summer. These results indicate high freshwater input to the Godthåbsfjord region may drive the low abundance of L . glaciale , and could decrease species distribution as climate change increases freshwater input to the Arctic marine system via enhanced ice sheet runoff and glacier calving.     

Anaerobic extremely thermophilic carboxydotrophic bacteria in hydrotherms of Kuril Islands

A new group of extremely thermophilic, obligately anaerobic, carboxydotrophic eubacteria is described. The organisms are characterized by a novel type of chemotrophic metabolism in thermophilic environments. They grow at temperatures up to 80–85°C chemolithotrophically with 100% CO in the gas phase as the sole energy source. The CO oxidation is coupled to H₂ and CO₂ formation according to the equation CO+H₂O → H₂+CO₂. No other products of metabolism are produced. The group of CO-utilizing, H₂-producing anaerobes includes diverse bacteria. They are non-sporeforming rods differing in morphology, CO uptake rates, habitats, and maximum growth temperatures. The new carboxydotrophic thermophilic anaerobes are widely distributed in freshwater and coastal marine hydrotherms of the Kuril Islands. Offprint requests to: V. A. Svetlichny.     

Effect of initial bacteria concentration on hydrogen gas production from cheese whey powder solution by thermophilic dark fermentation

Dark fermentative hydrogen gas production from cheese whey powder solution was realized at 55°C. Experiments were performed at different initial biomass concentrations varying between 0.48 and 2.86 g L^?1 with a constant initial substrate concentration of 26 ± 2 g total sugar (TS) per liter. The highest cumulative hydrogen evolution (633 mL, 30°C), hydrogen yield (1.56 mol H₂ mol^?1 glucose), and H₂ formation rate (3.45 mL h^?1) were obtained with 1.92 g L^?1 biomass concentration. The specific H₂ production rate decreased with increasing biomasss concentration from the highest value (47.7 mL g^?1 h^?1) at 0.48 g L^?1 biomass concentration. Total volatile fatty acid concentration varied beetween 10 and 14 g L^?1 with the highest level of 14.2 g L^?1 at biomass concentration of 0.48 g L^?1 and initial TS content of 28.4 g L^?1. The experimental data were correlated with the Gompertz equation and the constants were determined. The most suitable initial biomass to substrate ratio yielding the highest H₂ yield and formation rate was 0.082 g biomass per gram of TS. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 28: 931–936, 2012     

Abundance,distribution, and secondary production of the apple snail Pomacea flagellata (Say, 1829) in Bacalar Lake,a tropical karstic system in southern Mexico

Pomacea flagellata is a gastropod conspicuous in freshwater environments, and represents a fishing resource. To assess their abundance, distribution, and secondary production, monthly samplings were carried out in Bacalar Lake from June 2012 to May 2013 at 12 sampling sites. In each site, three random transects were marked parallel to the shore. All snails on transect were collected and shell length and wet weight measured. The highest density occurred in September (1.27 ind.m^?2), lowest in October (0.47 ind.m^?2). Shell lengths ranged from 2 to 56 mm, with recruitment in January–March. Growth parameters were L_∞ 59.50 mm, K 0.65.year^?1; the lifetime span was 3 years. Average biomass reached 5.57 wet g.m^?2 and secondary production was 6.025 wet g.m^?2.year^-1; annual renewal rate P/B 1.08. Highest abundance and secondary production was contributed by individuals between 31 and 41 mm in length. A potential biomass of 25.06 tons of snails was estimated in the lake. Snail densities, secondary production, and turnover were very low during the year, indicating that it is not viable to consider a commercial catch without affecting the population. A ban of 10 years is proposed, and aquaculture practices of snails are recommended to recover the resource.     

Bacterial dynamics in two high-arctic lakes

The heterotrophic planktonic bacteria in two high-arctic lakes were studied by direct microscope count and the enzymatic uptake of ¹⁴C labelled glucose which generally conformed to Michaelis-Menten kinetics. Bacterial numbers and activity in oligo-trophic Char Lake ranged from 0.1 to 2.0×10^?3 bacteria/l and a maximum uptake velocity (V_max) of 1.8 × 10^?3μg glucose l^? h^?1. Nearby Meretta Lake received waste water from the Department of Transport Base at Resolute and this eutrophication was reflected in higher bacterial numbers of 2-80 × 10⁸/1 and Kmax of 0.1 × 10^?1-7.5 × 10^?1 fig glucose l^?1 h^?1 The Kmax per cell in Char Lake was 3 × 10^?11μg glucose l^?1 h^?1 and changed little between the period of solid ice cover in May and ice-free conditions in August. Bacterial cycles could not be related to phytoplankton cycles in either lake. Comparison of kinetic data from several lakes suggests a relationship between the bacterial uptake rate of glucose and phytoplankton production. Both bacterial numbers and activity in Char Lake may be very close to the minima to be expected in undisturbed freshwater environments.     

Benthic bacterial biomass and production in the Hudson River estuary

Bacterial biomass, production, and turnover were determined for two freshwater marsh sites and a site in the main river channel along the tidally influenced Hudson River. The incorporation of [methyl-³H]thymidine into DNA was used to estimate the growth rate of surface and anaerobic bacteria. Bacterial production at marsh sites was similar to, and in some cases considerably higher than, production estimates reported for other aquatic wetland and marine sediment habitats. Production averaged 1.8–2.8 mg C·m^–2·hour^–1 in marsh sediments. Anaerobic bacteria in marsh sediment incorporated significant amounts of [methyl-³H]thymidine into DNA. Despite differences in dominant vegetation and tidal regime, bacterial biomass was similar (1×10³±0.08 mg C·m^–2) inTrapa, Typha, andNuphar aquatic macrophyte communities. Bacterial abundance and productivity were lower in sandy sediments associated withScirpus communities along the Hudson River (0.2×10³±0.05 mg C·m^–2 and 0.3±0.23 mg C·m^–2·hour^–1, respectively).     

Comparing the potential production and value of high-energy liquid fuels and protein from marine and freshwater macroalgae

The biomass production and biochemical properties of marine and freshwater species of green macroalgae (multicellular algae), cultivated in outdoor conditions, were evaluated to assess the potential conversion into high-energy liquid biofuels, specifically biocrude and biodiesel and the value of these products. Biomass productivities were typically two times higher for marine macroalgae (8.5–11.9 g m⁻² d⁻¹, dry weight) than for freshwater macroalgae (3.4–5.1 g m⁻² d⁻¹, dry weight). The biochemical compositions of the species were also distinct, with higher ash content (25.5–36.6%) in marine macroalgae and higher calorific value (15.8–16.4 MJ kg⁻¹) in freshwater macroalgae. Lipid content was highest for freshwater Oedogonium and marine Derbesia. Lipids are a critical organic component for biocrude production by hydrothermal liquefaction (HTL) and the theoretical biocrude yield was therefore highest for Oedogonium (17.7%, dry weight) and Derbesia (16.2%, dry weight). Theoretical biocrude yields were also higher than biodiesel yields for all species due to the conversion of the whole organic component of biomass, including the predominant carbohydrate fraction. However, all marine species had higher biomass productivities and therefore had higher projected biocrude productivities than freshwater species, up to 7.1 t of biocrude ha⁻¹ yr⁻¹ for Derbesia. The projected value of the six macroalgae was increased by 45–77% (up to US$7700 ha⁻¹ yr⁻¹) through the extraction of protein prior to the conversion of the residual biomass to biocrude. This study highlights the importance of optimizing biomass productivities for high-energy fuels and targeting additional coproducts to increase value.     

Partitioning of river metabolism identifies phytoplankton as a major contributor in the regulated Murray River (Australia)

1. River metabolism was measured over an annual cycle at three sites distributed along a 1000 km length of the lowland Murray River, Australia. 2. Whole system metabolism was measured using water column changes in dissolved oxygen concentrations while planktonic and benthic metabolism were partitioned using light‐dark bottles and benthic chambers. 3. Annual gross primary production (GPP) ranged from 775 to 1126 g O₂ m^?2 year^?1 which in comparison with rivers of similar physical characteristics is moderately productive. 4. Community respiration (CR) ranged from 872 to 1284 g O₂ m^?2 year^?1 so that annual net ecosystem production (NEP) was near zero, suggesting photosynthesis and respiration were balanced and that allochthonous organic carbon played a minor role in fuelling metabolism. 5. Planktonic rates of gross photosynthesis and respiration were similar to those of the total channel, indicating that plankton were responsible for much of the observed metabolism. 6. Respiration rates correlated with phytoplankton standing crop (estimated as the sum of GPP plus the chlorophyll concentration in carbon units), yielding a specific respiration rate of ?1.1 g O₂ g C^?1 day^?1. The respiration rate was equivalent to 19% of the maximum rate of phytoplankton photosynthesis, which is typical of diatoms. 7. The daily GPP per unit phytoplankton biomass correlated with the mean irradiance of the water column giving a constant carbon specific photon fixation rate of 0.35 gO₂ g Chl a^?1 day^?1 per μmole photons m^?2 s^?1 (ca. 0.08 per mole photons m^?2 on a carbon basis) indicating that light availability determined daily primary production. 8. Annual phytoplankton net production (NP) estimates at two sites indicated 25 and 36 g C m^?2 year^?1 were available to support riverine food webs, equivalent to 6% and 11% of annual GPP. 9. Metabolised organic carbon was predominantly derived from phytoplankton and was fully utilised, suggesting that food‐web production was restricted by the energy supply.     

Temporal dynamics and growth of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic lake

1. The in situ abundance, biomass and mean cell volume of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic German mining lake (Lake 111; pH 2.65), were determined over three consecutive years (spring to autumn, 2001–03). 2. Actinophrys sol exhibited pronounced temporal and vertical patterns in abundance, biomass and mean cell volume. Increasing from very low spring densities, maxima in abundance and biomass were observed in late June/early July and September. The highest mean abundance recorded during the study was 7 × 10³ Heliozoa L^?1. Heliozoan abundance and biomass were higher in the epilimnion than in the hypolimnion. Actinophrys sol cells from this acidic lake were smaller than individuals of the same species found in other aquatic systems. 3. We determined the growth rate of A. sol using all potential prey items available in, and isolated and cultured from, Lake 111. Prey items included: single‐celled and filamentous bacteria of unknown taxonomic affinity, the mixotrophic flagellates Chlamydomonas acidophila and Ochromonas sp., the ciliate Oxytricha sp. and the rotifers Elosa worallii and Cephalodella hoodi. Actinophrys sol fed over a wide‐size spectrum from bacteria to metazoans. Positive growth was not supported by all naturally available prey. Actinophrys sol neither increased in cell number (k) nor biomass (k_b) when starved, with low concentrations of single‐celled bacteria or with the alga Ochromonas sp. Positive growth was achieved with single‐celled bacteria (k = 0.22 ± 0.02 d^?1; k_b = ?0.06 ± 0.02 d^?1) and filamentous bacteria (k = 0.52 ± <0.01 d^?1; k_b = 0.66 d^?1) at concentrations greater than observed in situ, and the alga C. acidophila (up to k = 0.43 ± 0.03 d^?1; k_b = 0.44 ± 0.04 d^?1), the ciliate Oxytricha sp. (k = 0.34 ± 0.01 d^?1) and in mixed cultures containing rotifers and C. acidophila (k = 0.23 ± 0.02–0.32 ± 0.02 d^?1; maximum k_b = 0.42 ± 0.05 d^?1). The individual‐ and biomass‐based growth of A. sol was highest when filamentous bacteria were provided. 4. Existing quantitative carbon flux models for the Lake 111 food web can be updated in light of our results. Actinophrys sol are omnivorous predators supported by a mixed diet of filamentous bacteria and C. acidophila in the epilimnion. Heliozoa are important components in the planktonic food webs of ‘extreme’ environments.