Role of planktonic bacteria in productivity and cycling of organic matter in the Eastern Pacific Ocean

Total number, biomass, production, and respiration of bacterioplankton were measured in oligotrophic, mesotrophic and eutrophic waters of the Eastern Pacific. Total number of bacteria in the upper mixed layer and in the upper thermocline boundary layers varied from 30–60.10³ ml^-1 in oligotrophic waters to 100–400.10³ ml^-1 in mesotrophic waters of fronts and divergences, and to 1–2,5.10⁶ ml^-1 in eutrophic waters of coastal upwellings. Wet biomass varied from 5–10 mg l^-1 in oligotrophic waters, to 50–200 mg l^-1 in mesotrophic waters, and to 1–2 g m^-3 in eutrophic waters. Below the layer of maximum temperature gradient i.e. below 35–50 m, bacterioplankton density decreased 5–10 times. P/B coefficients per day were highest in the oligotrophic surface water ( 1), and lowest in the eutrophic ones (0.2–0.4). In mesotrophic waters they were intermediate (0.4–1.0). the stock of labile organic matter (LOM) accessible to microbial action varied from 0.3 to 1.6 mg Cl^-1. Its highest value occurred in the upwelling area. The stock of LOM does not noticeably decrease from the euphotic zone to a depth of 2 000 m. Its turnover time varied from 5 to 45 days in surface waters, and 30–50 years in deep oceanic waters. The role of bacterioplankton in productivity and in cycling of organic matter in surface — and deep oceanic waters is discussed.     

Bacterioplankton Abundance and Activity in a Small Hypertrophic Stratified Lake

Bacterioplankton abundance and production were followed during one decade (1991–2001) in the hypertrophic and steeply stratified small Lake Verevi (Estonia). The lake is generally dimictic. However, a partly meromictic status could be formed in specific meteorological conditions as occurred in springs of 2000 and 2001. The abundance of bacteria in Lake Verevi is highly variable (0.70 to 22 × 10⁶ cells ml⁻¹) and generally the highest in anoxic hypolimnetic water. In 2000–2001, the bacterial abundance in the hypolimnion increased probably due to meromixis. During a productive season, heterotrophic bacteria were able to consume about 10–40% of primary production in the epilimnion. Our study showed that bacterioplankton in the epilimnion was top-down controlled by predators, while in metalimnion bacteria were dependent on energy and carbon sources (bottom-up regulated). Below the thermocline hypolimnetic bacteria mineralized organic matter what led to the depletion of oxygen and created anoxic hypolimnion where rich mineral nutrient and sulphide concentrations coexisted with high bacterial numbers.     

Bacterial response to extracellular dissolved organic carbon released from healthy and senescent Fragilaria crotonensis (Bacillariophyceae) in experimental systems

Responses of bacteria to dissolved organic carbon (DOC) released from healthy and senescent Fragilaria crotonensis (Bacillariophyceae) were examined in experimental systems. The alga released DOC actively, although the concentration fluctuated greatly in both the axenic (the alga alone) and the mixed (the alga plus the enriched bacteria) cultures. In the control (the bacteria alone) cultures, both DOC concentration and bacterial density were low and almost constant throughout the experiment: 5.0 mg C 1^–1 and 1.1 × 10⁵ cells ml^–1, respectively. In the mixed cultures, bacterial growth was negligible during the exponential growth phase of the alga, but rapid proliferation of the bacteria occurred after the onset of the stationary growth phase. As the bacterial population grew, the density of senescent algal cells also increased. When the bacteria were fed on the DOC from healthy algae, their growth rate was relatively low (0.44 d^–1), but the maximum cell density was high (6.4 × 10⁵ cells ml^–1). Conversely, when the bacteria fed on the DOC of senescent algae, they grew at a relatively high rate (0.51 d^–1), but the maximum cell density was low (2.8 × 10⁵ cells ml^–1). These results suggest that DOCs released from dominant phytoplankton species in different physiological states affect the biomass and activity of bacteria.     

Production in the Sea of Okhotsk

Primary production, microbial production and the density of planktonic microheterotrophs were estimated at 40 stations in the Okhotsk Sea in July-August 1992 during the seasonal phytoplankton minimum. The primary production by phytoplankton remained rather high even during this minimum. At most stations it was >0.6-0.8 g m^-2 day^-1, and in leftover patches of spring diatom 'bloom' it reached >5 g C m^-2 day^-1. The deep maxima of phytoplankton at the upper boundary of the seasonal thermocline were an ordinary phenomenon. The depth of the euphotic zone was normally 30-50 m in the open sea and 12-25 m at the shelf station. Any correlations between the phosphate contents in the upper mixed layer and primary production were absent at the stations. There was no adaptation of the phytoplankton to the light deficiency in deep maxima layers. The total numbers of bacterioplankton were 1-1.5 x 10⁶ ml^-1 and its biomass was close to 100 mg m^-3 in the open sea. All these numbers were 2-3 times greater at the shelf stations. In deep waters, the bacterioplankton biomass decreased to 10-40 mg m^-3. The microbial production in the upper layer was high, at 50-100 mg m^-3, decreasing 50-100 times in the deep waters. The numbers of ciliates in the upper water layer varied from 3 to 6 x 10³ l^-1 and were 1.5-2 times greater than in the shelf areas. Ciliate biomass was 60-100 mg m^-3 in the upper mixed layer, and per square metre varied to 1.5-2.5 g. The dominant ciliate taxa belonged to the naked oligotrichid genera Strombidium and Tontonia. Tentative calculations were made of the basin's annual primary production and for the analysis of energy balance in the ecosystem.      

Size-selective grazing of coastal bacterioplankton by natural assemblages of pigmented flagellates,colorless flagellates,and ciliates

Fluorescently-labelled bacteria (FLB) were used to study the feeding strategies of a natural assemblage of estuarine protozoans and to examine whether the protozoan grazing could account for the in situ size structure of the bacterioplankton. The FLB, DTAF-stained enterococci, ranging in volume from 0.01 to 0.30 × 10^–1 µm³, were added to a natural planktonic assemblage at a density of 5.5% of the natural bacterioplankton. Initial densities (individuals ml^–1) were as follows: total natural bacteria, 2.2 × 10⁶; FLB, 1.2 × 10⁵; pigmented flagellates, 300; colorless flagellates, 250; and ciliates, 30. FLB consumption rates were determined by examining the contents of protozoan food vacuoles, and the long-term effect of grazing (over a period of 100 hours) was determined by monitoring the decline in the FLB density in experimental vessels. The average consumption rates of FLB by pigmented flagellates were similar to those by flagellates that lacked chloroplasts (0.9 and 0.6 FLB protozoan^–1 hour^–1, respectively). The ciliates consumed bacteria at an average rate that was 17-fold higher (per cell) than flagellates, and they displayed a greater preference for larger bacteria than did the flagellates. FLB of the mid-size classes (0.025–0.100 µm³) were heavily grazed by the entire protozoan assemblage; the smallest (<0.025 µm³) and the largest (>0.100 µm³) FLB escaped protozoan grazing. This had a profound effect on the resulting size distribution of FLB. At the end of a 100-hour incubation, the percentage of mid-size FLB (0.025 to 0.100 µm³) decreased 2.0–2.2-fold, while the percentage of the smallest and the largest FLB increased 2.0–2.5-fold. Resultant densities of FLB were consistent with initial clearance rates determined for the protozoan groups. The grazing rates of protozoans on FLB were species-specific; whereas some species consumed FLB, others did not demonstrate bacterivory. The results suggest that protozoan grazing has a major effect on the size distribution of coastal bacterioplankton. By selectively feeding on a particular size-class of bacteria, planktonic ciliates may consume 15–90% day^–1 of the standing stock of largest size classes of bacterioplankton. Thus, ciliates, which were present in low abundance in the field, could not balance the production of the entire bacterial community, but they may strongly influence the portion of the bacterial community represented by the largest bacterial class. The direct effect of flagellates (e.g., grazing) was limited to smaller bacteria.Offprint requests to: M. P. Shiaris.     

Role of plankton in the turnover of organic matter on the Great Barrier Reef,Australia

Number, biomass and production of phytoplankton, bacteria, micro- and mesozooplankton and turnover of labile and stable organic matter were measured in waters over some Capricornia round reefs, and over the reefs of Lizard Island. Primary production was 10 to 40 mg C m^–3 d^–1 but was lower over the living reefs. Microbial wet biomass in reef waters varied from 100 to 500 mg m^–3, and production from 4 to 68 mg C m^–3 d^–1, which was commensurable with primary production. The biomass of microzooplankton (ciliates, zooflagellates and larvae) in waters of Lizard Island reefs reached 100–300 mg m^–3. Mesozooplankton biomass at night in reef waters of Heron Island varied from 200 to 800 mg m^–3. Its composition depended upon the tide phase. PB coefficients in bacterioplankton were 0.3 to 1.2 per day. The food demand of bacterioplankton in waters over the reefs was 5 to 20 times higher than the primary phytoplankton production. Labile organic matter (LOM) doubled in waters after it stayed over living reef for several hours. The turnover time of LOM in reef waters was as short as 1–2 weeks.     

Natural populations of bacteria in Lake Kinneret: Observations with scanning electron and epifluorescence microscopy

The bacterioplankton assemblage in Lake Kinneret, Israel, sampled on 6 occasions representative of different seasonal conditions was studied with scanning electron microscopy (SEM) and epifluorescence microscopy after acridine-orange staining. In near-surface (1–3 m) samples taken in October 1981 and March 1983, several unusual types of budding, appendaged, and filamentous cells were found. During lake stratification, typical large anaerobic forms (including photosynthetic green sulphur bacteria) were observed in samples from the metalimnion and deep (40 m) hypolimnion. Epifluorescence counts indicated that bacteria in the water column ranged from 0.55 to 2.67 × 10⁶ cells ml^–1.     

Distribution of microaerophilic bacteria through the oxic-anoxic transition zone of lagoon sediments

In marine sediments, where soluble gases diffuse only very slightly, many organisms struggle for molecular oxygen. Microaerophilic bacteria, able to grow at reduced pO₂ between 0.2 and 12%, have an advantage. Distribution of aerobes, microaerophiles and anaerobes was compared with the oxygen gradient in seawater and sediment samples collected in a northern Mediterranean lagoon. In the near bottom seawater and in the 0–10 mm upperlayer of sediment, the microaerophilic counts were less than 1% of aerobe densities. In the 10–15 mm zone, these two groups were equivalent in density (1 × 10⁵ cells ml^–1). As expected, the microaerophiles took advantage of their low oxygen tension requirements in the subsurface sediments, between the well aerated zone (0–5 mm depth) and the low redox potential zone. Then, beyond a depth of 20 mm, the anaerobes prevailed in this sandy clay.     

Bacterial activity in sea ice and open water of the Weddell Sea,Antarctica: A microautoradiographic study

Metabolic activity of bacteria was investigated in open water, newly forming sea ice, and successive stages of pack ice in the Weddell Sea. Microautoradiography, using [³H]leucine as substrate, was compared with incorporation rates of [³H]leucine into proteins. Relation of [³H]leucine incorporation to the biomass of active bacteria provides information about changes of specific metabolic activity of cells. During a phytoplankton bloom in an ice-free, stratified water column, total numbers of bacteria in the euphotic zone averaged 2.3 × 10⁵ ml^–1, but only about 13% showed activity via leucine uptake. Growth rate of the active bacteria was estimated as 0.3–0.4 days^–1. Total cell concentration of bacteria in 400 m depth was 6.6 × 10⁴ ml^–1. Nearly 50% of these cells were active, although biomass production and specific growth rate were only about one-tenth that of the surface populations. When sea ice was forming in high concentrations of phytoplankton, bacterial biomass in the newly formed ice was 49.1 ng C ml^–1, exceeding that in open water by about one order of magnitude. Attachment of large bacteria to algal cells seems to cause their enrichment in the new ice, since specific bacterial activity was reduced during ice formation, and enrichment of bacteria was not observed when ice formed at low algal concentration. During growth of pack ice, biomass of bacteria increased within the brine channel system. Specific activity was still reduced at these later stages of ice development, and percentages of active cells were as low as 3–5%. In old, thick pack ice, bacterial activity was high and about 30% of cells were active. However, biomass-specific activity of bacteria remained significantly lower than that in open water. It is concluded that bacterial assemblages different to those of open water developed within the ice and were dominated by bacteria with lower average metabolic activity than those of ice-free water.     

Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layerof the Bering Sea during the late spring-early summer periodranged between 1 and {small tilde}4 x 10⁶ ml^–1 (biomass10–40mg C m^–3). In the northern Pacific, along 47–52⁶N,the corresponding characteristics of the bacterioplankton densityin the upper mixed water layer were: total number 1–2x 10⁶ cells ml^–1 and biomass 15–46mg C m^–3Below the thermocline at 50–100 m, the density of bacterioplanktonrapidly decreased. At 300 m depth, it stabilized at 0.1–0.2x 106 cells ml^–1. The integrated biomass of bacterioplanktonin the open Bering Sea ranged between 1.2 and 3.6 g C m^–2(wet biomass 6–18 g m^–2) Its production per dayvaried from 2 to 23 mg C m^–3 days^–1 in the upper0–100 m. The numerical abundance of planktonic ciliatesin this layer was estimated to be from 3 to l0 x 10³ cells l^–1,and in the northern Pacific from 0.4 to 4.5 x 10³ l^–2.Their populations were dominated by naked forms of Strombidium,Strombilidium and Tontonia. In some shelf areas, up to 40% ofthe total ciliate population was represented by the symbioticciliate Mesodinium rubrum. The data on the integrated biomassof basic groups of planktonic microheterotrophs are also presented,and their importance in the trophic relationships in pelagiccommunities of subarctic seas is discussed.     

Phytoplankton, bacterial production and protozoan bacterivory in stratified, brackish-water Lake Shira (Khakasia, Siberia)

Rates of oxygenic and anoxygenic photosynthesis, chemoautotrophic and heterotrophic bacterial production and protozoan bacterivory were measured in the pelagic zone of the stratified brackish-water lake with the purpose to determine the vertical distribution of these processes and to estimate their significance in the functioning of planktonic community of the lake. In midsummer, total daily primary productivity was about 1.3 g C m^–2, of which 72% was produced by the phytoplankton, 24% by the chemoautotrophic bacteria, and only 4% by the phototrophic sulphur bacteria. Thus anoxygenic photosynthesis is a negligible source of organic matter in the lake. The production of heterotrophic bacteria averaged 1.5 g C m^–2 d^–1 and exceeded the total photosynthesis of phytoplankton and photosynthetic bacteria by a factor of 1.5. The estimated total primary production was too low to sustain the bacterial production. Probably the carbon cycle in the lake is dependent on the input of allochthonous organic matter. As a rule, the maximal rates of primary production and heterotrophic bacterial production were found in the chemocline or at the upper boundary of the chemocline. Heterotrophic flagellates dominated among the protozoan populations and were the major consumers of the bacterioplankton production in the lake. They showed maximal ingestion rates from 2.3 to 2.9 mg C m^–3 h^–1 at the upper boundary of the chemocline, where they consumed from 50 to 54% of the production of heterotrophic bacteria. Data obtained indicate that in Lake Shira the oxic-anoxic interface is the site of the most intensive production and mineralization of organic matter.     

Optimal cell density and multiplicity of infection for the propagation of human rotavirus in monkey kidney cells

The human rotavirus titer was optimal at an infection cell density of about 4–8 × 10⁴ cells cm^–2 in monkey kidney cell cultures. The highest viral titers (3.8 × 10⁷ TCID₅₀ ml^–1 and 3.7 × 10⁷ TCID₅₀ ml^–1) were obtained at an multiplicity of infection of 0.05 in well plate and T-flask, respectively.     

Microbial diversity of Minnesota peatlands

Microbial diversity, numbers, and metabolic activities in Minnesota peatlands were investigated using a variety of microbial enrichment and enumeration procedures together with radioisotopic measurements of microbial degradative processes. Minnesota peatlands were shown to contain large microbial populations of wide metabolic diversity. Direct counts of bacteria using epifluorescence microscopy indicated bacterial populations of about 10⁸ ml^–1 of peatland water, irrespective of depth. Radioisotopic most-probable-number (MPN) counts of heterotrophs able to mineralize¹⁴C-labeled substrates to¹⁴CO₂ showed significant populations of glucose degraders (10⁴–10⁶ ml^–1) as well as degraders of benzoate (10²–10³ ml^–1), 2,4-dichlorophenoxyacetate (10²–10⁵ ml^–1), and sphagnum (10³–10⁷ ml^–1) in the various peatlands examined. The MPNs of NO₃ ^– reducers varied from 10³–10⁶ ml^–1, SO₄ ^– reducers from 10²–10³ ml^–1, methanogenic bacteria from 10³–10⁶ ml^–1, and methane oxidizers from 10³–10⁴ ml^–1, depending on sampling site and depth. Eighty pure cultures of aerobic bacteria and fungi were isolated from Minnesota peats. Most of those cultures tested were able to grow on at least 20 organic compounds (carbohydrates, aromatic molecules, hydrocarbons, etc.) as sole sources of carbon and energy. One isolate, aBacillus, was able to fix atmospheric N₂. Several of the isolates were able to mineralize¹⁴C-labeled lignin.     

Spatial and temporal distribution of virioplankton and bacterioplankton in a Brackish Environment (Lake of Ganzirri,Italy)

Temporal and spatial changes of viral and bacterial abundance were examined in relation to environmental factors and hydrography at five stations between May 2000 and July 2001 in the brackish lake of Ganzirri (Sicily, Italy). Virioplankton abundance ranged from 5.26 × 10⁴ to 7.54 × 10⁸VLP ml^–1 (on average 1.38 × 10⁸particles ml^–1) and was significantly higher at the three eutrophic stations located in the lake of Ganzirri (Stations 1, 2, and 3) than in the channel connecting the lake with the Straits of Messina. The virus-to-bacterium abundance ratio (VBR range, 0.4–117; average:14) showed the highest values in channel connecting the lake of Ganzirri with the meromictic lake of Faro. VBR values <1.0 were found in summer 2000 in relation with peculiar hydrographic constraints. Virioplankton distribution was dependent on salinity, and on dilution of the oligotrophic waters flowing from the Straits. Virioplankton was closely related with bacterioplankton indicating a close coupling between viruses and host cell abundance.     

Microbial populations and sludge characteristics in thermophilic aerobic sewage sludge digestion

Summary Estimates of bacterial numbers from raw sewage sludge and sludge treated by thermophilic aerobic digestion were compared with simple indicators of sludge quality and concentrations of potential substrates. Significant differences were found between sludge types for all but one of the variables examined (frequency of dividing cells). During a stable period of digestor operation, the average number of viable obligate thermophiles present in digested sludge (1.63 × 10⁶ ml^–1) was approximately 10²-fold greater than in feed sludge (1.10 × 10⁴ ml^–1). Total numbers of bacteria were slightly greater in digested sludge (3.24 × 10¹⁰ ml^–1) than in feed sludge (2.39 × 10 ml^–10), as were viable counts of bacteria at incubation temperatures of 37°C and 55°C. Significant correlation was found between viable counts of bacteria at 37°C and 55°C for digested sludge, and 65°C and 55°C for feed sludge. The numbers of obligate thermophiles present and the total of bacteria present were related to the temperature and pH of the digested sludge and inversely related to the numbers ofEscherichia coli and coliforms present, which were not detected at temperatures greater than 50°C.     

The Distribution of Bacterio- and Mesozooplankton in the Coastal Waters of the White and Barents Seas

The total population density and the biomass of bacterioplankton, mesozooplankton, and phosphate-accumulating bacteria (PAB) were estimated during the 2000–2001 summer–autumn seasons in the coastal waters of the White and Barents Seas, which are subject to the action of tidal and sea currents, the inflow of riverine waters, and anthropogenic impact. In the shallow estuarine waters with salinities of 6.5–32 near the Chernaya, Pesha, and Pechora River mouths, the population of PAB fluctuated from 0.1 to 9.1 million cells/ml (0–36% of the total bacterial population). In pelagic seawaters, which are low in phosphates (12–50 g/l) and are characterized by an increased iron/phosphorus ratio (2.0–3.6), bacterioplankton amounted to 0.1–1.6 million cells/ml and was mainly represented by small organisms with a volume of 0.08–0.15 m³, commonly lacking intracellular polyphosphates. In the pelagic zone of the Barents Sea, the biomass of mesozooplankton (B _z) was comparable with that of bacterioplankton (B _b = 39–175 mg/m³), the B _b/B _z ratio being 1.4–4.6. Off the Varandeiskii, Pechora, and Kolguyev oil terminals, B _b increased to 155–300 mg/m³ and the B _b/B _z ratio rose to 1.4 to 50.3 (with an average value of 20.9), presumably due to the severe anthropogenic impact on these waters. In this case, the dense population of bacterioplankton (0.9–7.6 million cells/ml) was mainly represented by large cells (0.12–0.76 m³ in volume), most of which (3–43% of the total bacterioplankton population) contained polyphosphates. The chemical composition of these waters was characterized by an elevated content of the total phosphorus (65–128 g/l) and by a low iron/phosphorus ratio (0.9–1.2).     

Examining the relationship between bacteria and heterotrophic nanoflagellates in Funka Bay (Japan) using the size-fractionation method

The chemical and biological conditions, and the bacteria-heterotrophic nanoflagellate (HNF) relationship were investigated in the vicinity of Funka Bay, southwest of Hokkaido, Japan during early spring 1999. At the time of sampling, chlorophyll a concentration, bacteria, phycoerythrin rich-cyanobacteria, and HNF abundance were in the following ranges: 0.3–3.6 g l^–1, 2.5–5.6 × 10⁵ cells ml^–1, 0.6–1.2 × 10³ cells ml^–1, and 2.2–4.2 × 10³ cells ml^–1, respectively. Dissolved inorganic nitrogen, phosphate and silicate concentrations were in the ranges: 8.7–12.2 M, 0.9–2.0 M, and 21.6–25.5 M, respectively. Primary production ranged from 6.4 to 76.3 mg C m^–3 d^–1. Using water samples from regions of different productivity levels (in and outside bay), the bacteria - HNF relationship was uncoupled experimentally by the size-fractionation technique. Higher primary production (19.9 mg C m^–3 d^–1) in the bay supported higher bacterial growth rate (0.029 h^–1). However, outside the bay both primary production (6.4 mg C m^–3 d^–1) and bacterial growth rate (0.007 h^–1) were lower. The HNF growth rates and grazing rates were similar for both but by comparing both HNF grazing capacity and bacterial production, there was net decrease in bacterial abundance outside the bay and net increase inside the bay. The microbial parameters (rates and abundance) and the amount of carbon flow estimated through the phytoplankton – dissolved organic matter (DOM) – bacteria loop were different between the coastal station and the open ocean station. However HNF grazing and growth rates was similar for both stations.     

Removal of bacteria and nutrient dynamics within the coral reef framework of Curaçao (Netherlands Antilles)

The authors studied removal rates of bacteria and the regeneration of inorganic nutrients in coral reef cavities in the reef slope of Curaçao, Netherlands Antilles. We found that in cavities the hard substratum surface area (=ca 68% of cavity surface area) is 65% covered with sessile filter feeders. The cryptic cavity surface area exceeds the projected surface area of the reef by 1.5–8 times. Consequently, the organisms living in these cryptic habitats have potentially a large impact on pico- and nano-plankton densities and are important in reef water nutrient dynamics. We closed cavities (±70 l volume, 15 m depth) in seven experiments to study changes in bacterial densities and dissolved inorganic nutrients (DIN, DIP, and silicate) over time. Water samples were taken from the middle of the cavity at 5-min intervals, for 30 min, and analyzed for heterotrophic bacterial abundance and nutrient concentrations. After closure, bacterial abundance dropped rapidly. Of the initial bacterial concentration in the cavities, 50–60% had disappeared after 30 min, an average disappearance rate of 1.43×10⁴ bacteria ml^–1 min^–1 (0.62 mg C l^–1 d^–1; or 30.1 mg C m^–2 cavity surface area d^–1). NO_x concentrations increased significantly during the time of closure. Efflux rates varied between 1.02–9.77 mmol m^–2 cavity surface area d^–1. NH₄⁺ and PO₄^3– concentrations were variable and did not show a consistent change over time in the experiments. Comparison of bacterial organic nitrogen disappearance rates and DIN (NO_x+NH₄⁺) release rates suggests that on average only 30–40% of additional sources of N besides bacteria were required to balance the nitrogen budget. This highlights the importance of heterotrophic bacterioplankton as food for cryptic filter feeders on coral reefs. Silicate concentrations significantly decreased after closure with 0.50 mmol m^–2 cavity surface area d^–1, suggesting the net deposition of SiO₄^2– in spicules of cryptic filter feeding sponges. We conclude that coral reef cavities are a major sink for heterotrophic bacteria, a sink for dissolved silicon (DSi), and a source for NO_x. That reef cavities are a source for NO_x suggests strong remineralization and nitrification in cavities with a potential role for sponge-symbiotic microbial nitrification.Communicated by K.S. Sealey     

Microbiological characteristics and sanitary status of Lake Peipsi-Pihkva and its inflows in 1980s

In 1980, 1982 and 1984–1987 the total count of bacteria (TC), the number of saprophytic bacteria (plate count, PC), total Coliforms and Enterococci were determined at 22 to 30 sampling sites in the pelagial of Lake Peipsi-Pihkva. Bacterioplankton production was investigated seasonally at two locations from May to November 1985–1987. Eleven inflows and the outflowing River Narva were studied three times in the vegetation period 1985–1987 and in winter 1987.The average TC was highest in L. Pihkva (4.3 × 10⁶ cells ml^–1), lower in L. Lämmijärv (3.9 × 10⁶ cells ml^–1) and the lowest in the northern part of L. Peipsi (2.2 × 10⁶ cells ml^–1). According to these data, L. Pihkva and L. Lämmijärv were similar to typical Estonian eutrophic lakes. L. Peipsi had mesotrophic features with a tendency to eutrophy in its southern part.The numbers of saprophytic bacteria (PC) in L. Pihkva and L. Lämmijärv fluctuated from 110 to 360 cells ml^–1, in L. Peipsi from 98 to 290 cells ml^–1, and up to 5400 and 5900 cells ml^–1 in the mouths of the Rivers Velikaja and Suur Emajõgi, respectively. The average production value per vegetation period was 37.9 g C m^–2.The numbers of Coliforms and Enterococci indicated that the pelagial was in a good sanitary state. Enterococci and high total Coliform numbers (up to 1000 per 100 ml) were determined at the mouths of the Rivers Suur Emajõgi and Velikaja.In comparison with the early 1960s, 1.5–2.0-fold increase in the total amount of bacteria in the 1980s was revealed in the southern more eutrophic regions of L. Peipsi-Pihkva where the fluctuation of parameters was greater.     

New and Fast Method To Quantify Respiration Rates of Bacterial and Plankton Communities in Freshwater Ecosystems by Using Optical Oxygen Sensor Spots

A new method of respiration rate measurement based on oxygen luminescence quenching in sensor spots was evaluated for the first time for aquatic bacterial communities. The commonly used Winkler and Clark electrode methods to quantify oxygen concentration both require long incubation times, and the latter additionally causes signal drift due to oxygen consumption at the cathode. The sensor spots proved to be advantageous over those methods in terms of precise and quick oxygen measurements in natural bacterial communities, guaranteeing a respiration rate estimate during a time interval short enough to neglect variations in organism composition, abundance, and activity. Furthermore, no signal drift occurs during measurements, and respiration rate measurements are reliable even at low temperatures and low oxygen consumption rates. Both a natural bacterioplankton sample and a bacterial isolate from a eutrophic river were evaluated in order to optimize the new method for aquatic microorganisms. A minimum abundance of 2.2 × 10⁶ respiring cells ml⁻¹ of a bacterial isolate was sufficient to obtain a distinct oxygen depletion signal within 20 min at 20°C with the new oxygen sensor spot method. Thus, a culture of a bacterial isolate from a eutrophic river (OW 144; 20 × 10⁶ respiring bacteria ml⁻¹) decreased the oxygen saturation about 8% within 20 min. The natural bacterioplankton sample respired 2.8% from initially 94% oxygen-saturated water in 30 min. During the growth season in 2005, the planktonic community of a eutrophic river consumed between 0.7 and 15.6 μmol O₂ liter⁻¹ h⁻¹. The contribution of bacterial respiration to the total plankton community oxygen consumption varied seasonally between 11 and 100%.