Population dynamics of bacteria in Arctic sea ice

The dynamics of bacterial populations in annual sea ice were measured throughout the vernal bloom of ice algae near Resolute in the Canadian Arctic. The maximum concentration of bacteria was 6.0·10¹¹ cells·m^–2 (about 2.0·10¹⁰ cells·l^–1) and average cell volume was 0.473 m³ in the lower 4 cm of the ice sheet. On average, 37% of the bacteria were epiphytic and were most commonly attached (70%) to the dominant alga,Nitzschia frigida (58% of total algal numbers). Bacterial population dynamics appeared exponential, and specific growth rates were higher in the early season (0.058 day^–1), when algal biomass was increasing, than in the later season (0.0247 day^–1), when algal biomass was declining. The proportion of epiphytes and the average number of epiphytes per alga increased significantly (P<0.05) through the course of the algal bloom. The net production of bacteria was 67.1 mgC·m^–2 throughout the algal bloom period, of which 45.5 mgC·m^–2 occurred during the phase of declining algal biomass. Net algal production was 1942 mgC·m^–2. Sea ice bacteria (both arctic and antarctic) are more abundant than expected on the basis of relationships between bacterioplankton and chlorophyll concentrations in temperate waters, but ice bacteria biomass and net production are nonetheless small compared with the ice algal blooms that presumably support them.     

Degradation of surface-water dissolved organic matter: influences of DOM chemical characteristics and microbial populations

The degree to which biodegradation of dissolved organic matter (DOM) depends on microbial community structure and source remains unknown. In this study, we concentrated the microbial biomass from two streams in northern Michigan and a dystrophic bog lake in northern Wisconsin with varying initial DOM concentration (6.7–78.8 mg C l^–1) and DOM chemical characteristics (e.g. DOM average molecular weights from 808–1887 Da). Each of the three microbial inocula was added to each of the three DOM sources at in situ population levels for a total of nine treatments. Changes in DOM concentration and bacterial productivity, along with chemical characteristics, were examined over 308 h. The [³H]-leucine incorporation method was used to measure microbial production. In two of three sampling sites, bacterial communities were most productive when metabolizing DOM in their native waters. A variable peak in productivity was seen between 16–48 h after inoculation, followed by a drop in productivity in most treatments, with periods of DOM production most likely due to microbial turnover. These data suggest that microbial communities are better able to degrade the DOM of their native habitats, suggesting that biodegradation of DOM is influenced by source-specific microbial species and DOM chemical characteristics.     

Bacterial production in a mesohumic lake estimated from [14C]leucine incorporation rate

Incorporation of [¹⁴C]leucine into proteins of bacteria was studied in a temperate mesohumic lake. The maximum incorporation of [¹⁴C] leucine was reached at a concentration of 30 nm determined in dilution cultures. Growth experiments were used to estimate factors for converting leucine incorporation to bacterial cell numbers or biomass. The initially high conversion factors calculated by the derivative method decreased to lower values after the bacteria started to grow. Average conversion factors were 7.09 × 10¹⁶ cells mol^–1 and 7.71 × 10¹⁵ m³ mol^–1, if the high initial values were excluded. Using the cumulative method, the average conversion factor was 5.38 × 10¹⁵ m^–3 mol^–1 I . The empirically measured factor converting bacterial biomass to carbon was 0.36 pg C m^–3 or 33.1 fg C cell^–1. Bacterial production was highest during the growing season, ranging between 1.8 and 13.2 g C liter^–1 day^–1, and lowest in winter, at 0.2–2.9 g C liter^–1 day^–1. Bacterial production showed clear response to changes in the phytoplankton production, which indicates that photosynthetically produced dissolved compounds were used by bacteria. In the epilimnion bacterial production was, on average, 19–33% of primary production. Assuming 50% growth efficiency for bacteria, the allochthonous organic carbon could have also been an additional energy and carbon source for bacteria, especially in autumn and winter. In winter, a strong relationship was found between temperature and bacterial production. The measuring of [¹⁴C]leucine incorporation proved to be a simple and useful method for estimating bacterial production in humic water. However, an appropriate amount of [¹⁴C]leucine has to be used to ensure the maximum uptake of label and to minimize isotope dilution.     

Limnology of Crater Lakes in Los Tuxtlas,Mexico

Investigations on the abundance, biomass and position of heterotrophic flagellates (HF) in the benthic microbial food web of a melt water stream on King George Island, Antarctic Peninsula, were undertaken during the Antarctic summer from 23rd December 1997 until 13th March 1998. Abundance and biomass of potential HF resources (picophotoautotrophic and non-photoautotrophic bacteria) as well as potential predators on HF (ciliates and meiofauna) were also investigated. HF abundance ranged from approximately 9 × 10³ to 81 × 10³ cells cm^–3, values which fall into the same range as those found in lower latitudes. Numerically important benthic HF were euglenids, kinetoplastids, thaumatomastigids and especially chrysomonads. Most species identified have been shown to have a worldwide distribution. Abundance of the benthic ciliates ranged from 27 to 950 cells cm^–3. Mean bacterial abundance was 1.9 × 10⁷ and 5.2 × 10⁸ cells cm^–3 for picophotoautotrophic and non-photoautotrophic benthos, respectively. The well-developed microbial community was able to support the large number of nematods, gastotrichs, tardigrads and rotifers with abundances reaching more than 1000 individuals cm^–3. The largest portion of heterotrophic biomass was formed by the meiofauna with a mean of 63 g C cm^–3, followed by that of the heterotrophic bacteria with 4.80 g C cm^–3. Picophotoautotrophic bacteria contributed a mean of 1.37 g C cm^–3. HF and ciliates mean biomass was 0.61 and 1.99 g C cm^–3, respectively, with the HF biomass comprising between <10 and 70% of the total protozoan biomass. The data obtained in this study identify the melt water stream as a hot-spot of heterotrophic microbial and meiofaunal activity during the austral summer. The HF in the melt water stream formed a diverse group in terms of taxa and potential feeding types. Chrysomonads, kinetoplastids, euglenids and thaumatomastigida were the most abundant taxa. A classification into feeding types identified an average of 34% of the total HF as bacterivorous while all others were able to utilise other, larger organisms as resources. Potential trophic interactions between HF and bacteria and higher trophic levels are discussed.     

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.     

The Soil Flagellate Heteromita globosa Accelerates Bacterial Degradation of Alkylbenzenes through Grazing and Acetate Excretion in Batch Culture

The impact of grazing by soil flagellates Heteromita globosa on aerobic biodegradation of benzene by Pseudomonas strain PS+ was examined in batch culture. Growth of H. globosa on these bacteria obeyed Monod kinetics (_max, 0.17 ± 0.03 h^–1; K_s, 1.1 ± 0.2 × 10⁷ bacteria mL^–1) and was optimal at a bacteria/ flagellate ratio of 2000. Carbon mass balance showed that 5.2% of total [ring-U-¹⁴C]benzene fed to bacteria was subsequently incorporated into flagellate biomass. Growth-inhibiting concentrations (IC₅₀) of alkylbenzenes (benzene, toluene, ethylbenzene) were inversely related with their octanol/ water partitioning coefficients, and benzene was least toxic for bacteria and flagellates with IC₅₀ values of 4392 (± 167) M and 2770 (± 653) M, respectively. The first-order rate constant for benzene degradation (k₁, 0.48 ± 0.12 day^–1) was unaffected by the presence or absence of flagellates in cultures. However, the rate of benzene degradation by individual bacteria averaged three times higher in the presence of flagellates (0.73 ± 0.13 fmol cell^–1 h^–1) than in their absence (0.26 ± 0.03 fmol cell^–1 h^–1). Benzene degradation also coincided with higher levels of dissolved oxygen and a higher rate of nitrate reduction in the presence of flagellates (p < 0.02). Grazing by flagellates may have increased the availability of dissolved oxygen to a smaller surviving population of bacteria engaged in the aerobic reactions initiating benzene degradation. In addition, flagellates may also have increased the rate of nitrate reduction through the excretion of acetate as an additional electron donor for these bacteria. Indeed, acetate was shown to progressively accumulate in cultures where flagellates grazed on heat-killed bacteria. This study provided evidence that grazing flagellates stimulate bacterial degradation of alkylbenzenes and provide a link for carbon cycling to consumers at higher trophic levels. This may have important implications for bioremediation processes.     

Bacterial productivity in ponds used for culture of penaeid prawns

The quantitative role of bacteria in the carbon cycle of ponds used for culture of penaeid prawns has been studied. Bacterial biomass was measured using epifluorescence microscopy and muramic acid determinations. Bacterial growth rates were estimated from the rate of tritiated thymidine incorporation into DNA. In the water column, bacterial numbers ranged from 8.3×10⁹ 1^–1 to 2.57×10¹⁰ 1^–1 and production ranged from 0.43 to 2.10 mg Cl^–1 d^–1. In the 0–10 mm zone in sediments, bacterial biomass was 1.4 to 5.8 g C m^–2 and production was 250 to 500 mg C m^–2 d^–1. The results suggested that most organic matter being supplied to the ponds as feed for the prawns was actually being utilized by the bacteria. When the density of meiofauna increased after chicken manure was added, bacterial biomass decreased and growth rates increased.     

Dissolved organic matter under native Cerrado and Pinus caribaea plantations in the Brazilian savanna

The transformation of native Cerrado into Pinus caribaea Morelet plantations changes the DOM dynamics including changed rates of mineralisation, denitrification, and C export to the groundwater. To examine the differences in quantity, temporal dynamics, and quality of DOM between Cerrado and Pine plantations we collected rainfall, throughfall, stemflow, litter leachate (under pine only) and soil solution at 15, 80, and 200 cm depth in weekly intervals during the rainy seasons 1997/98 and 1998/99. We determined total dissolved organic carbon (DOC) concentrations and assessed DOM quality by separating hydrophilic and hydrophobic fractions and by NMR analysis of organic layer extracts. The rainfall had a mean DOC concentration of 2.6 mg L^–1. The mean concentrations of DOC in the throughfall of the pine plantations (5.0–10.5 mg L^–1) were significantly above those of Cerrado (2.6–4.9 mg L^–1). During the first part of the rainy seasons (October–December), the concentrations of DOC in the soil solution (15–200 cm depth) under Cerrado and pine did not differ significantly. During the second part of both rainy seasons (January–April), the concentrations of DOC in the soil solution under Cerrado (4.4–5.1 mg L^–1) exceeded those under PI (1.4–2.7 mg L^–1). Possible explanations of the latter include higher DOM input into the Cerrado soil and a stronger retention and/or faster mineralisation of the pine DOM than of the Cerrado DOM in the mineral soil. As the structural composition of DOM extracted from the organic layer under Cerrado and pine did not differ significantly, faster mineralisation was the most likely explanation for partly lower DOC concentrations in the soil solution under pine than under Cerrado. This assumption was supported by increasing contributions of hydrophobic DOM to total DOM with increasing depth under pine while, under Cerrado, the DOM composition did not change with depth. The reason for DOM mineralisation under pine was probably the higher N availability because total N concentrations were 11–23 times higher under pine than under Cerrado.     

Factors affecting the horizontal patchiness of coastal Antarctic seawater bacteria

Summary Previous Antarctic studies have pointed out the ecological importance of ornithogenic soils. However, few data exist to determine the impact of the bird's manuring on surrounding seawater microbial populations. In order to evaluate the influence of birds manuring, the relationships between the spatial distributions of seawater bacterial microflora and some related biological (chlorophyll pigments) and physicochemical (seston, NH₄ ⁺ & NO₃ ^–) parameters were studied during the Antarctic summer 1988 in the Terre Adelie land area. The clearly decreasing gradient from the shore towards the open sea previously reported for bacterial microflora (from 10⁴ to 1 CFU ml^–1 for heterotrophic bacteria and from 10⁵ to 5.010³ cells ml^–1 for total bacteria) was also observed for organic and mineral nutrients (from 1.09 mg Cl^–1 to 0.1 mg Cl^–1 for POC and from 196 to 17 mole l^–1 for NH ₄ ⁺ ) but not for chlorophyll pigments. The absence of any observable phytoplankton enrichment in the coastal area suggests a direct interaction between the birds manuring and the bacterial seawater microflora.     

Seasonal and diel relationships between the isotopic compositions of dissolved and particulate organic matter in freshwater ecosystems

A study of the isotopic composition of organic matter was conducted in a freshwater marsh over seasonal and diel time scales to determine the sources of dissolved organic matter (DOM) and the processes leading to its formation. Bulk C and N isotopic compositions of the bacterial fraction (0.2–0.7 m) and particulate organic matter (POM; 0.7–10 m) were compared on a seasonal basis with the change in ¹³C of DOM. The bulk isotopic data support the idea that DOM was, in part, derived from the breakdown of larger organic matter fractions. The bacterial fraction and POM were compositionally similar throughout the year, based on a comparison of the ¹³C of individual amino acids in each fraction. Annual variation in the ¹³C of amino acids in DOM was greater relative to the variation in larger fractions indicating that microbial reworking was an important factor determining the proteinaceous component of DOM. The ¹³C enrichment of serine and leucine in each organic matter fraction suggested microbial reworking was an important factor determining organic matter composition during the most productive times of year. Changes in the bulk ¹³C of DOM were more significant over daily, relative to seasonal, time scales where values ranged by 6 and followed changes in chlorophyll a concentrations. Although bulk ¹³C values for POM ranged only from –29 to –28 during the same diel period, the ¹³C of alanine in POM ranged from –30 to –22. Alanine is directly synthesized from pyruvate and is therefore a good metabolic indicator. The ¹³C of individual amino acids in DOM revealed the diel change in the importance of autotrophic versus heterotrophic activity in influencing DOM composition. Diel changes in the ¹³C of phenylalanine, synthesized by common pathways in phytoplankton and bacteria, were similar in both DOM and POM. The diel change in ¹³C of isoleucine and valine, synthesized through different pathways in phytoplankton and bacteria, were distinctly different in DOM versus POM. This disparity indicated a decoupling of the POM and DOM pools, which suggests a greater source of bacterial-derived organic matter at night. The results of this study demonstrate the use of the isotopic composition of individual amino acids in determining the importance of microbial reworking and autotrophic versus heterotrophic contributions to DOM over both diel and seasonal time scales.     

Biomass change in an Atlantic tropical moist forest: the ENSO effect in permanent sample plots over a 22-year period

There are a number of controversies surrounding both biomass estimation and carbon balance in tropical forests. Here we use long-term (from 1978 through 2000) data from five 0.5-ha permanent sample plots (PSPs) within a large tract of relatively undisturbed Atlantic moist forest in southeastern Brazil to quantify the biomass increment (M_I), and change in total stand biomass (M_stand), from mortality, recruitment, and growth data for trees 10 cm diameter at breast height (DBH). Despite receiving an average of only 1,200 mm annual precipitation, total forests biomass (334.5±11.3 Mg ha^–1) was comparable to moist tropical forests with much greater precipitation. Over this relatively long-term study, forest biomass experienced rapid declines associated with El Niño events, followed by gradual biomass accumulation. Over short time intervals that overlook extreme events, these dynamics can be misinterpreted as net biomass accumulation. However for the 22 years of this study, there was a small reduction in forest biomass, averaging –1.2 Mg ha^–1 year^–1 (±3.1). Strong climatic disturbances can severely reduce forest biomass, and if the frequency and intensity of these events increases beyond historical averages, these changing disturbance regimes have the capacity to significantly reduce forest biomass, resulting in a net source of carbon to the atmosphere.     

Heterotrophic bacterial production in Lake Xolotlán (Managua) during 1988–1989

The biomass and production (thymidine incorporation) of heterotrophic bacterioplankton has been assessed from July, 1988, to October, 1989. in Lake Xolotlán, Nicaraqua. Bacterial abundance was high, 2–3.10¹⁰ cells.l^–1, and bacterial biomass averaged ca. 0.75 mg C.l^–1, or roughly 20% of the partivculate organic carbon. Bactrial production averaged between 3.5–5 g C.l^–1.h^–1 and on a areal basis was 650–959 mg C.m^–2.d^–1 or 13–20% ofthe primary production. Although bacterial production (volumetric basis) was typical for eutrophic lakeks, the bacterial specific growth rate was low, the bacteial population doubling time was ca. 1 week, perhaps indicating that there was a low grazing pressure on the bacteria.     

Integrated cultivation of salmonids and seaweeds in open systems

Bacterial abundance and production in a vertical profile in Lake Kariba (17dgS), Zimbabwe, were affected by solar irradiance. At the surface, 1.87 × 10⁹ bacteria 1^–1 were found and abundance peaked at 10 m (2.5 × 10⁹ bacteria l^-1), then decreasing with depth. Bacterial reproduction at the surface(0.145 µg C1^–1 h^–1) was nearly four times less than the production at 10 m although bacterial numbers were only 26% less. Thus, bacterial production per cell was lower at the surface than deeper down, suggesting that bacterial production is inhibited at the surface.Bacterial production in GF/F filtered lake water in Whirl Pack bags showed an exponential decrease down to 3 m depth. The inhibition was well in accordance with light extinction in the UV region. Phosphatase activity was low in light exposed bags compared to dark, indicating photolysis of extracellular enzymes, or phototransformation of recalcitrant DOM, which substitutes enzyme activity. Hypolimnetic enzyme activity was less affected by solar light than epilimnetic.     

Thymidine and leucine incorporation in soil bacteria with different cell size

Thymidine and leucine incorporation into macromolecules of soil bacteria extracted by homogenization-centrifugation were measured after size-fractionation of the bacterial suspension through different sized filters (1.0, 0.8, 0.6, 0.4 m). The specific thymidine incorporation rate was highest for the unfiltered and 1.0 m filtered suspensions (approximately 10 × 10^–21 mol thymidine bacteria^–1 h^–1), but decreased to 1.39 × 10^–21 mol bacteria^–1 h^–1 for bacteria passing the 0.4 m filter. The proportion of culturable bacteria (percent colony forming units/acridine orange direct counts) also decreased with bacterial cell size from 5.0% for the unfiltered bacterial suspension to 0.8% in the 0.4 µm filtrate. A strong linear correlation (r ² = 0.995) was found between the specific thymidine incorporation rate and the proportion of culturable bacteria. Leucine incorporation gave similar results to the thymidine incorporation. No effects of cell size on the degree of isotope dilution or unspecific labeling of other macromolecules were found either for the thymidine or the leucine incorporation technique. These data indicate that small bacteria, although more numerous than larger ones, not only constitute a smaller proportion of the soil bacterial biomass than larger bacteria, but also contribute to a lesser degree to carbon transformations in soil.     

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.     

Heterotrophic bacterial activity in Roskilde Fjord sediment during an autumn sedimentation peak

Total oxygen uptake, bacterial oxygen uptake, total bacterial biomass and active bacterial biomass were determined at the sediment-water interface at two stations in the brackish Roskilde Fjord between September and December in 1986 before, during and after sedimentation of a phytoplankton bloom. Bacterial oxygen consumption was separated from total oxygen consumption by addition of cycloheximide. The fractional and the absolute bacterial oxygen uptake were greatest at the most eutrophic station, where total oxygen uptake was 870–1740 mg O₂ m^–2 d^–1 and the bacterial oxygen uptake was 232–870 mg O₂ m^–2 d^–1. At the less eutrophic station, total oxygen uptake was 725–1740 mg O₂ m^–2 d^–1. and bacterial oxygen uptake was 200–550 mg O₂ m^–2 d^–1.Active bacterial biomass was separated from total bacterial biomass by addition of the terminal electron acceptor INT-formazan. The active bacterial biomass was 70–120 µg C mg^–1 ww of sediment at the most eutrophic station and 50–90 µg C g^–1 ww of sediment at the other station. Differences in capacity of bacterial oxygen uptake between the two stations correlated to the active bacterial biomass. The non-temperature dependent bacterial oxygen uptake correlated with the sedimentation rate.     

Responses of physiological groups of tropical heterotrophic bacteria to temperature and dissolved organic matter additions: food matters more than warming

Compared to higher latitudes, tropical heterotrophic bacteria may be less responsive to warming because of strong bottom-up control. In order to separate both drivers, we determined the growth responses of bacterial physiological groups to temperature after adding dissolved organic matter (DOM) from mangroves, seagrasses and glucose to natural seawater from the Great Barrier Reef. Low (LNA) and high (HNA) nucleic acid content, membrane-intact (Live) and membrane-damaged (Dead) plus actively respiring (CTC+) cells were monitored for 4 days. Specific growth rates of the whole community were significantly higher (1.9 day^-1) in the mangrove treatment relative to the rest (0.2–0.4 day^-1) at in situ temperature and their temperature dependence, estimated as activation energy, was also consistently higher. Strong bottom-up control was suggested in the other treatments. Cell size depended more on DOM than temperature. Mangrove DOM resulted in significantly higher contributions of Live, HNA and CTC+ cells to total abundance, while the seagrass leachate reduced Live cells below 50%. Warming significantly decreased Live and CTC+ cells contributions in most treatments. Our results suggest that only in the presence of highly labile compounds, such as mangroves DOM, can we anticipate increases in heterotrophic bacteria biomass in response to warming in tropical regions.     

Photosynthetic bacteria in meromictic lakes and stratified fjords of the Vestfold Hills,Antarctica

Thirteen meromictic lakes and two permanently stratified fjords in the Vestfold Hills, Antarctica, were surveyed in 1983 for photosynthetic bacteria. Burton Lake and Ellis Fjord were sampled throughout the year to determine seasonal variations. Physical and chemical parameters were recorded and related to the species present. The dominant species in waters with salinities of 100.7 g kg^–1 were Chlorobium vibrioforme and Chlorobium limicola with populations at the O₂–H₂S interface in the range 0.3 to 6.7 × 10⁶ ml^–1. Neither of these species was found at higher salinities. Thiocapsa roseopersicina and a Chromatium sp. were found in low numbers (< 10⁵ ml^–1) in most of the same waters as the Chlorobium spp. These bacterial phototrophs developed in a narrow band below the O₂–H₂S interface where both light and H₂S were available. Very low numbers (< 10² ml^–1) of Rhodopseudomonas palustris were found in both oxic and anoxic waters having salinity 148 g kg^–1. The dominance of the Chlorobium spp. is ascribed to their more efficient maintenance metabolism during the darkness, their faster growth at low light intensities (< 1 µE m^–2 s^–1) and the lack of selective filtering of incident light. The Chlorobium spp. grew well at –2 °C, but not –5°C in hypersaline waters. The concentration of H₂S had no apparent effect on the development of the bacterial flora. Viable cells were found to depths of 100 m in Ellis Fjord indicating that viability in total darkness could have been maintained for periods of the order of 1700 days.     

Effects of intensity and quality of light on phycocyanin production by a marine cyanobacterium Synechococcus sp. NKBG 042902

Among 150 strains, including marine cyanobacteria isolated from coastal areas of Japan and a freshwater cyanobacterium from the IAM collection, Spirulina platensis IAM M-135, the marine cyanobacterium Synechococcus sp. NKBG 042902 contained the highest amount of phycocyanin (102 mg/g dry cell weight). We have proposed that the cyanobacterium could be an alternative producer for phycocyanin. The effects of light intensity and light quality on the phycocyanin content in cells of Synechococcus sp. NKBG 042902 were investigated. When the cyanobacterium was cultured under illumination of 25 mol m^–2 s^–1 using a cool-white fluorescent lamp, the phycocyanin content was highest, and the phycocyanin and biomass productivities were 21 mg 1^–1 day^–1 and 100 mg 1^–1 day^–1 respectively. Red light was essential for phycocyanin production by this cyanobacterium. Phycocyanin and biomass production were carried out by the cyanobacterium cultures grown under only red light (peak wavelength at 660 nm) supplied from light-emitting diodes (LED). Maximum phycocyanin and biomass productivities were 24 mg 1^–1 day^–1 and 130 mg 1^–1 day^–1 when the light intensity of the LED was 55 mol m^–2 s^–1.     

Light-dependent phagotrophy in the freshwater mixotrophic chrysophyte Dinobryon cylindricum

The mixotrophic (bacterivorous), freshwater chrysophyte Dinobryon cylindricum was cultured under a variety of light regimes and in bacterized and axenic cultures to investigate the role of phototrophy and phagotrophy for the growth of this alga. D. cylindricum was found to be an obligate phototroph. The alga was unable to survive in continuous darkness even when cultures were supplemented with high concentrations of bacteria, and bacterivory ceased in cultures placed in the dark for a period longer than one day. Axenic growth of the alga was poor even in an optimal light regime. Live bacteria were required for sustained, vigorous growth of the alga in the light. Carbon (C), nitrogen (N), and phosphorus (P) budgets determined for the alga during growth in bacterized cultures indicated that bacterial biomass ingested by the alga may have contributed up to 25% of the organic carbon budget of the alga. Photosynthesis was the source of most (75%) of the organic carbon of the alga. D. cylindricum populations survived but did not grow when cultured in a continuous low light intensity (30 E m^–2 sec^–1), or in a light intensity of 150 E m^–2 sec^–1 for only two hours each day. Net efficiency of incorporation of bacterial C, N, and P into algal biomass under these two conditions was zero (i.e., no net algal population growth). We conclude that the primary function of bacterivorous behavior in D. cylindricum may be to provide essential growth factor(s) or major nutrients for photosynthetic growth, or to allow for the survival of individuals during periods of very low light intensity or short photoperiod. Offprint requests to: David A. Caron