A study of the diatom-dominated microplankton summer assemblages in coastal waters from Terre Adélie to the Mertz Glacier, East Antarctica (139°E–145°E)

In January 2004 the microplankton community from the coastal waters of Terre Adélie and Georges V Land (139°E–145°E) was studied. Results showed a diatom-dominated bloom with chlorophyll a levels averaging 0.64 μg l⁻¹ at 5 m depth (range 0.21–1.57 μg l⁻¹). Three geographic assemblages of diatoms were identified, based on principal diatom taxa abundances. The stratified waters near the Mertz Glacier presented highest phytoplankton biomasses (0.28–1.57 μg Chl a l⁻¹ at 5 m) and diatom abundances (6,507–70,274 cells l⁻¹ at 5 m), but low diversity, dominated by Fragilariopsis spp. Lower biomasses (0.38–0.94 μg Chl a l⁻¹ at 5 m) and abundances (394–9,058 cells l⁻¹ at 5 m) were observed in the mixed waters around the Astrolabe Glacier with a diverse diatom community characterised by larger species Corethron pennatum and Rhizosolenia spp. Finally an intermediate zone between them over the shallower shelf waters of the Adélie Bank represented by Chaetoceros criophilus, where biomasses (0.21–0.35 μg Chl a l⁻¹ at 5 m) and abundances (1,190–5,431 cells l⁻¹ at 5 m) were lowest, coinciding with the presence of abundant herbivorous zooplankton.     

Abundance and composition of the summer phytoplankton community along a transect from the Barguzin River to the central basin of Lake Baikal

The abundance and composition of phytoplankton were investigated at six stations along a transect from the Barguzin River inflow to the central basin of Lake Baikal in August 2002 to clarify the effect of the river inflow on the phytoplankton community in the lake. The water temperature in the epilimnion was high near the shore at Station 1 (17.3°C), probably due to the higher temperature of the river water, and gradually decreased offshore at Station 6 (14.5°C). Thermal stratification developed at Stations 2–6, and a thermocline was observed at a 17–22-m depth at Stations 2–4 and an 8–12-m depth at Stations 5 and 6. The concentrations of nitrogen and phosphorus nutrients in the epilimnion at all stations were <1.0 μmol N l⁻¹ and <0.16 μmol P l⁻¹, respectively. Relatively high concentrations of nutrients (0.56–7.38 μmol N l⁻¹ and 0.03–0.28 μmol P l⁻¹) were detected in the deeper parts of the euphotic zone. Silicate was not exhausted at all stations (>20 μmol Si l⁻¹). The chlorophyll a (chl. a) concentration was high (>10 μg l⁻¹) near the shore at Station 1 and low (<3 μg l⁻¹) at five other stations. The <2 μm fraction of chl. a in Stations 2–6 ranged between 0.80 and 1.85 μg l⁻¹, and its contribution to total chl. a was high (>60%). In this fraction, picocyanobacteria were abundant at all stations and ranged between 5 × 10⁴ and 5 × 10⁵ cells ml⁻¹. In contrast, chl. a in the >2 μm fraction varied significantly (0.14–11.17 μg l⁻¹), and the highest value was observed at Station 1. In this fraction, the dominant phytoplankton was Aulacoseira and centric diatoms at Station 1 and Cryptomonas, Ankistrodesmus, Asterionella, and Nitzschia at Stations 2–6. The present study demonstrated the dominance of picophytoplankton in the pelagic zone, while higher abundance of phytoplankton dominated by diatoms was observed in the shallower littoral zone. These larger phytoplankters in the littoral zone probably depend on nutrients from the Barguzin River.     

Drift-ice and under-ice water communities in the Gulf of Bothnia (Baltic Sea)

The algal, protozoan and metazoan communities within different drift-ice types (newly formed, pancake and rafted ice) and in under-ice water were studied in the Gulf of Bothnia in March 2006. In ice, diatoms together with unidentified flagellates dominated the algal biomass (226 ± 154 μg ww l⁻¹) and rotifers the metazoan and protozoan biomass (32 ± 25 μg ww l⁻¹). The under-ice water communities were dominated by flagellates and ciliates, which resulted in lower biomasses (97 ± 25 and 21 ± 14 μg ww l⁻¹, respectively). The under-ice water and newly formed ice separated from all other samples to their own cluster in hierarchical cluster analysis. The most important discriminating factors, according to discriminant analysis, were chlorophyll-a, phosphate and silicate. The under-ice water/newly formed ice cluster was characterized by high nutrient and low chlorophyll-a values, while the opposite held true for the ice cluster. Increasing trends in chlorophyll-a concentration and biomass were observed with increasing ice thickness. Within the thick ice columns (>40 cm), the highest chlorophyll-a concentrations (6.6–22.2 μg l⁻¹) were in the bottom layers indicating photoacclimation of the sympagic community. The ice algal biomass showed additional peaks in the centric diatom-dominated surface layers coinciding with the highest photosynthetic efficiencies [0.019–0.032 μg C (μg Chl-a ⁻¹ h⁻¹) (μE m⁻² s⁻¹)⁻¹] and maximum photosynthetic capacities [0.43-1.29 μg C (μg Chl-a ⁻¹ h⁻¹)]. Rafting and snow-ice formation, determined from thin sections and stable oxygen isotopic composition, strongly influenced the physical, chemical and biological properties of the ice. Snow-ice formation provided the surface layers with nutrients and possibly habitable space, which seemed to have favored centric diatoms in our study.     

Winter-time ecology in the Bothnian Bay, Baltic Sea: nutrients and algae in fast ice

Landfast ice algal communities were studied in the strongly riverine-influenced northernmost part of the Baltic Sea, the Bothnian Bay, during the winter-spring transition of 2004. The under-ice river plume, detected by its low salinity and elevated nutrient concentrations, was observed only at the station closest to the river mouth. The bottommost ice layer at this station was formed from the plume water (brine volume 0.71%). This was reflected by the low flagellate-dominated (93%) algal biomass in the bottom layer, which was one-fifth of the diatom-dominated (74%) surface-layer biomass of 88 μg C l⁻¹. Our results indicate that habitable space plays a controlling role for ice algae in the Bothnian Bay fast ice. Similarly to the water column in the Bothnian Bay, average dissolved inorganic N:P-ratios in the ice were high, varying between 12 and 265. The integrated chlorophyll a (0.1–2.2 mg m⁻²) and algal biomass in the ice (1–31 mg C m⁻²) correlated significantly (Spearman ρ = 0.79), with the highest values being measured close to the river mouth in March and during the melt season in April. Flagellates <20 μm generally dominated in both the ice and water columns in February–March. In April the main ice-algal biomass was composed of Melosira arctica and unidentified pennate diatoms, while in the water column Achnanthes taeniata, Scrippsiella hangoei and flagellates dominated. The photosynthetic efficiency (0.003–0.013 (μg C [μg chl a ⁻¹] h⁻¹)(μE m⁻²s⁻¹)⁻¹) and maximum capacity (0.18–1.11 μg C [μg chl a ⁻¹] h⁻¹) could not always be linked to the algal composition, but in the case of a clear diatom dominance, pennate species showed to be more dark-adapted than centric diatoms.     

Eutrophication of moderately deep Dutch lakes during the past century: flaws in the expectations of water management?

We studied the trophic development of the past 30–100 years in eight moderately deep Dutch lakes based on their sedimentary fossil diatom assemblages. The dominant diatoms indicating meso- to eutrophic conditions were Aulacoseira subarctica, Cyclotella ocellata, C. cyclopuncta, C. meneghiniana, Puncticulata bodanica, Aulacoseira granulata, Cyclostephanos dubius, C. invisitatus, Stephanodiscus hantzschii, S. medius, and S. parvus. Ordination of diatom data separated the lakes into four groups according to their total phosphorus concentrations (TP), water supply, water management, and origin. The first group consists of dike-breach lakes, which were in stable eutrophic to hypertrophic conditions throughout the past century with diatom-inferred TP (DI-TP) concentrations of between 70 and 300 μg l⁻¹. The main factors influencing these dike-breach lakes are river management, ground water supply of riverine origin, and local land use. The second group are artificial lakes of fluctuating oligo- to mesotrophic conditions and DI-TP concentrations of 10–30 μg l⁻¹. Only one of the artificial lakes showed a DI-TP increase due to changes in catchment agricultural practice. A third group includes an artificial moat and an inland dike-breach lake with DI-TP concentrations of 50–100 μg l⁻¹. The fourth group contains an individual dike-breach lake with stable mesotrophic conditions of 50 μg l⁻¹ throughout the past century. Rather than showing a regional pattern, the studied lakes behave very individualistically with regard to their trophic history, reflecting changes in the local hydrology and in their nutrient sources.     

Phytoplankton biomass in the sub-Antarctic area of the Straits of Magellan (53°S), Chile during spring-summer 1997/1998

In order to provide a better understanding of the dynamics of phytoplankton in the coastal regions of high latitudes, a study was carried out to estimate the dynamics of carbon biomass of autotrophic and heterotrophic algal groups over the austral spring-summer 1997/1998 period. At a fixed station located in the central basin (Paso Ancho) of the Straits of Magellan (53°S), surface water samples were collected at least once a week from September 1997 (early spring) to March 1998 (late summer). Quantitative analysis of biomass of phytoplankton was estimated from geometric volumes, using non-linear equations, and converted to biomass. The pattern of chlorophyll a showed a strong temporal variability, with maximum values (mean 2.8 mg m⁻³) at the austral spring phytoplankton increase or bloom (October/November) and minimum values during early spring (September: <0.5 mg m⁻³) and summer (January/March: 0.5–1.0 mg m⁻³). During the spring bloom, diatoms made up to 90% of the total phytoplankton carbon (0.01–189 μg l⁻¹), followed by a maximum of thecate dinoflagellates (0.08–34 μg l⁻¹), and sporadic high biomass of phytoflagellates during summer. Heterotrophic algal groups such as Gymnodinium and Gyrodinium spp. dominated (70%, in the 5- to 25-μm size range) shortly before the main diatom bloom, and small peaks were observed within spring and early summer periods (0–0.4 μg l⁻¹). Phytoflagellates dominated earlier (spring) with higher carbon biomass (8 μg l⁻¹) and post-bloom periods (summer) when carbon biomass ranged between 1 and 4 μg l⁻¹. Accepted: 6 September 2000     

Bioremoval of hexavalent chromium from water by a salt tolerant bacterium, Exiguobacterium sp. GS1

Pollution of terrestrial surfaces and aquatic systems by hexavalent chromium, Cr(VI), is a worldwide public health problem. A chromium resistant bacterial isolate identified as Exiguobacterium sp. GS1 by 16S rRNA gene sequencing displayed high rate of removal of Cr(VI) from water. Exiguobacterium sp. GS1 is 99% identical to Exiguobacterium acetylicum. The isolate significantly removed Cr(VI) at both high and low concentrations (1–200 μg mL⁻¹) within 12 h. The Michaelis–Menten K _m and V _max for Cr(VI) bioremoval were calculated to be 141.92 μg mL⁻¹ and 13.22 μg mL⁻¹ h⁻¹, respectively. Growth of Exiguobacterium sp. GS1 was indifferent at 1–75 μg mL⁻¹ Cr(VI) in 12 h. At initial concentration of 8,000 μg L⁻¹, Exiguobacterium sp. GS1 displayed rapid bioremoval of Cr(VI) with over 50% bioremoval in 3 h and 91% bioremoval in 8 h. Kinetic analysis of Cr(VI) bioremoval rate revealed zero-order in 8 h. Exiguobacterium sp. GS1 grew and significantly reduced Cr(VI) in cultures containing 1–9% salt indicating high salt tolerance. Similarly the isolate substantially reduced Cr(VI) over a wide range of temperature (18–45  °C) and initial pH (6.0–9.0). The T _opt and initial pH_opt were 35–40  °C and 7–8, respectively. Exiguobacterium sp. GS1 displayed a great potential for bioremediation of Cr(VI) in diverse complex environments.     

Antimicrobial activity of crude extracts from mangrove fungal endophytes

The aim of this work was to select endophytic fungi from mangrove plants that produced antimicrobial substances. Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) or minimal fungicidal concentrations (MFC) of crude extracts from 150 isolates were determined against potential human pathogens by a colorimetric microdilution method. Ninety-two isolates (61.3%) produced inhibitory compounds. Most of the extracts (28–32%) inhibited Staphylococcus aureus (MIC/MBC 4–200/64–200 μg ml⁻¹). Only two extracts inhibited Pseudomonas aeruginosa (MIC/MBC 200/>200 μg ml⁻¹). 25.5 and 11.7% inhibited Microsporum gypseum and Cryptococcus neoformans (MIC/MFC 4–200/8–200 μg ml⁻¹ and 8–200/8–200 μg ml⁻¹, respectively), while 7.5% were active against Candida albicans (MIC/MFC 32–200/32–200 μg ml⁻¹). None of the extracts inhibited Escherichia coli. The most active fungal extracts were from six genera, Acremonium, Diaporthe, Hypoxylon, Pestalotiopsis, Phomopsis, and Xylaria as identified using morphological and molecular methods. Phomopsis sp. MA194 (GU592007, GU592018) isolated from Rhizophora apiculata showed the broadest antimicrobial spectrum with low MIC values of 8–32 μg ml⁻¹against Gram-positive bacteria, yeasts and M. gypseum. It was concluded that endophytic fungi from mangrove plants are diverse, many produce compounds with antimicrobial activity and could be suitable sources of new antimicrobial natural products.     

Seasonal variations of microphytoplankton assemblages and environmental variables in the coastal zone of Bozcaada Island in the Aegean Sea (NE Mediterranean Sea)

Seasonal changes in the microphytoplankton assemblages were examined in the coastal zone of Bozcaada Island with regard to some major physical and chemical variables. Samples were collected from May 2000 to December 2001 at four stations. A total of 108 dinoflagellates, 102 diatoms, 1 chrysophycean, 3 dictyochophycean, and 1 prasinophycean species were identified and quantified during the study period. Diatoms and dinoflagellates were the most important in terms of species number and abundance. The maximum values of total microphytoplankton were observed at 0.5 m depth (46.2 × 10³ cells l⁻¹ at st. 3) in May as this was the month when the diatom Pseudo-nitzschia pungens bloomed. Chlorophyll (chl) a concentration ranged between 0.08 (August) and 0.78 μg l⁻¹ (February). May was another important month in which chlorophyll a increased (0.41–0.47 μg l⁻¹). Species diversity values (H′_log2) ranged from 1.66 bits (June, 20 m) to 4.11 bits (November, 0.5 m). The increase was attributed to a more balanced distribution of abundance among species. The amounts of nitrate + nitrite (0.6−3.7 μg-at N l⁻¹), phosphate (0.2−0.6 μg-at P l⁻¹) and silicate (0.7−2.5 μg-at Si l⁻¹) were recorded on each sampling occasion. Nutrient concentrations and chl a values of the research area were found to be poorer than those of the many other coastal areas in the northeastern Mediterranean. The mean atomic ratio of nitrogen to phosphorus varied from 1.3 (June) to 12.9 (February). This ratio was lower than the Redfield ratio of 16 for ocean phytoplankton, and phytoplankton was potentially limited by nitrogen for most of the months. The result of this study confirms and emphasizes the oligotrophic nature of the eastern Mediterranean.     

Microbial processes in a high-latitude fjord (Kongsfjorden, Svalbard): II. Ciliates and dinoflagellates

The composition and ecological role of ciliates and dinoflagellates were investigated at one station in Kongsfjorden, Svalbard, during six consecutive field campaigns between March and December 2006. Total ciliate and dinoflagellate abundance mirrored the seasonal progression of phytoplankton, peaking with 5.8 × 10⁴ cells l⁻¹ in April at an average chlorophyll a concentration of 10 μg l⁻¹. Dinoflagellates were more abundant than ciliates, dominated by small athecates. Among ciliates, aloricate oligotrichs dominated the assemblage. A large fraction (>60%) of ciliates and dinoflagellates contained chloroplasts in spring and summer. The biomass of the purely heterotrophic fraction of the ciliate and dinoflagellate community (protozooplankton) was with 14 μg C l⁻¹ highest in conjunction with the phytoplankton spring bloom in April. Growth experiments revealed similar specific growth rates for heterotrophic ciliates and dinoflagellates (<0–0.8 d⁻¹). Food availability may have controlled the protozooplankton assemblage in winter, while copepods may have exerted a strong control during the post-bloom period. Calculations of the potential grazing rates of the protozooplankton indicated its ability to control or heavily impact the phytoplankton stocks at most times. The results show that ciliates and dinoflagellates were an important component of the pelagic food web in Kongsfjorden and need to be taken into account when discussing the fate of phytoplankton and biogeochemical cycling in Arctic marine ecosystems.     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

Bioremediation of Fungicides by Spent Mushroom Substrate and Its Associated Microflora

Experiments were conducted both under in vitro and in situ conditions to determine the biodegradation potential of button mushroom spent substrate (SMS) and its dominating microbes (fungi and bacteria) for carbendazim and mancozeb, the commonly used agricultural fungicides. During 6 days of incubation at 30 ± 2°C under broth culture conditions, highest degradation of carbendazim (17.45%) was recorded with B-1 bacterial isolate, while highest degradation of mancozeb (18.05%) was recorded with Trichoderma sp. In fungicide pre-mixed sterilized SMS, highest degradation of carbendazim (100.00–66.50 μg g⁻¹) was recorded with mixed inoculum of Trichoderma sp. and Aspergillus sp., whereas highest degradation of mancozeb (100.00–50.50 μg g⁻¹) was with mixed inoculum of Trichoderma sp., Aspergillus sp. and B–I bacterial isolate in 15 days of incubation at 30 ± 2°C. All these microbes both individually as well as in different combinations grew well and produced extracellular lignolytic enzymes on SMS, which helped in fungicides degradation. Under in situ conditions, among three different proportions of SMS (10, 20 and 30%, w/w) mixed with fungicide pre-mixed soil (100 μg g⁻¹ of soil), the degradation of carbendazim was highest in 30% SMS treatment, while for mancozeb it was in 20% SMS treatment. The residue levels of both fungicides decreased to half of their initial concentration after 1 month of SMS mixing.     

Growth retardants stimulate guggulsterone production in the presence of fungal elicitor in fed-batch cultures of <Emphasis Type="Italic">Commiphora wightii</Emphasis>

Guggulsterone, a hypolipidemic natural agent, is produced in resin canals of the plant Commiphora wightii. In this study, the stimulatory effects of growth retardants [ALAR (N,N-dimethylaminosuccinamic acid) and CCC (chlormequat chloride)] and fungal elicitor on guggulsterone accumulation in cell cultures of C. wightii are reported. CCC at 1 mg l⁻¹ enhanced guggulsterone content (~123 μg l⁻¹) when added on the fifth day after inoculation, while ALAR at 2.5 mg l⁻¹ increased guggulsterone content (~116 μg l⁻¹) when added on the tenth day. In a two-stage fed-batch process, combined treatment with fungal elicitor and growth retardant caused a significant increase (~353 μg l⁻¹) in guggulsterone content in cell cultures after 17 days of growth. This represents an approximately fivefold increase over the guggulsterone contents in initial cultures of this plant.     

Net Primary Production and Carbon Stocks for Subarctic Mesic–Dry Tundras with Contrasting Microtopography, Altitude, and Dominant Species

Mesic–dry tundras are widespread in the Arctic but detailed assessments of net primary production (NPP) and ecosystem carbon (C) stocks are lacking. We addressed this lack of knowledge by determining the seasonal dynamics of aboveground vascular NPP, annual NPP, and whole-ecosystem C stocks in five mesic–dry tundras in Northern Sweden with contrasting microtopography, altitude, and dominant species. Those measurements were paralleled by the stock assessments of nitrogen (N), the limiting nutrient. The vascular production was determined by harvest or in situ growing units, whereas the nonvascular production was obtained from average species growth rates, previously assessed at the sites. Results showed that aboveground vascular NPP (15–270 g m⁻²), annual NPP (214–282 g m⁻² or 102–137 g C m⁻²) and vegetation biomass (330–2450 g m⁻²) varied greatly among communities. Vegetation dominated by Empetrum hermaphroditum is more productive than Cassiope tetragona vegetation. Although the large majority of the apical NPP occurred in early-mid season (85%), production of stems and evergreen leaves proceeded until about 2 weeks before senescence. Most of the vascular vegetation was belowground (80%), whereas most of the vegetation production occurred aboveground (85%). Ecosystem C and N stocks were 2100–8200 g C m⁻² and 80–330 g N m⁻², respectively, stored mainly in the soil turf and in the fine organic soil. Such stocks are comparable to the C and N stocks of moister tundra types, such as tussock tundra. Author Contributions  Matteo Campioli, Anders Michelsen, Roeland Samson, Raoul Lemeur—conceived and designed study, Matteo Campioli, Anders Michelsen, Andreas Demey, Annemie Vermeulen—performed research, Matteo Campioli—analyzed data, and Matteo Campioli—wrote the paper.     

Inhibition of Clostridium butyricum by 1,3-propanediol and diols during glycerol fermentation

1,3-Propanediol inhibition during glycerol fermentation to 1,3-propanediol by Clostridium butyricum CNCM 1211 has been studied. The initial concentration of the 1,3-propanediol affected the growth of the bacterium more than the glycerol fermentation. μ _max was inversely proportional to the initial concentration of 1,3-propanediol (0–65 g l⁻¹). For glycerol at 20 g l⁻¹, the growth and fermentation were completely stopped at an initial 1,3-propanediol concentration of 65 g l⁻¹. However, for an initial 1,3-propanediol concentration of 50 g l⁻¹ and glycerol at 70 g l⁻¹, the final concentration (initial and produced) of 1,3-propanediol reached 83.7 g l⁻¹(1.1 M), with complete consumption of the glycerol. Therefore, during the fermentation, the strain tolerated a 1,3-propanediol concentration higher than the initial inhibitory concentration (65 g l⁻¹). The addition of 1,2-propanediol or 2,3-butanediol (50 g l⁻¹) in the presence of glycerol (50–100 g l⁻¹), showed that 2-diols reduced the μ _max in a similar way to 1,3-propanediol. The measurement of the osmotic pressure of glycerol solutions, diols and diol/glycerol mixtures did not indicate any differences between these compounds. The hypothesis of diol inhibition was discussed. Taking into account the strain tolerance of highly concentrated 1,3-propanediol during fermentation, the fermentation processes for optimising production were considered. Received: 15 November 1999 / Revision received: 1 February 2000 / Accepted: 4 February 2000     

Physical and chemical limnology of a wind-swept tropical highland reservoir

Valle de Bravo (VB) is a tropical reservoir located (19°21′30″ N, 100°11′00″ W) in the highlands of Mexico. The reservoir is daily swept by strong (7.4 m s⁻¹ mean speed) diurnal (12:00–19:00 h) winds that blow along its two main arms. As expected from its fetch (6.9 km) and its depth (21.1 m mean), the reservoir behaves as a warm monomictic water body. During 2001, VB was stratified from February to October, and well mixed from November to January. Its mean temperature was 19.9°C; the maximum found was 23.8°C in the epilimnion, while a minimum of 17.8°C was registered during mixing. VB exhibited a thermal regime similar to other water bodies of the Mexican tropical highlands, except for a steady increase of its hypolimnetic temperature during stratification, which is attributed to entrainment of epilimnetic water into the hypolimnion. During stratification, the hypolimnion was anoxic, while the whole water column remained under-saturated (60%) during mixing. The flushing time is 2.2 years. Mineralization and total alkalinity are low, which allows strong changes in pH. Ammonia remained low (2.4 μmol l⁻¹ mean) in the epilimnion, but reached up to 60 μmol l⁻¹ in the hypolimnion. Soluble reactive phosphorous had a mean of 0.28 μmol l⁻¹ in the epilimnion and a mean of 1.25 μmol l⁻¹ in the hypolimnion. Nitrate exhibited maxima (up to 21 μmol l⁻¹) during mixing, and also in the metalimnion (2 μmol l⁻¹) during stratification. Low dissolved inorganic nitrogen indicated nitrogen limitation during stratification. Eutrophication is an emerging problem in VB, where cyanobacteria dominate during stratification. At VB chlorophyll a is low during mixing (mean of 9 μg l⁻¹), and high during stratification (mean 21 μg l⁻¹), when blooms (up to 88 μg l⁻¹) are frequent. This pattern is similar to that found in other eutrophic tropical water bodies. We propose that in VB the wind regime causes vertical displacements of the thermocline (0.58–1.10 m hr⁻¹) and boundary mixing, enhancing the productivity during the stratification period in this tropical reservoir.     

Enhanced regeneration of haploid plantlets from microspores of <Emphasis Type="Italic">Brassica napus</Emphasis> L. using bleomycin,PCIB, and phytohormones

The effects of three periods of incubation (10, 20 and 30 min) at different levels of bleomycin (0, 0.1, 0.2, 0.3, 0.4 and 0.5 μg ml⁻¹), as well as three periods of exposure (12, 24 and 48 h) to different levels of the anti-auxin p-chlorophenoxyisobutyric acid (PCIB), including 1, 2, 3, 4 and 5 mg l⁻¹, on microspore embryogenesis of rapeseed cv. ‘Amica’ were investigated. Microspore embryogenesis was significantly enhanced following 20 min treatment with 0.2 μg ml⁻¹ bleomycin compared with untreated cultures. Highest embryo yield (163 embryos Petri dish⁻¹) was observed with 24 h treatment of 4 mg l⁻¹ PCIB. The highest percentage of secondary embryogenesis was observed on B₅ medium containing 0.15 mg l⁻¹ of gibberellic acid (GA₃) and 0.2 mg l⁻¹ 6-benzyladenine (BA) in 4–6 mm microspore-derived embryos (MDEs). Most callus formed on B₅ medium containing 0.15 mg l⁻¹ GA₃, 0.1 mg l⁻¹ BA and 0.1 mg l⁻¹ indole-3-acetic acid (IAA) when 4–6 mm embryos were used. Regeneration was highest on B₅ medium containing 0.05 mg l⁻¹ GA₃ or 0.1 mg l⁻¹ BA and 0.2 mg l⁻¹ IAA with 2–4 mm embryos. Microspore embryogenesis and plant regeneration could be improved by both bleomycin and PCIB when the appropriate MDE length and phytohormone level were selected.     

Diatom composition and biomass variability in nearshore waters of Maxwell Bay, Antarctica, during the 1992/1993 austral summer

 Diatom composition and biomass were investigated in the nearshore water (<30 m in depth) of Maxwell Bay, Antarctica during the 1992/1993 austral summer. Epiphytic or epilithic diatoms such as Fragilaria striatula, Achnanthes brevipes var. angustata and Licmophora spp. dominated the water column microalgal populations. Within the bay, diatom biomass in surface water was several times higher at the nearshore (2.4–14 μg C l^-1) than at the offshore stations (>100 m) (1.2–3.2 μg C l^-1) with a dramatic decrease towards the bay mouth. Benthic forms accounted for >90% of diatom carbon in all nearshore stations, while in the offshore stations planktonic forms such as Thalassiosira antarctica predominated (50–>90%). Microscopic examination revealed that many of these diatoms have become detached from a variety of macroalgae growing in the intertidal and shallow subtidal bottoms. Epiphytic diatoms persistently dominated during a 19-day period in the water column at a fixed nearshore station, and the biomass of these diatoms fluctuated from 0.86 to 53 μg C l^-1. A positive correlation between diatom biomass and wind speed strongly suggests that wind-driven resuspension of benthic forms is the major mechanism increasing diatom biomass in the water column. Received: 28 April 1995/Accepted: 1 April 1996     

Phytoplankton and particulate matter at the Weddell / Scotia Confluence (47°W) in summer 1989, as a final step of a temporal succession (EPOS project)

During January 1989, phytoplankton biomass and species composition were studied in a north / south transect at the Weddell / Scotia Confluence (47°W), between 57° and 61°30′S. Results showed a diatom bloom in the Scotia Sea (chlorophyll a 1.9 μg l⁻¹, particulate organic carbon 239 μg l⁻¹), dominated by Fragilariopsis cylindrus, Dactyliosolen antarcticus and Chaetoceros dichaeta. Low chlorophyll a / phaeopigments ratios (about 1.4) and silicate concentrations (15 μmol l⁻¹) suggested that this was an advanced bloom phase, probably linked to high grazing pressure. Minimum chlorophyll a values of 0.1–0.2 μg l⁻¹ and particulate organic carbon 46 μg l⁻¹ were found at the Weddell / Scotia Front and in a subsurface layer of the Weddell Sea Water. In the southern part of the transect (61°30′S), in the Weddell Sea, a second surface maximum was found (chlorophyll a 0.9 μg l⁻¹, particulate organic carbon 120 μg l⁻¹), but with a different species composition, with Cryptomonas sp. dominant. Our results show a succession within the diatom community in the Weddell / Scotia Confluence Waters when comparing the three EPOS legs. In the Weddell Sea from spring to summer, nanoflagellates, with only a minor contribution from diatoms, persist over a long period with little change in the community structure. We suggest that the frontal system, together with the receding ice edge and the grazing pressure of either krill or protozooplankton, are mainly responsible for the phytoplankton distribution patterns found. Received: 3 July 1996 / Accepted: 3 November 1996     

Ethanol and xylitol production from glucose and xylose at high temperature by <Emphasis Type="Italic">Kluyveromyces</Emphasis> sp. IIPE453

A yeast strain Kluyveromyces sp. IIPE453 (MTCC 5314), isolated from soil samples collected from dumping sites of crushed sugarcane bagasse in Sugar Mill, showed growth and fermentation efficiency at high temperatures ranging from 45°C to 50°C. The yeast strain was able to use a wide range of substrates, such as glucose, xylose, mannose, galactose, arabinose, sucrose, and cellobiose, either for growth or fermentation to ethanol. The strain also showed xylitol production from xylose. In batch fermentation, the strain showed maximum ethanol concentration of 82 ± 0.5 g l⁻¹ (10.4% v/v) on initial glucose concentration of 200 g l⁻¹, and ethanol concentration of 1.75 ± 0.05 g l⁻¹ as well as xylitol concentration of 11.5 ± 0.4 g l⁻¹ on initial xylose concentration of 20 g l⁻¹ at 50°C. The strain was capable of simultaneously using glucose and xylose in a mixture of glucose concentration of 75 g l⁻¹ and xylose concentration of 25 g l⁻¹, achieving maximum ethanol concentration of 38 ± 0.5 g l⁻¹ and xylitol concentration of 14.5 ± 0.2 g l⁻¹ in batch fermentation. High stability of the strain was observed in a continuous fermentation by feeding the mixture of glucose concentration of 75 g l⁻¹ and xylose concentration of 25 g l⁻¹ by recycling the cells, achieving maximum ethanol concentration of 30.8 ± 6.2 g l⁻¹ and xylitol concentration of 7.35 ± 3.3 g l⁻¹ with ethanol productivity of 3.1 ± 0.6 g l⁻¹ h⁻¹ and xylitol productivity of 0.75 ± 0.35 g l⁻¹ h⁻¹, respectively.