Structural and functional properties of sympagic communities in the annual sea ice at Terra Nova Bay (Ross Sea, Antarctica)

Studies on the chemical and biological properties of annual pack ice at a coastal station in Terra Nova Bay (74°41.72′S, 164°11.63′E) were carried out during austral spring at 3-day intervals from 5 November to 1 December 1997. Temporal changes of nutrient concentrations, algal biomasses, taxonomic composition, photosynthetic pigment spectra and P–E relationships were studied. Quantity, composition and degradation rates of organic matter in the intact sea ice were also investigated. In addition, microcosm experiments were carried out to evaluate photosynthetic and photo-acclimation processes of the sympagic flora in relation to different light regimes. High concentrations of ammonia were measured in four ice-cores (weighted mean values of the cores ranged from 4.3 ± 1.9 μM to 7.2 ± 3.4 μM), whereas nitrate and phosphate displayed high concentrations (up to 35.9 μM and 7.6 μM, respectively) only in the bottom layer (135–145 cm depth). Particulate carbohydrate and protein concentrations in the intact sea ice ranged from 0.5 to 2.3 mg l⁻¹ and 0.2 to 2.0 mg l⁻¹, respectively, displaying a notable accumulation of organic matter in the bottom colored layer, where bacterial enzymatic activities also reached the highest values. Aminopeptidase activity was extremely high (up to 19.7 μM l⁻¹ h⁻¹ ± 0.05 in the bottom layer), suggesting a rapid turnover rate of nitrogen–enriched organic compounds (e.g. proteins). By contrast, bacterial secondary production was low, suggesting that only a very small fraction of mobilized organic matter was converted into bacterial biomass (<0.01‰). The sympagic autotrophic biomass (in terms of chlorophaeopigments) of the bottom layer was high, increasing during the sampling period from 680 to 2480 μg l⁻¹. Analyses of pigments performed by HPLC, as well as microscope observations, indicated that diatoms dominated bottom communities. The most important species were Amphiprora sp. and Nitschia cfr. stellata. Bottom sympagic communities showed an average P ^B _max of 0.12 mgC mg Chl⁻¹ and low photoadaptation index (E _k=18 μE m⁻² s⁻¹, E _m=65 μE m⁻² s⁻¹). Results of the microcosm experiment also indicated that communities were photo-oxidized when irradiance exceeded 100 μE m⁻² s⁻¹. This result suggests that micro- autotrophs inhabiting sea ice might have a minor role in the pelagic algal blooms. Accepted: 4 August 1999     

Seasonal variation of phytoplankton community structure and nitrogen uptake regime in the Indian Sector of the Southern Ocean

This study investigates the dynamics of phytoplankton communities and nitrogen uptake in the Indian sector of the Southern Ocean during spring and summer. The study area is oligotrophic (Chl a stocks <50 mg m⁻²); nevertheless, a large spatial variation of phytoplankton biomass and community structure was observed. During both seasons the phytoplankton community in the seasonal ice zone showed higher biomasses and was mainly composed of large diatom cells. However, in the permanently open ocean zone the community had low biomass and was chiefly composed of nano- and picoflagellates. In the polar front zone, although biomass was higher, the community structure was similar to the open ocean zone. The results suggest that the variation in phytoplankton community structure on a larger scale resonates with gradients in water column stability and nutrient distribution. However, significant changes in biomass and nutrient stocks but little change in community structure were observed. Absolute nitrogen uptake rates were generally low, but their seasonal variations were highly significant. During spring the communities displayed high specific nitrate uptake (mean rate = 0.0048 h⁻¹), and diatoms (in the seasonal ice zone) as well as nano- and picoflagellates (in the permanently open ocean zone and polar front zone) were mainly based on new production (mean ƒ-ratio = 0.69). The transition to summer was accompanied by a significant reduction in nitrate uptake rate (0.0048 h⁻¹ → 0.0011 h⁻¹) and a shift from predominantly new to regenerated production (ƒ-ratio 0.69 → 0.39). Ammonium played a major role in the seasonal dynamics of phytoplankton nutrition. The results emphasize that, despite a large contrast in community structure, the seasonal dynamics of the nitrogen uptake regime and phytoplankton community structure in all three subsystems were similar. Additionally, this study supports our previous conclusion that the seasonal shift in nitrogen uptake regime can occur with, as well as without, marked changes in community structure. Received: 2 December 1997 / Accepted: 20 April 1998     

Preliminary investigation of Okhotsk Sea ice algae; taxonomic composition and photosynthetic activity

Okhotsk Sea pack ice from Shiretoko in northern Hokkaido, sampled in March 2007, contained microalgal communities dominated by the centric diatoms Thalassiosira nordenskioeldi and T. punctigera. Domination by this genus is very unusual in sea ice. Communities from nearby fast ice at Saroma-ko lagoon were dominated by Detonula conferavea and Odontella aurita. Average microalgal biomass of the Okhotsk Sea pack ice (surface and bottom) was 1.59 ± 1.09 μg chla l⁻¹ and for fast ice (bottom only) at nearby Saroma-ko lagoon, 16.5 ± 3.2 μg l⁻¹ (=31.1 ± 5.0 mg chla m⁻²). Maximum quantum yield of the Shiretoko pack ice algal communities was 0.618 ± 0.056 with species-specific data ranging between 0.211 and 0.653. These community values are amongst the highest recorded for sea ice algae. Rapid light curves (RLC) on individual cells indicated maximum relative electron transfer rates (relETR) between 20.8 and 60.6, photosynthetic efficiency values (α) between 0.31 and 0.93 and onset of saturation values (E _k) between 33 and 91 μmol photons m⁻² s⁻¹. These data imply that the pack ice algal community at Shiretoko was healthy and actively photosynthesising. Maximum quantum yield of the Saroma-ko fast ice community was 0.401 ± 0.086, with values for different species between 0.361 and 0.560. RLC data from individual Saroma-ko fast ice algal cells indicated relETR between 55.3 and 60.6, α values between 0.609 and 0.816 and E _k values between 74 and 91 μmol photons m⁻² s⁻¹ which are consistent with measurements in previous years.     

Effect of temperature and irradiance on the uptake of ammonium and nitrate by <Emphasis Type="Italic">Laurencia brongniartii</Emphasis> (Rhodophyta,Ceramiales)

The effects of temperature (20, 24 and 28 °C) and irradiance (15 and 40 μmol photon m⁻² s⁻¹) on the nitrate and ammonium uptake rates of the subtropical red alga, Laurencia brongniartii, were investigated to prepare for tank cultivation. Nitrate uptake followed saturation kinetics and was faster at higher irradiances and temperatures. In contrast, ammonium uptake was linear over the experimental range and was not affected by an increase in temperature. A parameter, β, was calculated to compare substrate uptake rates of nitrate along the linear portion of the uptake curve with that of ammonium. For nitrate, β was lower at low irradiance and higher at high irradiance (β = 0.007 ± 0.003 and 0.030 ± 0.002 [μmol N L⁻¹ (μmol N g_ww⁻¹ d⁻)⁻¹], respectively). However, β was 0.023 ± 0.002 and 0.034 ± 0.002 [μmol N L⁻¹ (μmol N g_ww⁻¹ d⁻¹)⁻¹] for ammonium, suggesting a preference for ammonium over nitrate.     

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.     

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.     

Comparative study of periphyton community structure in long and short-hydroperiod Everglades marshes

The Florida Everglades is a mosaic of short and long-hydroperiod marshes that differ in the depth, duration, and timing of inundation. Algae are important primary producers in widespread Everglades’ periphyton mats, but relationships of algal production and community structure to hydrologic variability are poorly understood. We quantified differences in algal biomass and community structure between periphyton mats in 5 short and 6 long-hydroperiod marshes in Everglades National Park (ENP) in October 2000. We related differences to water depth and total phosphorus (TP) concentration in the water, periphyton and soils. Long and short-hydroperiod marshes differed in water depth (73 cm vs. 13 cm), periphyton TP concentrations (172μg g⁻¹ vs. 107 μg g⁻¹, respectively) and soil TP (284 μg g⁻¹ vs. 145 μg g⁻¹). Periphyton was abundant in both marshes, with short-hydroperiod sites having greater biomass than long-hydroperiod sites (2936 vs. 575 grams ash-free dry mass m⁻²). A total of 156 algal taxa were identified and separated into diatom (68 species from 21 genera) and “soft algae” (88 non-diatom species from 47 genera) categories for further analyses. Although diatom total abundance was greater in long-hydroperiod mats, diatom species richness was significantly greater in short- hydroperiod periphyton mats (62 vs. 47 diatom taxa). Soft algal species richness was greater in long-hydroperiod sites (81 vs. 67 soft algae taxa). Relative abundances of individual taxa were significantly different among the two site types, with soft algal distributions being driven by water depth, and diatom distributions by water depth and TP concentration in the water and periphyton. Periphyton communities differ between short and long-hydroperiod marshes, but because they share many taxa, alterations in hydroperiod could rapidly promote the alternate community. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Palm oil mill effluent (POME) cultured marine microalgae as supplementary diet for rotifer culture

Malaysia is the world’s leading producer of palm oil products that contribute US$ 7.5 billion in export revenues. Like any other agro-based industries, it generates waste that could be utilized as a source of organic nutrients for microalgae culture. Present investigation delves upon Isochrysis sp. culture in POME modified medium and its utilization as a supplement to Nanochloropsis sp. in rotifer cultures. The culture conditions were optimized using a 1 L photobioreactor (Temp: 23°C, illumination: 180 ∼ 200 μmol photons m⁻²s⁻¹, n = 6) and scaled up to 10 L outdoor system (Temp: 26–29°C, illumination: 50 ∼ 180 μmol photons m⁻²s⁻¹, n = 3). Algal growth rate in photobioreactor (μ = 0.0363 h⁻¹) was 55% higher compared to outdoor culture (μ = 0.0163 h⁻¹), but biomass production was 1.3 times higher in outdoor culture (Outdoor = 91.7 mg m⁻²d⁻¹; Photobioreactor = 69 mg m⁻²d⁻¹). Outdoor culture produced 18% higher lipid; while total fatty acids (FA) was not significantly affected by the change in culture systems as both cultures yield almost similar concentrations of fatty acids per gram of sample (photobioreactor = 119.17 mg g⁻¹; outdoor culture = 104.50 mg g⁻¹); however, outdoor cultured Isochrysis sp. had 26% more polyunsaturated fatty acids (PUFAs). Rotifers cultured in Isochrysis sp./ Nanochloropsis sp. (1:1, v/v) mixture gave similar growth rate as 100% Nanochoropsis sp. culture (μ = 0.40 d⁻¹), but had 45% higher counts of rotifers with eggs (t = 7, maximum). The Isochrysis sp. culture successfully lowered the nitrate (46%) and orthophosphate (83%) during outdoor culture.     

Biogeochemical conditions and ice algal photosynthetic parameters in Weddell Sea ice during early spring

Physical, biogeochemical and photosynthetic parameters were measured in sea ice brine and ice core bottom samples in the north-western Weddell Sea during early spring 2006. Sea ice brines collected from sackholes were characterised by cold temperatures (range −7.4 to −3.8°C), high salinities (range 61.4–118.0), and partly elevated dissolved oxygen concentrations (range 159–413 μmol kg⁻¹) when compared to surface seawater. Nitrate (range 0.5–76.3 μmol kg⁻¹), dissolved inorganic phosphate (range 0.2–7.0 μmol kg⁻¹) and silicic acid (range 74–285 μmol kg⁻¹) concentrations in sea ice brines were depleted when compared to surface seawater. In contrast, NH₄ ⁺ (range 0.3–23.0 μmol kg⁻¹) and dissolved organic carbon (range 140–707 μmol kg⁻¹) were enriched in the sea ice brines. Ice core bottom samples exhibited moderate temperatures and brine salinities, but high algal biomass (4.9–435.5 μg Chl a l⁻¹ brine) and silicic acid depletion. Pulse amplitude modulated fluorometry was used for the determination of the photosynthetic parameters F _v/F _m, α, rETR_max and E _k. The maximum quantum yield of photosystem II, F _v/F _m, ranged from 0.101 to 0.500 (average 0.284 ± 0.132) and 0.235 to 0.595 (average 0.368 ± 0.127) in the sea ice internal and bottom communities, respectively. The fluorometric measurements indicated medium ice algal photosynthetic activity both in the internal and bottom communities of the sea ice. An observed lack of correlation between biogeochemical and photosynthetic parameters was most likely due to temporally and spatially decoupled physical and biological processes in the sea ice brine channel system, and was also influenced by the temporal and spatial resolution of applied sampling techniques.     

Pelagic primary production during summer along 65 to 72°N off West Greenland

The distribution of phytoplankton biomass and primary production were studied during summer 1993 at 16 stations from 65 to 72°N off West Greenland, ranging more than 900 km. Hydrography, nutrients and chlorophyll a profiles revealed a significant change in structure from south to north. Nitrate was depleted in the euphotic zone at most stations except close to the ice edge (West Ice) or close to outflow from large glaciers. The vertical distribution of phosphate followed that of nitrate, but was never depleted. Despite two stations with relatively high surface concentrations, silica showed the same distribution as the other two nutrients. In the south, chlorophyll a concentration and primary production were lower than north of Disko Bay (69°N), associated with a well-mixed versus a salinity-generated stratification, respectively. In Vaigat, a high-production station was identified, (st. 910, 69°52′69N–51°30′61W) with a chlorophyll a concentration in the euphotic zone of >13 μg l⁻¹ and an area primary production of 3.2 g C m⁻² day⁻¹. This is seldom encountered in arctic waters and was presumably due to nutrient-rich melt-water originating from the Iluliíssat Glacier. The overall primary production for the studied area was 67–3207 mg C m⁻² day⁻¹ (mean ± SD=341± 743 mg C m⁻² day⁻¹), which is within the range of the few results published for West Greenland and eastern Canadian Arctic waters. Accepted: 24 October 1998     

Cell growth and nutritive value of the tropical benthic diatom, <Emphasis Type="Italic">Amphora</Emphasis> sp., at varying levels of nutrients and light intensity,and different culture locations

Two series of experiments were conducted to determine suitable growth factors for the mass propagation of the local algal isolate Amphora sp. A higher growth rate of 0.2 doubling (μ) day⁻¹ was attained at a lower photosynthetic photon flux density (PPFD; 11.4 μmol photon m⁻²s⁻¹) compared to cultures exposed to higher levels of PPFD (16.1 μmol photon m⁻²s⁻¹, −0.1 μ day ⁻¹; 31.3 μmol photon m⁻²s⁻¹, 0.0 μ day⁻¹). Cultures located inside the laboratory had a significantly higher cell density (133 × 10⁴ cells cm⁻²) and growth rate (0.3 μ day⁻¹) compared to those located outdoors (100 × 10⁴ cells cm⁻², 0.2 μ day⁻¹). A comparison of nutrient medium across two locations showed that lipid content was significantly higher in cultures enriched with F/2MTM (macronutrients + trace metals) and F/2MV (macronutrients + vitamins). Saturated fatty acids were also present in high concentrations in cultures enriched with F/2M (macronutrients only). Significantly higher amounts of saturated fatty acids were observed in cultures located outdoors (33.1%) compared to those located indoors (26.6%). The protein, carbohydrates and n-6 fatty acid content of Amphora sp. were influenced by the location and enrichment of the cultures. This study has identified growth conditions for mass culture of Amphora sp. and determined biochemical composition under those culture conditions. Presented at the 6th Meeting of the Asian Pacific Society of Applied Phycology, Manila, Philippines.     

Arctic Sea ice biota: design and evaluation of a mesocosm experiment

A mesocosm experiment (enclosure volume 220 l) was designed such that sea ice inhabited by Arctic Sea ice organisms was formed and maintained under natural conditions at 66°N in Rovaniemi, Finland. The experiment was run from natural freezing in December 1994 to melting in April 1995. The ice was inhabited by diatoms, chlorophyceae, heterotrophic flagellates, ciliates, nematodes and turbellarians. Biomass in the ice, expressed as Chlorophyll a concentration, was 20–110 μg l⁻¹; total cell densities varied from 5 × 10⁶ to 35 × 10⁶ cells l⁻¹. Amongst phototrophic organisms, a succession from a flagellate-dominated community (Chlamydomonas sp.) to a multi-species diatom-dominated community was observed. Typical Arctic species such as Nitzschia frigida and Melosira arctica were present in the ice. Bacterial concentration varied between 2 × 10⁸ and 7 × 10⁸ cells l⁻¹. At least two trophic levels were present in the ice. Received: 3 April 1997 / Accepted: 9 September 1997     

Grazing by the Antarctic sea-ice ciliate Pseudocohnilembus

Potential uptake and clearance rates of fluorescent microspheres (FM) from 0.25 to 4.05 μm diameter were determined for the non-loricate ciliate Pseudocohnilembus sp. from Antarctic sea ice. The percentage of ciliate cells that ingested FM after 20 min incubation decreased with increasing particle diameter. Pseudocohnilembus sp. ingested FM between 0.25 and 4.05 μm in diameter. We offered FM at concentrations less than natural concentrations for plankton plus detrital material and obtained clearance rates less than those previously reported for bactivorous ciliates. Clearance rates were 3.6–5.4 nl cell⁻¹ h⁻¹ for FM 0.5 and 1 μm diameter, respectively, but decreased to 1.1 nl cell⁻¹ h⁻¹ for 1.97 μm diameter and 1.4 nl cell⁻¹ h⁻¹ for 4.05-μm-diameter FM. Clearance and uptake rates of FM 0.5 and 1 μm diameter indicate that Pseudocohnilembus sp. principally grazes on bacteria-sized particles. However, it can also ingest organisms as large as nanoplankton and may graze particles as small as femtoplankton and colloids. This suggests a feeding strategy that may suit the temporal and spatial changes in food availability in the sea-ice habitat. Accepted: 13 August 2000     

CO2 exchange of the endolithic lichen Verrucaria baldensis from karst habitats in northern Italy

CO₂ exchange of the endolithic lichen Verrucaria baldensis was measured in the laboratory under different conditions of water content, temperature, light, and CO₂ concentration. The species had low CO₂ exchange rates (maximum net photosynthesis: c. 0.45 μmol CO₂ m⁻² s⁻¹; maximum dark respiration: c. 0.3 μmol CO₂ m⁻² s⁻¹) and a very low light compensation point (7 μmol photons m⁻² s⁻¹ at 8°C). The net photosynthesis/respiration quotient reached a maximum at 9–15°C. Photosynthetic activity was affected only after very severe desiccation, when high resaturation respiratory rates were measured. Microclimatic data were recorded under different weather conditions in an abyss of the Trieste Karst (northeast Italy), where the species was particularly abundant. Low photosynthetically active radiation (normally below 40 μmol photons m⁻² s⁻¹), very high humidities (over 80%), and low, constant temperatures were measured. Thallus water contents sufficient for CO₂ assimilation were often measured in the absence of condensation phenomena. Received: 22 September 1996 / Accepted: 26 April 1997     

Biodegradation of the pesticide 4,6-dinitro-ortho-cresol by microorganisms in batch cultures and in fixed-bed column reactors

A mixed culture of microorganisms able to utilize 4,6-dinitro-ortho-cresol (DNOC) as the sole source of carbon, nitrogen and energy was isolated from soil contaminated with pesticides and from activated sludge. DNOC was decomposed aerobically in batch cultures as well as in fixed-bed column reactors. Between 65% and 84% of the substrate nitrogen was released as nitrate into the medium, and 61% of the carbon from uniformly ¹⁴C-labelled DNOC was recovered as ¹⁴CO₂. The mixed microbial culture also decomposed 4-nitrophenol and 2,4-dinitrophenol but not 2,3-dinitrophenol, 2,6-dinitrophenol, 2,4-dinitrotoluene, 2,4-dinitrobenzoic acid or 2-sec-butyl-4,6-dinitrophenol (Dinoseb). Maximal degradation rates for DNOC by the bacterial biofilm immobilized on glass beads in fixed-bed column reactors were 30 mmol day⁻¹ (l reactor volume)⁻¹, leaving an effluent concentration of less than 5 μg l⁻¹ DNOC in the outflowing medium. The apparent K _s value of the immobilized mixed culture for DNOC was 17 μM. Degradation was inhibited at DNOC concentrations above 30 μM and it ceased at 340 μM, possibly because of the uncoupling action of the nitroaromatic compound on the cellular energy-transducing mechanism. Received: 27 March 1997 / Received revision: 5 June 1997 / Accepted: 7 June 1997     

A preliminary study of the bioremediation potential of Codium fragile applied to seaweed integrated multi-trophic aquaculture (IMTA) during the summer

In integrated multi-trophic aquaculture (IMTA), seaweeds have the capacity to reduce the environmental impact of nitrogen-rich effluents in coastal ecosystems. To establish such bioremediation systems, selection of suitable seaweed species is important. The distribution and productivity of seaweeds vary seasonally based on water temperature and photoperiod. In Korea, candidate genera such as Pophyra, Laminaria, and Undaria grow from autumn to spring. In contrast, Codium grows well at relatively high water temperatures in summer. Thus, aquaculture systems potentially could capitalize on Codium’s capacity for rapid growth in the warm temperatures of late summer and early fall. In this study, we investigated ammonium uptake and removal efficiency by Codium fragile. In laboratory experiments, we grew C. fragile under various water temperatures (10, 15, 20, and 25°C), irradiances (dark, 10, and 100 μmol photons m⁻² s⁻¹), and initial ammonium concentrations (150 and 300 μM); in all cases, C. fragile exhausted the ammonium supply for 6 h. At 150 μM of , ammonium removal efficiency was greatest (99.5 ± 2.6%) when C. fragile was incubated at 20°C under 100 μmol photons m⁻² s⁻¹. At 300 μM of , removal efficiency was greatest (86.3 ± 2.1%) at 25°C under 100 μmol photons m⁻² s⁻¹. Ammonium removal efficiency was significantly greater at 20 and 25°C under irradiance of 100 μmol photons m⁻² s⁻¹ than under other conditions tested.     

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

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

Effect of light and temperature on the cyanobacterium <Emphasis Type="Italic">Arthronema africanum</Emphasis> - a prospective phycobiliprotein-producing strain

The effect of light intensity (50–300 μmol photons m⁻² s⁻¹) and temperature (15–50°C) on chlorophyll a, carotenoid and phycobiliprotein content in Arthronema africanum biomass was studied. Maximum growth rate was measured at 300 μmol photons m⁻² s⁻¹ and 36°C after 96 h of cultivation. The chlorophyll a content increased along with the increase in light intensity and temperature and reached 2.4% of dry weight at 150 μmol photons m⁻² s⁻¹ and 36°C, but it decreased at higher temperatures. The level of carotenoids did not change significantly under temperature changes at illumination of 50 and 100 μmol photons m⁻² s⁻¹. Carotenoids were about 1% of the dry weight at higher light intensities: 150 and 300 μmol photons m⁻² s⁻¹. Arthronema africanum contained C-phycocyanin and allophycocyanin but no phycoerythrin. The total phycobiliprotein content was extremely high, more than 30% of the dry algal biomass, thus the cyanobacterium could be deemed an alternative producer of C-phycocyanin. A highest total of phycobiliproteins was reached at light intensity of 150 μmol photons m⁻² s⁻¹ and temperature of 36°C, C-phycocyanin and allophycocyanin amounting, respectively, to 23% and 12% of the dry algal biomass. Extremely low (<15°C) and high temperatures (>47°C) decreased phycobiliprotein content regardless of light intensity.     

Decrease of phosphoribulokinase activity by antisense RNA in transgenic tobacco: definition of the light environment under which phosphoribulokinase is not in large excess

 To test the hypothesis that the contribution of phosphoribulokinase (PRK) to the control of photosynthesis changes depending on the light environment of the plant, the response of transgenic tobacco (Nicotiana tabacum L.) transformed with antisense PRK constructs to irradiance was determined. In plants grown under low irradiance (330 μmol m⁻² s⁻¹) steady-state photosynthesis was limited in plants with decreased PRK activity upon exposure to higher irradiance, with a control coefficient of PRK for CO₂ assimilation of 0.25 at and above 800 μmol m⁻² s⁻¹. The flux control coefficient of PRK for steady-state CO₂ assimilation was zero, however, at all irradiances in plant material grown at 800 μmol m⁻² s⁻¹ and in plants grown in a glasshouse during mid-summer (alternating shade and sun 300–1600 μmol m⁻² s⁻¹). To explain these differences between plants grown under low and high irradiances, Calvin cycle enzyme activities and metabolite content were determined. Activities of PRK and other non-equilibrium Calvin cycle enzymes fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase and ribulose-1,5-bisphosphate carboxylase-oxygenase were twofold higher in plants grown at 800 μmol m⁻² s⁻¹ or in the glasshouse than in plants grown at 330 μmol m⁻² s⁻¹. Activities of equilibrium enzymes transketolase, aldolase, ribulose-5-phosphate epimerase and isomerase were very similar under all growth irradiances. The flux control coefficient of 0.25 in plants grown at 330 μmol m⁻² s⁻¹ can be explained because low ribulose-5-phosphate content in combination with low PRK activity limits the synthesis of ribulose-1,5-bisphosphate. This limitation is overcome in high-light-grown plants because of the large relative increase in activities of sedoheptulose-1,7-bisphosphatase and fructose-1,6-bisphosphatase under these conditions, which facilitates the synthesis of larger amounts of ribulose-5-phosphate. This potential limitation will have maintained evolutionary selection pressure for high concentrations of PRK within the chloroplast. Received: 15 November 1999 / Accepted: 27 January 2000     

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