Characterization of an aerated submerged hollow fiber ultrafiltration device for efficient microalgae harvesting

The present work characterizes a submerged aerated hollow fiber polyvinylidene fluorid (PVDF) membrane (0.03 μm) device (Harvester) designed for the ultrafiltration (UF) of microalgae suspensions. Commercial baker''s yeast served as model suspension to investigate the influence of the aeration rate of the hollow fibers on the critical flux (CF, J _c) for different cell concentrations. An optimal aeration rate of 1.25 vvm was determined. Moreover, the CF was evaluated using two different Chlorella cultures (axenic and non‐axenic) of various biomass densities (0.8–17.5 g DW/L). Comparably high CFs of 15.57 and 10.08 L/m/²/h were measured for microalgae concentrations of 4.8 and 10.0 g DW/L, respectively, applying very strict CF criteria. Furthermore, the J _c‐values correlated (negative) linearly with the biomass concentration (0.8–10.0 g DW/L). Concentration factors between 2.8 and 12.4 and volumetric reduction factors varying from 3.5 to 11.5 could be achieved in short‐term filtration, whereat a stable filtration handling biomass concentrations up to 40.0 g DW/L was feasible. Measures for fouling control (aeration of membrane fibers, periodic backflushing) have thus been proven to be successful. Estimations on energy consumption revealed very low energy demand of 17.97 kJ/m³ treated microalgae feed suspension (4.99 × 10⁻³ kWh/m³) and 37.83 kJ/kg treated biomass (1.05 × 10⁻² kWh/kg), respectively, for an up‐concentration from 2 to 40 g DW/L of a microalgae suspension.     

Optimization of production of C-phycocyanin and extracellular polymeric substances by Arthrospira sp.

The key factors influencing the production of C-phycocyanin (C-PC) and extracellular polymeric substances (EPS) by photoautotrophic culture of Arthrospira sp. were optimized using Taguchi method. Six factors were varied at either three or two levels as follows: light intensity at three levels; three initial culture pHs; two species of Arthrospira; three concentrations of Zarrouk’s medium; three rates of aeration of the culture with air mixed with 2% v/v carbon dioxide; and two incubation temperatures. All cultures ran for 14 days. The optimal conditions for the production of C-PC and EPS were different. For both products, the best cyanobacterium proved to be Arthrospira maxima IFRPD1183. The production of C-PC was maximized with the following conditions: a light intensity of 68 µmol photons m⁻² s⁻¹ (a diurnal cycle of 16-h photoperiod and 8-h dark period), an initial pH of 10, the full strength (100%) Zarrouk’s culture medium, an aeration rate of 0.6 vvm (air mixed with 2% v/v CO₂) and a culture temperature of 30 °C. The concentration of Zarrouk’s medium was the most important factor influencing the final concentration of C-PC. The optimal conditions for maximal production of EPS were as follows: a light intensity of 203 µmol photons m⁻² s⁻¹ with the earlier specified light–dark cycle; an initial pH of 9.5; a 50% strength of Zarrouk’s medium; an aeration rate of 0.2 vvm (air mixed with 2% v/v CO₂); and a temperature of 35 °C. Production of C-PC and EPS in raceway ponds is discussed.

     

Photosynthetic performance of a cyanobacterium in a vertical flat-plate photobioreactor for outdoor microalgal production and fixation of CO2

A vertical flat-plate photobioreactor was developed for the outdoor culture of microalgae using sunlight as the light source. The ability for biomass production and CO₂ fixation was evaluated by using a cyanobacterium, Synechocystis aquatilis SI-2. The average areal productivity was 31 g biomass m^–2 d^–1, which corresponded to a CO₂ fixation rate of 51 g CO₂ m^–2 d^–1, sustainable in the northern region of Japan during the winter time (January and February). The relationships between the efficiency of solar energy utilization of the reactor and its effect factors (cell concentration and irradiation) were investigated.     

Mixotrophic growth ofChlorella sorokiniana in outdoor enclosed photobioreactor

Chlorella sorokiniana was cultured in heterotrophic or mixotrophic mode in outdoor enclosed tubular photobioreactor. The culture temperature was maintained at 32–35 °C. At night, theChlorella culture grew heterotrophically, and 0.1 M glucose was completely consumed. The biomass growth yield of glucose was 0.35 ± 0.001 g-biomass g-glucose^–1. During the day, the algal culture grew mixotrophically and the biomass growth yield was 0.49 g-biomass g-glucose^–1 in low density culture (initial biomass concentration, X_o = 2 g l^–1), 0.56 g-biomass g-glucose^–1 in medium density culture (X_o = 4 g l^–1) and 0.46 g-biomass g-glucose^–1 in high density culture (X_o = 7 g l^–1). The daily area productivity of the culture, with X_o = 4 g l^–1 corresponded to 127 g-biomass m^–2 d^–1 during the day and 79 g-biomass m^–2 d^–1 during the night. In all the cultures, the dissolved O₂ concentration increased in the morning, reached the maximum value at noon, and then decreased in the afternoon. The dissolved CO₂ concentration remained at 3 mBar in the morning and increased in the afternoon. Glycolate was not found to accumulate in culture medium.     

Mixotrophic culture of Chlorella pyrenoidosa with olive-mill wastewater as the nutrient medium

With olive-mill wastewater (`alpechín') as the nutrient medium, theinfluence of specific rate of aeration and initial alpechín concentrationhave been analysed in cultures of Chlorella pyrenoidosa, exposed bothto continuous and intermittent illumination (12/12 h light/dark cycles). The stirring rate in the bioreactor, as well as pH and temperature werefixed previously at 180 rpm, 6.5 and 30 ^°C, respectively. Themaximum specific growth rate (_m) and biomass productivity(b) were determined as kinetic parameters. The chlorophyll, protein andcarbohydrate contents were evaluated, as well as the fatty-acid compositionof the lipid fraction. The experimental conditions most conducive to abalanced biomass composition with regard to proteins and lipids were: initial alpechín concentration of 10% (v/v), continuous illumination,and aeration rate of 1 L (litre cell suspension)^-1 min^-1. Under these conditions, the highest values of _m and b wereclose to 0.04 h^-1 and 1.4 10^-3 g L^-1 h^-1, respectively.     

Optimization of a Propionibacterium acidipropionici continuous culture utilizing sucrose

To produce propionic acid and vitamin B₁₂ from sucrose, the strain Propionibacterium acidipropionici NRRL B3569 was selected by screening a number of Propionibacterium strains. The nutrient composition and the fermentation conditions for this strain were optimized in continuous culture. The investigations show that within a concentration range of 30–170 g l^–1 of sucrose in the fermentation medium, no significant substrate inhibition occurred. For the production of propionic acid and vitamin B₁₂, concentrations of 1.5 mg FeSO₄·7H₂O g^–1 dry biomass, 0.75 mg cobalt ions g^–1 dry biomass, 0.3 mg 5,6-dimethylbenzimidazole g^–1 dry biomass, and 12 g yeast extract 1^–1 were necessary additions to the sources of nitrogen, phosphate, and magnesium ions. The extra addition of up to 2.8 g betaine g^–1 dry biomass significantly increases the production of vitamin B₁₂. In the optimization of the pH value, temperature, and aeration, it was established that the conditions for propionic acid production and vitamin B₁₂ production are different. Whereas the optimal production of propionic acid took place under completely anaerobic conditions with a pH value of 6.5 and a temperature of 37°C, optimal vitamin B₁₂ production required a temperature of 40°C and aerobic conditions (0.5 vvm aeration at 100 rpm) with a pH value of 6.5.     

Effect of culturing parameters on the vegetative growth of Haematococcus alpinus (strain lcr‐cc‐261f) and modeling of its growth kinetics

The present study investigated the effect of different culture conditions on the vegetative growth of a new species, Haematococcus alpinus (strain LCR‐CC‐261f) using airlift photobioreactors. The influence of culture medium, aeration rates, CO₂ concentration in air‐gas mixture, temperature, light intensities, and wavelengths were investigated to achieve sustainable high cell density cultures. Growth parameters were determined by fitting the data to a form of the logistic equation that included a lag phase. The shear‐sensitive vegetative cells favored lower aeration rates in the photobioreactors. MLA medium increased to 40 mM nitrate produced high density cultures. Temperatures between 12°C and 18°C, 3% (v/v) CO₂ concentration and a narrow photon flux density ranging between 37 and 48 μmol photons · m^?2 · s^?1 were best suited for growth. The wavelength of the light source also impacted growth and a high cell density of 9.6 × 10⁵ cells · mL^?1 was achieved using a mixture of red and blue compared to warm white, red, or blue LEDs.     

Physico-chemical characterization of exomannan from <Emphasis Type="Italic">Rhodotorula acheniorum</Emphasis> MC

The batch fermentation of Rhodotorula acheniorum MC on a culture medium containing 5% sucrose, mineral salts and yeast extract at 26 °C for 96 h, with aeration at 0.75 v/v/m and agitation at 500 rev min ⁻¹ resulted in the synthesis of an exopolysaccharide (6.2 g l ⁻¹) which formed two fractions upon precipitation. The fractions were purified to a carbohydrate content of 98.2% for fraction I and 87.3% for fraction II. Mannose was the main monosaccharide component in a 92.8% concentration in fraction I and a 90.6% concentration in Fraction II. The exopolysaccharide was thus a mannan. The gel chromatograms confirmed the chemical composition of both fractions. The molecular weight of mannan I was 310 kD, whereas that of mannan II was 249 kD. The mannan I intrinsic viscosity [η]=6.23 dl g⁻¹ was higher than that of mannan II [η]=2.73 dl g⁻¹. The water-binding capacity of the mannan samples was established within the 1.2–3.5 g g⁻¹ range. The multiplicative model [η]=387.22. D_r^−0.1913. T^−1.095. C^1.814 describing the effect of the velocity gradient D_r, the exomannan concentration C and the temperature T on the dynamic viscosity values η of polymer solutions was obtained.     

Carbon dioxide biofixation by Chlorella vulgaris at different CO2 concentrations and light intensities

In order to develop an effective CO₂ mitigation process using microalgae for potential industrial application, the growth and physiological activity of Chlorella vulgaris in photobioreactor cultures were studied. C. vulgaris was grown at two CO₂ concentrations (2 and 13% of CO₂ v/v) and at three incident light intensities (50, 120 and 180 μmol m^?2 s^?1) for 9 days. The measured specific growth rate was similar under all conditions tested but an increase in light intensity and CO₂ concentration affected the biomass and cell concentrations. Although carbon limitation was observed at 2% CO₂, similar cellular composition was measured in both conditions. Light limitation induced a net change in the growth behavior of C. vulgaris. Nitrogen limitation seemed to decrease the nitrogen quota of the cells and rise the intracellular carbon:nitrogen ratio. Exopolysaccharide production per cell appeared to be affected by light intensity. In order to avoid underestimation of the CO₂ biofixation rate of the microalgae, exopolysaccharide production was taken into account. The maximum CO₂ removal rate (0.98 g CO₂ L^?1 d^?1) and the highest biomass concentration (4.14 g DW L^?1) were determined at 13% (v/v) CO₂ and 180 μmol m^?2 s^?1. Our results show that C. vulgaris has a real potential for industrial CO₂ remediation.     

Monitoring microalgal growth of Chlorella minutissima with a new all solid-state contact nitrate selective sensor

As third generation feedstock, microalgae are microorganisms that can grow only in the optimum conditions. There are parameters including the concentration of macro and microelements in nutrient solution, pH, temperature and light intensity that have significant impact on microalgal growth. In recent years, various sensing devices have been developed for sensitive measurement of these parameters during microalgal growth. In this study, a new potentiometric nitrate selective sensor was developed to indicate the nitrate uptake of microalgae and the effect of nitrate nutrient on microalgal growth, specifically, and this sensor was successfully applied to determine nitrate concentration in medium during microalgal growth. Moreover, the effects of nitrate, carbonate and phosphate concentration in the growth medium on biomass production of Chlorella minutissima were investigated by using Box–Behnken design method, and optimum conditions were determined for the highest biomass production of microalgae. As a result of the experiments, it was seen that the highest C. minutissima production was achieved using the medium consist of 2.63 g/L NaNO₃, 0.35 g/L Na₂CO₃ and 0.4 g/L KH₂PO_4. Statistically, it was observed that there was a proportional relationship between the microalgae production and investigated parameters such as carbon, nitrogen and phosphate amounts of culture mediums. The electrode showed a wide linear range between 1.0 × 10⁻¹ and 5.0 × 10⁻⁵ M with a detection limit of the 5 × 10⁻⁶ M and the response time was found as 10 s. The results showed that developed nitrate selective sensor could be successfully applied for continuous measurement of nitrate in microalgal productions at reduced cost.     

Effect of light intensity on the proximate biochemical and fatty acid composition of Isochrysis sp. and Nannochloropsis oculata for use in tropical aquaculture

The total protein, carbohydrate, lipid and ash compositions, and fatty acid contents of two species of marine microalgae, the eustigmatophyte Nannochloropsis oculata (formerly ‘Chlorella sp., Japan’) and the chrysophyte Isochrysis sp. (Tahitian) used in tropical Australian mariculture, were studied. The microalgae were grown under a range of culture conditions (41 and 601 laboratory culture, 3001 bag culture, and 80001 outdoor culture) and four light regimes (100 to 107 μ E m⁻² s⁻¹, 240 to 390 μ E m⁻² s⁻¹, 340 to 620 μ E m⁻² s⁻¹, and 1100 to 1200 μE m⁻² s⁻¹ respectively) to determine the effect of light intensity on the chemical composition of large scale outdoor cultures. Laboratory and bag cultures were axenic and cultured in Walne medium while outdoor cultures were grown in a commercial medium designed for optimum nutrition in tropical outdoor aquaculture operations. Change in growth medium and photon flux density produced only small changes in the proximate biochemical composition of both algae. N. oculata and Isochrysis sp. both showed a trend towards slightly lower carbohydrate and higher chlorophyll a in shaded outdoor culture. Isochrysis sp. showed significant concentrations of the essential polyunsaturated fatty acid 22:6(n−3) (docosahexaenoic acid) from 5.3 to 10.3% of total fatty acid, and 20:5(n−3) (eicosapentaenoic acid) ranged from 0.6 to 4.1%. In contrast, N. oculata had high concentrations of 20:5(n−3) (17.8 to 39.9%) and only traces of 22:6(n−3). The fatty acid composition of Isochrysis sp. grown at high photon flux density (1100–1200 μE m⁻² s⁻¹) under outdoor culture showed a decrease in the percentage of several highly unsaturated fatty acids, including 20:5(n−3), and an increase in 22:6(n−3). N. oculata showed a similar decrease in the percentage of 20:5(n−3). High light intensity caused a decrease in the ratio of total C₁₆ unsaturated fatty acids to saturated 16:0 in N. oculata, and a decrease in the ratio of total C₁₈ unsaturated fatty acids to saturated 18:0 together with a decrease in the ratio of total unsaturated fatty acids to total saturated fatty acids in both microalgae.     

Synergistic Interaction Between Anabaena and Zoogloea spp. in Carbon Dioxide-Limited Continuous Cultures

Flocs consisting of Anabaena and Zoogloea spp. were used as a model system for the study of planktonic phototroph-heterotroph interactions. In CO₂-limited continuous culture (3.2 μmol of NaHCO₃ liter⁻¹ h⁻¹, 1.5 μmol of glucose liter⁻¹ h⁻¹, pH 8.5, D = 0.026 h⁻¹), the biomass of the phototroph increased 8.6-fold due to association. However, direct CO₂ exchange accounted for only a 3.8-fold increase. When the glucose supply rate was increased to 7.5 μmol liter⁻¹ h⁻¹, there was a 26-fold increase in biomass. When CO₂ was supplied in excess, there was no difference due to association. In batch culture, using the same medium, the specific growth rate was 0.029 h⁻¹ for the phototroph alone and 0.047 h⁻¹ for the phototroph in association with the heterotroph. The stimulatory effect of the heterotroph was found only under CO₂-limiting conditions and was directly related to the concentration of organic matter supplied in the medium. Both the biomass and the growth rate of the Anabaena sp. were increased by association with the Zoogloea sp. Thus, dissolved organic matter may substitute for CO₂ to maximize both growth rate and biomass production by phototrophs when heterotrophic bacteria are present.     

Selective production of bikaverin in a fluidized bioreactor with immobilized Gibberella fujikuroi

The best culture medium composition for the production of bikaverin by Gibberella fujikuroi in shake-flasks, i.e. 100 g glucose l^–1; 1 g NH₄Cl l^–1; 2 g rice flour l^–1; 5 g KH₂PO₄ l^–1 and 2.5 g MgSO₄ l^–1, was obtained through a fractional factorial design and then scaled-up to a fluidized bioreactor. The effects of carbon and nitrogen concentrations, inoculum size, aeration, flow rate and bead sizes on batch bikaverin production using immobilized G. fujikuroi in a fluidized bioreactor were determined by an orthogonal experimental design. Concentrations of up to 6.83 g bikaverin l^–1 were obtained when the medium contained 100 g glucose l^–1 and 1 g NH₄Cl l^–1 with an inoculum ratio of 10% v/v, an aeration rate of 3 volumes of air per volume of medium min^–1, and a bead size of 3 mm. Based on dry weight, the bikaverin production was 30–100 times larger than found in submerged culture and approximately three times larger than reported for solid substrate fermentation.     

Model aided design of repeated fed-batch penicillin fermentation

Structured models of antibiotic fermentation that quantify maturation and aging of product forming biomass are fitted to experimental data. Conditions of superiority of repeated fed batch cultivation are characterized on the basis of a performance criterion that includes penicillin productivity and costs of operation. Emphasis is placed on the relevance of such research to the model aided design of optimal cyclic operation.List of Symbols c IU/mg cost factor - D s^–1 dilution rate - J IU · cm^–3 · h^–1 net productivity - k _p IU · mg^–11 · h^–1 specific product formation rate - k _pm IU · mg^–1 · h^–1 maximum specific product formation rate - p IU/cm³ concentration of penicillin - T s final time of fermentation - t s fermentation time - X kg/m³ concentration of biomass dry weight - X ₁kg/m³ concentration of young, immature biomass - X ₂ kg/m³ concentration of mature product forming biomass - X _c kg/m³ biomass concentration of the end of growth phase - X _mkg/m³ maximum biomass concentration Greek Letters s^–1 specific maturation rate - s^–1 specific aging rate - s^–1 specific growth rate - _m s^–1 maximum specific growth rate - _p s^–1 specific growth rate during the product formation phase - s cycle time - % volume fraction of draw-off Abbreviations CC chemostat culture - RFBC repeated fed batch culture - RBC repeated batch culture     

Optimization of eicosapentaenoic acid production byPhaeodactylum tricornutumin the flat panel airlift (FPA) reactor

Biomass and eicosapentaenoic acid (EPA) productivities were investigated in a flat panel airlift loop reactor ideally mixed by static mixers. Growth with ammonium, urea and nitrate as nitrogen source were performed at different aeration rates. Cultures grew on ammonium but the decay of pH strongly inhibited biomass increase. On urea biomass productivity reached 2.35 g L^–1d^–1at an aeration rate of 0.66 vvm (24 h light per day, 1000 mol photon m^–2s^–1). Aeration rates between 0.33 vvm and 0.66 vvm and maximal productivities on urea were linearly dependent. Productivity on nitrate never exceeded 1.37 g L^–1d^–1. In the range of maximum productivity photosynthesis efficiency of 10.6% was reached at low irradiance (250 mol photon m^–2s^–1). Photosynthesis efficiency decreased to 4.8% at 1000 mol photon m^–2s^–1. At these high irradiances the flat panel airlift reactor showed a 35% higher volume productivity than the bubble column. At continuous culture conditions the influence of CO₂concentration in the supply air was tested. Highest productivities were reached at 1.25% (v/v) CO₂where the continuous culture yielded 1.04 g L^–1d^–1(16 h light per day, 1000 mol photon m^–2s^–1). The average EPA content amounted to 5.0% of cell dry weight, that resulted in EPA productivities of 52 mg L^–1d^–1(continuous culture, 16 h light per day) or 118 mg L^–1d^–1(batch culture, 24 h light per day).     

Anthocyanin production in callus cultures of Cleome rosea: Modulation by culture conditions and characterization of pigments by means of HPLC-DAD/ESIMS

Leaf and stem explants of Cleome rosea formed calluses when cultured on MS medium supplemented with different concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) or 4-amino-3,5,6-trichloropicolinic acid (PIC). The highest biomass accumulation was obtained in the callus cultures initiated from stem explants on medium supplemented with 0.90 μM 2,4-D. Reddish-pink regions were observed on callus surface after 6–7 months in culture and these pigments were identified as anthocyanins. Anthocyanins production was enhanced by reducing temperature and increasing light irradiation. Pigmented calluses transferred to MS1/2 with a 1:4 ratio NH₄⁺/NO₃⁻, 70 g L⁻¹ sucrose and supplementation with 0.90 μM 2,4-D maintained a high biomass accumulation and showed an increase of 150% on anthocyanin production as compared with the initial culture conditions. Qualitative analysis of calluses was performed by high performance liquid chromatography coupled to diode array detector and electrospray ionization mass spectrometry (HPLC-DAD/ESIMS). Eleven anthocyanins were characterized and the majority of them were identified as acylated cyanidins, although two peonidins were also detected. The major peak was composed by two anthocyanins, whose proposed identity were cyanidin 3-(p-coumaroyl) diglucoside-5-glucoside and cyanidin 3-(feruloyl) diglucoside-5-glucoside.     

Propagation of Rhizopus javanicus Biosorbent

After propagation of Rhizopus javanicus in defined media containing glucose, urea, and mineral salts in deionized distilled water, the ability of the nonliving biomass to sequester cupric ion was assayed. Growth, uptake capacity (saturation uptake at >1 mM Cu²⁺ concentration in solution), and biosorptive yield (biomass concentration × uptake capacity) were increased by augmentation of the growth medium with mineral salts once growth was under way. In the stationary phase, the uptake capacity of mycelia, which were normally a poor biosorbent, was improved within 4 h of trace metal addition to the growth medium. Growth of the culture was inhibited by excessive concentrations (0.04 to 40 μM) of metals in the medium in the following order: Cu > Co ≥ Ni > Mn > Mo; zinc was not inhibitory at 40 μM, and chromium was stimulatory at 0.53 μM but slightly inhibitory at higher levels. Iron and potassium phosphate stimulated growth at levels of 0.53 and 40 mM, respectively. When R. javanicus was propagated in a medium with a high salt concentration, exponential growth (0.23 h⁻¹) to a biomass concentration of >3 g/liter and a biosorptive yield of >500 μmol/liter was achieved. It is evident that the powerful biosorbent characteristics of Rhizopus biomass led to depletion of available trace minerals in suspension culture, which in turn limited growth.     

The air‐lift photobioreactors with flow patterning for high‐density cultures of microalgae and carbon dioxide removal

A photobioreactor containing microalgae is a highly efficient system for converting carbon dioxide (CO₂) into biomass. Using a microalgal photobioreactor as a CO₂ mitigation system is a practical approach to the problem of CO₂ emission from waste gas. In this study, a marine microalga, Chlorella sp. NCTU‐2, was applied to assess biomass production and CO₂ removal. Three types of photobioreactors were designed and used: (i) without inner column (i.e. a bubble column), (ii) with a centric‐tube column and (iii) with a porous centric‐tube column. The specific growth rates (μ) of the batch cultures in the bubble column, the centric‐tube and the porous centric‐tube photobioreactor were 0.180, 0.226 and 0.252 day^?1, respectively. The porous centric‐tube photobioreactor, operated in semicontinuous culture mode with 10% CO₂ aeration, was evaluated. The results show that the maximum biomass productivity was 0.61 g/L when one fourth of the culture broth was recovered every 2 days. The CO₂ removal efficiency was also determined by measuring the influent and effluent loads at different aeration rates and cell densities of Chlorella sp. NCTU‐2. The results show that the CO₂ removal efficiency was related to biomass concentration and aeration rate. The maximum CO₂ removal efficiency of the Chlorella sp. NCTU‐2 culture was 63% when the biomass was maintained at 5.15 g/L concentration and 0.125 vvm aeration (volume gas per volume broth per min; 10% CO₂ in the aeration gas) in the porous centric‐tube photobioreactor.     

Variation in fatty acid composition of <Emphasis Type="BoldItalic">Arthrospira</Emphasis> (Spirulina) strains

A study of the fatty acid composition was made for 35 Arthrospira strains, concentrating on the most abundant fatty acids, the two polyunsaturated C₁₈ acids, linoleic and γ-linolenic acid, and palmitic acid. When grown at 30 ^∘C and low irradiance (10 μmol photon m⁻² s⁻¹), these three acids together formed 88–92% of total fatty acids. There were considerable differences in the composition of the two polyunsaturated acids. Depending on the strain, linoleic acid formed 13.1–31.5% and γ-linolenic acid formed 12.9–29.4% total fatty acids. In contrast, the range for palmitic acid was narrow: 42.3–47.6% of total fatty acids. Repeat experiments on several strains under defined conditions led to closely similar results for any particular environment, suggesting that fatty acid composition can be used as an aid in differentiating between strains. Five additional strains, which had apparently originated from the same original stock cultures as 3 of the 35 in the main study, but from different culture collections, were also assayed. With four strains the results were similar, irrespective of culture source, but with one strain marked differences occurred, especially in the polyunsaturated C₁₈ fatty acid fraction. These differences were independent of the age of the culture. In addition, straight morphotypes derived during repeat subcultures of four strains; each showed a similar fatty acid composition to that of the helical morphotypes of the same strains. A decrease in temperature from 30 to 20 ^∘C, an increase in irradiance (at 30 ^∘C) from 10 to 70 μmol photon m⁻² s⁻¹ and transfer to dark heterotrophy all favoured an increase in polyunsaturated C₁₈ fatty acids. The highest γ-linolenic acid content of any conditions was found for three strains grown heterotrophically on glucose in the dark at 30 ^∘C. A comparative study of six strains of Spirulina confirmed a previous study showing the absence of γ-linolenic acid in all Spirulina strains, thus permitting the separation of these two genera.     

Benthic microalgae in coral reef sediments of the southern Great Barrier Reef,Australia

The abundance and productivity of benthic microalgae in coral reef sediments are poorly known compared with other, more conspicuous (e.g. coral zooxanthellae, macroalgae) primary producers of coral reef habitats. A survey of the distribution, biomass, and productivity of benthic microalgae on a platform reef flat and in a cross-shelf transect in the southern Great Barrier Reef indicated that benthic microalgae are ubiquitous, abundant (up to 995.0 mg chlorophyll (chl) a m^–2), and productive (up to 110 mg O₂ m^–2 h^–1) components of the reef ecosystem. Concentrations of benthic microalgae, expressed as chlorophyll a per surface area, were approximately 100-fold greater than the integrated water column concentrations of microalgae throughout the region. Benthic microalgal biomass was greater on the shallow water platform reef than in the deeper waters of the cross-shelf transect. In both areas the benthic microalgal communities had a similar composition, dominated by pennate diatoms, dinoflagellates, and cyanobacteria. Benthic microalgal populations were potentially nutrient-limited, based on responses to nitrogen and phosphorus enrichments in short-term (7-day) microcosm experiments. Benthic microalgal productivity, measured by O₂ evolution, indicated productive communities responsive to light and nutrient availability. The benthic microalgal concentrations observed (92–995 mg chl a m^–2) were high relative to other reports, particularly compared with temperate regions. This abundance of productive plants in both reef and shelf sediments in the southern Great Barrier Reef suggests that benthic microalgae are key components of coral reef ecosystems.Communicated by Environmental Editor, B.C. Hatcher