Effects of fungicides on Aspergillus fumigatus

Aspergillus fumigatus was the most frequently isolated thermophilous fungus from green leaf surfaces. The application of fungicides significantly reduced the frequency of its occurrence there. A. fumigatus was relatively tolerant to fungicides. On Captan-, Thiram-, and Verdasan-treated leaves, A. fumigatus constituted 66%–80% of the total number of isolates obtained at 45°C from each treatment while Dicloran did not depress the percentages. At 45°C, A. fumigatus was found to be strongly cellulolytic with a slow rate of radial extension on YpSs agar and rapid rate of mycelial growth in Czapek Dox liquid medium. Increasing concentrations of all four fungicides reduced or prevented growth, sporulation, starch depletion and cellulose clearing of A. fumigatus. The fungus could tolerate higher concentrations of HgCl₂ than of Verdasan. 0.5 g/ml of the four fungicides altered the rates of mycelial growth but not the maximum amount of mycelial dry weight attained.     

Respiration rates of Chiloplacus sp. and other arctic nematodes at low and high temperatures

Summary Respiration of an undescribed species of soil nematode of the genus Chiloplacus from the Canadian High Arctic was measured at 2°, 5°, 10°, 15°, 20° and 25°C. The corresponding metabolic rates were 0.2697×10^-3 l, 0.3406×10^-3 l, 0.8408×10^-3 l, 0.8539×10^-3 l, 1.8420×10^-3 l and 2.9360×10^-3 l O₂ ind^-1 h^-1, respectively, for a nematode of 1.0 g dry weight. The relationship between respiration and dry weight for Chiloplacus sp. at 10°C is described by the function log R=-3.0693+0.8844 log W. Q₁₀ values for the 2°–5°, 5°–10°, 10°–15°, 15°–20° and 20°–25°C temperature intervals were 2.18, 6.09, 1.03, 4.65 and 2.54, respectively. Chiloplacus sp. showed raised metabolic rates at low tempetatures compared with species from warmer environments. Metabolic rates of representative samples of the soil, nematode fauna (dominated by individuals of the genus Plectus) from the same location were 0.1593×10^-3 l, 0.3603×10^-3 l and 0.5332×10^-3 l O₂ ind^-1 h^-1 at 5°, 10° and 15°C for an average nematode of 0.4297 g dry weight.     

Cellulolytic and physiological properties of Clostridium thermocellum

Three strains of Clostridium thermocellum obtained from various sources were found to have nearly identical deoxyribonucleic acid guanosine plus cytosine contents that ranged from 38.1–39.5 mole-%. All strain examined fermented only cellulose and cellulose derivatives, but not glucose, or xylose or other sugars. The principal cellulose fermentation products were ethanol, lactate, acetate, hydrogen and carbon dioxide. Growth of C. thermocellum on cellulose resulted in the production of extracellular cellulase that was non-oxygen labile, was thermally stable at 70° C for 45 min and adsorbed strongly on cellulose. Production of cellulase during fermentation correlated linearly with growth and cellulose degradation. Both the yield and specific activity of crude cellulase varied considerably with the specific growth substrates. Highest cellulase yield was obtained when grown on native cellulose, -cellulose and low degree of polymerization cellulose but not carboxymethylcellulose or other carbohydrate sources. Cellulase activity was not detected when cells were grown on cellobiose. Crude extracellular protein preparations lacked proteolytic and cellobiase activity. The pH and temperafure optima for endoglucanase activity were 5.2 and 65° C, respectively, while that of the exoglucanase activity were 5.4 and 64° C, respectively. The specific activity at 60° c for exoglucanase and endoglucanase of crude cellulase obtained from cells grown on cellulose (MN 300) was 3.6 moles reducing sugar equivalents released per h (unit)/mg of protein and 1.5 mole reducing sugar equivalent released per min (unit)/mg of protein, respectively. The yield of endoglucanase was 125 units per g of cellulose MN 300 degraded and that of exoglucanase was 300 units per g of cellulose MN 300 degraded. Glucose and cellobiose were the hydrolytic end products of crude cellulase action on cellulose, cellotraose and cellotriose in vitro.     

Der Entwicklungscyclus mit Sexualphase bei der marinen Diatomee Coscinodiscus asteromphalus

Zusammenfassung Die Kultur der großen marinen Diatomee Coscinodiscus asteromphalus wird beschrieben. Die Synchronisation der vegetativen Stadien aus dem Entwicklungscyclus mit Sexualphase wird durch Messung der Valvendurchmesser charakterisiert. Die Art entwickelt sich von Stadien mit 200 m Valvendurchmesser (V.-D.), die nicht sexuell induzierbar sind, zu Stadien mit 80–90 m V.-D. mit einem Optimum der Induzierbarkeit und weiter zu Stadien mit 55–60 m V.-D. Bei dieser Größe ist keine weitere mitotische Zellteilung mehr möglich. Entwicklungsstadien mit 200–190 m, 140–130 m und 100–90 m. V.-D. zeigen bei 24°C und bei 18°C die gleiche Generationszeit im mitotischen Entwicklungscyclus von 1 bzw. 0,6 Zellteilungen pro Tag. Der Valvendurchmesser verringert sich bei dieser Art um 1,5 m bei 24°C und 1,4 m bei 18°C während einer Zellteilung.

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The life cycle with sexual phase in the marine diatom Coscinodiscus asteromphalus I. Culture and synchronisation of developmental stages

Summary Culture-conditions for the large marine centric diatom Coscinodiscus asteromphalus are described. The cells grow in a defined medium in a light-dark regime of 14: 10 h. Synchronization of different stages of the sexual life cycle is characterized by measuring the valve diameter (v.d.) of the cells. The cells develop from stages with 200 m v. d. (not sexually inducible) to stages with 80–90 m v. d. (optimum for sexual induction), and further to stages with 55–60 m v. d., where no following mitotic cell division is possible. The length of the pervalvar axis does not change during this development. Different stages (200–190m, 140–130 m and 100–90 m v. d.) grow with the same doubling time during their mitotic life cycle: 1 cell division per day at 24° C and 0.6 cell divisions per day at 18°C. During one cell division the valve diameter of this species decreases by about 1.5m at 24°C and by 1.4 m at 18°C. Therefore, the development from stages with 200 m v.d. to stages with 60 m v. d. takes between 90 days at 24°C and 165 days at 18°C.

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Teile einer Habilitationsschrift der Naturwissenschaftlichen Fakultät der Universität Marburg (Lahn).     

Grazing by a dominant rotifer Conochilus unicornis Rousselet in a mountain lake: in situ measurements with synthetic microspheres

Grazing rates of zooplankton were analysed in the summer of 1999 in Yellow Belly Lake, an oligotrophic system in the Sawtooth Mountains of Idaho (U.S.A.). The colonial rotifer Conochilus unicornis was a dominant species in the epilimnion, with densities reaching 20 colonies l^–1 (ca. 400 ind. l^–1). Clearance rates were measured with an in situ Haney Grazing chamber and synthetic microspheres 5, 9 and 23m in diameter. At epilimnetic temperatures of around 14 °C, mean clearance rates for 5m particles ranged from 30 to 65 l ind.^–1 h ^–1. Clearance rates were 2–9 times higher on the 5m spheres than on the 9 m spheres, and C. unicornis almost never fed on the 23 m spheres. Grazing rates did not change over the diel cycle. Clearance rates declined more than 10-fold as temperatures declined from 14 °C in the epilimnion to 7 °C in the metalimnion. In the epilimnion, grazing by C. unicornis was more important than grazing by crustaceans in the community, at least on particles 9m. The results show the importance of grazing by rotifers in lakes, and the significance of spatial variations that influence grazing rates.     

Growth of the thermotolerant yeast,Candida valida,on ethanol: Dependences of maximal growth rate and cell biomass yield on temperature

Summary Growth of Candida valida on ethanol in pH-auxostat and chemostat has been studied. Maximal growth rate, _m, and cell biomass yield, Y _s, display the Arrhenius dependence on temperature within the ranges 18°–30° C and 30°–36° C and an abrupt fall above 36° C. The temprature dependence of both parameters has breaks at 30° C and 36° C. Activation energies have been measured for both _m and Y _s. The reason for a weaker effect of temperature on Y _s than on _m is discussed.     

Effects of pre-storage conditions on storage of in vitro cultures of Nephrolepis exaltata (L.) Schott and Cordyline fruticosa (L.) A. Chev.

In vitro cultures of Nephrolepis exaltata and Cordyline fruticosa were stored at 5°, 9° or 13°C, at a low irradiance (3–5 mol m^–2 s^–1) or in darkness. Prior to storage the cultures were subjected to 18°, 21°, 24° or 27°C and 15, 30 or 45 mol m^–2 s^–1 in a factorial combination.The optimal storage conditions for Nephrolepis were 9°C in complete darkness. These cultures were still transferable to a peat/perlite mixture at the end of the experimental period of 36 months.The optimal storage conditions for Cordyline were 13°C and a low light level (±3–5 mol m^-2 s^-1). When the pre-storage conditions were normal growth room conditions (24°C and 30 mol m^-2 s^-1), in vitro cultures could be stored for 18 months. With the most favourable pre-storage treatment (18°C and 15 mol m^-2 s^-1) some cultures still had green shoots after 36 months of storage, but did not survive transfer to peat/perlite.Pre-conditioning before storage was most favourable for Nephrolepis, and not that important, but still favourable, for Cordyline. There was an interaction between pre-storage temperature and pre-storage irradiance. For both species a high irradiance level was less favourable than a low irradiance level when combined with high growth room temperatures.Abbreviations BA 6-benzyladenine - IAA indole-3-acetic acid - NOA 2-naphthoxyacetic acid     

Photosynthesis and growth rates of Laurencia brongniartii J. Agardh (Rhodophyta, Ceramiales) in preparation for cultivation

Laurencia brongniartii is usually found at depths below 4 m, but can be found in shallow subtidal areas in crevices and on the walls of a coral reef in Amami Oshima Island, Kagoshima Prefecture, Japan, where irradiances were significantly lower than those at similar depths in open water. In preparation for the possible cultivation of this species for its antibiotic compounds, the effects of temperature and irradiance on photosynthesis and growth were measured. Photosynthesis and growth rates of L. brongniartii explants were highest at 26 and 28 °C, which closely corresponded to temperatures found during August to late December when it was most abundant. The estimated maximum photosynthesis rate (P _max) was 4.41 mol photon m^–2 s^–1 at 26 °C and 4.07 mol photon m^–2 s^–1 at 28 °C. Saturating irradiance occurred at 95 mol photon m^–2 s^–1 at 26 °C and 65 mol photon m^–2 s^–1 at 28 °C. In contrast, growth experiments at 41.7 mol photon m^–2 s^–1 caused bleaching of explants and the maximum growth rate observed during the study was 3.02 ± 0.75% day^–1 at 28 °C and 25 mol photon m^–2 s^–1. The difference in the saturating irradiance for photosynthesis and the irradiance that caused bleaching in growth experiments suggests that long-term exposure to high irradiance was detrimental and should be addressed before the initiation of large scale cultivation.     

Characterization of dihydropyrimidinase from Pseudomonas stutzeri

Dihydropyrimidinase from Pseudomonas stutzeri ATCC 17588 was purified 100-fold and characterized. It was found that dihydrouracil, dihydrothymine and hydantoin could serve as substrates for the partially purified enzyme. The K _m values for dihydrouracil, dihydrothymine and hydantoin were determined to be 19.6 M, 21.3 M and 36.4 M, respectively, while their respective V _max values were 0.836 mol/min, 0.666 mol/min and 2.21 mol/min. Between pH 7.5 and 9.0, enzyme activity was shown to be maximal. The optimum temperature for enzyme activity was 45 °C. Using gel filtration, the molecular weight of the enzyme was calculated to be approximately 115000 Da. Metal ions were found to influence the level of enzyme activity. Dihydropyrimidinase activity was stimulated by magnesium ions and inhibited by either zinc or copper ions.     

Minimal growth in vitro conservation of coffee (Coffea spp.)

We have studied the influence of low concentrations of 6-benzyladenine on growth limitation, in order to preserve coffee germplasm through a microcutting collection. Concentrations of 0 M, 1.3 M and 4.4 M were compared in four species: Coffea congensis, C. canephora, C. liberica and C. racemosa. After six months, microcutting behaviour varied between the different treatments, and a species effect was observed. The slow growing species (C. liberica and C. congensis) needed 1.3 M; the others coffee species (C. canephora and C. racemosa) exhibited moderate caulogenesis on 6-benzyladenine-free medium. Zero and low concentrations did not affect survival rates. In conclusion 1.3 M seems most appropriate for conserving all four species.Abbreviation BA 6-benzyladenine     

Changes with age in the absorption spectrum of chlorophyll α in a diatom

Summary Absorption spectra of a young and an old culture of the diatom Pheodactylum tricornutum were measured in thin layers between two opal glass sheets. The spectra at 24° and at -196°C were replotted to give equal areas from 730–625 m to allow direct comparison. At 24°C the spectrum for the difference between the two cultures had a negative component of 18 m half width centered at 675 m and a positive region of W_0.5=26 m near 700 m.The spectra at -196°C may be somewhat distorted by clumping of the cells during freezing but nevertheless the 16 day culture clearly showed a smaller proportion of Ca 670 to Ca 680. This older culture has a shoulder due to a 707 m component. The difference curve at -196°C shows the decrease of an unsymmetrical band peaking at 669 m and an increase at 695 m in addition to the 707 m component. Due to the possibility of distortion, the presence of an actual component at 695 is doubtful in these particular cultures.The room temperature spectrum in the chloropyhll a region for the 5 day culture can be closely fitted by a single probability curve at 675 m having a half-width of 31 m. The sum of two components, with widths more reasonable for chlorophylls, also matched the data well enough. These two probability curves, of 22 m half width, centered on 669 and 683.2 m and had a height ratio, h₆₆₉/h₆₈₃ of 1.18. In the 16 day culture the ratio for these bands changed to 1.11 and there was extra absorption around 700 m.Dedicated to Professor C. B. van Niel on the occasion of his 70th birthday     

In vitro multiplication of Quercus leucotrichophora and Q. glauca: Important Himalayan oaks

Multiple shoots of Quercus leucotrichophora L. and Q. glauca Thunb. were induced from the intact embryos (decoated seeds) as well as from the cotyledonary nodes (with attached cotyledons but without radicle and primary shoot) of 3-weeks old in vitro grown seedlings on Woody Plant (WP; Lloyd and McCown, 1980) and Murashige and Skoog (MS; 1962) media supplemented with 6-benzyladenine (BA), either alone or in combination with gibberellic acid (GA₃)/ indole-3-butyric acid (IBA). BA (22.19 M) was effective for induction of multiple shoots and addition of GA₃ to the medium further enhanced the shoot number and shoot height but resulted in shoot thinness. High frequency shoot multiplication was achieved using cotyledonary nodes. Shoots were further multiplied from the original explant on WP medium supplemented with BA (22.19 M). Nearly 78% and 67% rooting was obtained in Q. leucotrichophora and Q. glauca microshoots (3–4 cm high), respectively on 1/2 strength WP medium supplemented with IBA (14.76 M). However, this was associated with basal callus formation. Treatment with IBA (25–100 M) for 24 or 48 h followed by transfer to PGR free 1/2 strength WP medium not only improved the rooting percentage but also avoided basal callus formation. IBA at 100 M for 24 h was most effective (90% and 100% rooting in Q. leucotrichophora and Q. glauca, respectively). In vitro rooted plants were hardened and established in garden soil.Growth performance of 6-month-old in vitro raised plants was compared with ex vitro plants (seedlings) of the same age. The photosynthesis and transpiration rates of eight months old in vitro and ex vitro raised plants of both species were measured under different light (0, 600, 900, 1200, 1500 and 2000 mol m^–2s^–1) and temperature (20, 25, 30, 35 and 40 °C). Light optimum for photosynthesis was around 2000 mol m^–2s^–1 in Q. leucotrichophora and around 1500 mol m^–2s^–1 in Q. glauca whereas optimum temperature for photosynthesis was 25 °C in Q. leucotrichophora and 30 °C in Q. glauca. The rate of transpiration at different temperatures (20–40 °C), in the two species, increased with increase in the light intensity up to the highest level, i.e., 2000 mol m^–2s^–1. Temperatures beyond 35 °C adversely affected the rate of transpiration in in vitro raised as well as ex vitro plants of both the species. In vitro raised and hardened plants of both the species were comparable to ex vitro plants in terms of gas and water vapour exchange characteristics, within the limits of this study.     

Pseudocyphellaria dissimilis: a desiccation-sensitive,highly shade-adapted lichen from New Zealand

Summary Pseudocyphellaria dissimilis, a foliose, cyanobacterial lichen, is shown not to fit into the normal ecological concept of lichens. This species is both extremely shade-tolerant and also more intolerant to drying than aquatic lichens previously thought to be the most desiccation-sensitive of lichens. Samples of P. dissimilis from a humid rain-forest site in New Zealand were transported in a moist state to Germany. Photosynthesis response curves were generated. The effect of desiccation was measured by comparing CO₂ exchange before and after a standard 20-h drying routine. Lichen thalli could be equilibrated at 15° C to relative humidities (RH) from 5% to almost 100%. Photosynthesis was saturated at a photosynthetically active radiation (PAR) level of 20 mol m^-2 s^-1 (350 bar CO₂) and PAR compensation was a very low 1 mol m^-2 s^-1. Photosynthesis did not saturate until 1500 bar CO₂. Net photosynthesis was relatively unaffected by temperature between 10° C and 30° C with upper compensation at over 40° C. Temporary depression of photosynthesis occurred after a drying period of 20 h with equilibration at 45–65% relative humidity (RH). Sustained damage occurred at 15–25% RH and many samples died after equilibration at 5–16% RH. Microclimate studies of the lichen habitat below the evergreen, broadleaf forest canopy revealed consistently low PAR (normally below 10–20 mol m^-2 s^-1) and high humidities (over 80% RH even during the day time). The species shows many features of an extremely deep shade-adapted plant including low PAR saturation and compensation, low photosynthetic and respiratory rates and low dry weight per unit area.     

Phytotoxic activity of selected water-soluble metabolites of Fusarium against Lemna minor L. (Duckweed)

Phytotoxicity and inhibitory effects of the fusarial toxins fumonisin B₁ (FB₁) [m.p. 103–105 °C], fusaric acid [m.p. 106–107 °C], butenolide (4-acetamido-4-hydroxy-2-butenoic acid lactone) [116–117 °C], 9, 10-dihydroxyfusaric acid [m.p. 150–155 ° C], and moniliformin on chlorophyll synthesis in the aquatic macrophyte Lemna minor (duckweed) were examined. FB₁ proved to be most active, reducing the growth of L. minor fronds and their ability to synthesize chlorophyll by 53% and 59%, respectively, at 0.7 g/ml. The growth rate of L. minor was reduced 59% by 6.7 g/ml fusaric acid, 62% by 66.7 g/ml butenolide, and 22% by 66.7 g/ml 9,10-dihydroxyfusaric acid. Moniliformin was the least phytotoxic to L. minor, with only a 16% suppression of growth rate and a 54% reduction in chlorophyll at 66.7 g/ml.The mention of firm names or trade products does not imply that they are endorsed or recommended by the US Department of Agriculture over other firms or similar products not mentioned.     

The combined relationship of temperature and molybdenum concentration to nitrogen fixation byAnabaena cylindrica

The joint effects of growth temperature, incubation temperature, and molybdenum concentration on the nitrogen fixation rate ofAnabaena cylindrica were determined using the acetylene-reduction technique. The nitrogen-fixation response to increased molybdenum concentration varied among three growth temperatures (15°, 23°, and 30° C). The pattern of rate change was similar within a growth temperature but increased overall in magnitude with the three incubation temperatures (also 15°, 23°, and 30° C). The maximum rate of nitrogen fixation occurred at 30°C regardless of previous growth temperature. The minimum molybdenum concentration necessary to yield substantial acetylene reduction varied with growth temperature: at 15°C, 15g 1^–1 was effective; at 23°C, less than 5g 1^–1 was effective; and at 30°C, 50g 1^–1 was effective. At all three growth temperatures, increases in molybdenum concentration above the minimum effective concentration produced increases in acetylene reduction. However, at higher molybdenum concentrations inhibition of nitrogen fixation occurred.     

Fervidobacterium islandicum sp. nov., a new extremely thermophilic eubacterium belonging to the “Thermotogales”

An extremely thermophilic anaerobic fermentative eubacterium growing at temperatures between 50 and 80°C (opt.: 65°C) was isolated from an Icelandic hot spring. The cells were Gram-negative motile rods, about 1.8 m in length, and 0.6 m in width occurring singly and in pairs. About 50% of the cells formed large spheroids at one end similar to Fervidobacterium nodosum. The new isolate H 21 differed from Fervidobacterium nodosum by a 6 mol % higher GC-content of its DNA (41 mol %), its ability to grow on cellulose, and insignificant DNA homology. The lipids of isolate H 21 were similar to that of members of Thermotogales. 16S rRNA sequencing of isolate H 21 and Fervidobacterium nodosum indicated (a) that isolate H 21 represents a new species of the genus Fervidobacterium which we name Fervidobacterium islandicum and (b) that the genus Fervidobacterium belongs to the Thermotogales branch.     

Aspects of cold hardiness in Steganacarus magnus (Acari: Cryptostigmata)

Supercooling points and the presence of antifreeze compounds were measured for both nymphs and adults of Steganacarus magnus (Nicolet) collected from a coniferous forest soil in southern England in March, June and November. The mean supercooling point of nymphs was –14.4°C and of adults, –11.7°C. Acclimation to low temperatures (1–2°C) did not alter these values significantly. The total concentration of antifreeze compounds in the nymphs was 4.46 g mg^-1 and in the adults 0.91 g mg^-1. These results are compared with similar data for other species of cryptostigmatic mites.     

Leaf photosynthetic characteristics of seedlings of actinorhizal Alnus spp. and Elaeagnus spp.

Single leaf photosynthetic characteristics of Alnus glutinosa, A. incana, A. rubra, Elaeagnus angustifolia, and E. umbellata seedlings conditioned to ambient sunlight in a glasshouse were assessed. Light saturation occurred between 930 and 1400 mol m^-2s^-1 PAR for all species. Maximum rates of net photosynthesis (Pn) measured at 25°C ranged from 12.8 to 17.3 mol CO₂m^-2s^-1 and rates of dark respiration ranged from 0.74 to 0.95 mol CO₂m^-2s^-1. These values of leaf photosynthetic variables are typical of early to midsuccessional species. The rate of Pn measured at optimal temperature (20°C) and 530mol m^-2s^-1 PAR was significantly (p<0.01) correlated with leaf nitrogen concentration (r=0.69) and negatively correlated with the mean area of a leaf (r=–0.64). We suggest that the high leaf nitrogen concentration and rate of Pn observed for Elaeagnus umbellata and to a lesser degree for E. angustifolia are genetic adaptations related to their crown architecture.Abbreviations Pn net photosynthesis     

Micropropagation of garlic through in vitro bulblet formation

We developed a novel micropropagation method for garlic (Allium sativum L.) by the combination of initial shoot-tip culture, shoot multiplication and in vitro bulblet formation. Garlic shoot-tips were cultured on LS medium containing 1 M indole-3-acetic acid (IAA) and 1 M 6-benzyladenine (BA) to regenerate proliferative shoots. These shoot-tips produced multiple shoots when transferred to modified LS medium containing 5 M 1-naphthaleneacetic acid (NAA) and 10 M BA, and cultured at 20°C under 12-h light conditions. Higher ratios of KNO₃/NH₄Cl in the media promoted multiple shoot formation, together with suppressing vitrification of these shoots. The proliferated shoots of early maturing cultivars produced bulblets by culture on LS growth regulator-free medium at 25°C under 16-h light. On the other hand, the late maturing cultivar, Howaito-roppen, formed bulblets after a low temperature treatment of the proliferated shoots for 6 months followed by culture on LS medium containing 6 to 12% sucrose for two months. The dormancy of the bulblets of cv. Howaito-roppen was broken by successive treatments at a high (35°C), a middle (20°C), and then a low (5°C) temperature.Abbreviations IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - BA 6-benzyladenine - LS Linsmaier and Skoog macro- and microelements     

The kinetics of photoinhibition and its recovery in the red alga Porphyridium cruentum

When Porphyridium cruentum cells were illuminated with high fluence rate between 1900 and 4800 mol photons m^-2s^-1, a decrease in the photosynthetic activity of the cells was observed. Within the time frame of 20 min, and under the fluence rates studied, the sum of photons to be absorbed by cells (mg of chlorophyll (Chl), sufficient to initiate photoinhibition was calculated to be 9235.8 mol. The minimal specific light absorption rate to initiate photoinhibition in P. cruentum ranges between 2.29 and 4.26 mol photons s^-1 mg^-1 chl.a. There was a linear relationship between the specific rate of photoinhibition and the specific light absorption rate. A photon number of 2.56×10⁴ mol mg^-1 chl.a photoinhibited photosynthesis instantaneously. At 15°C, no photoinhibitory effect was observed at 2300 mol photons m^-2 s^-1 even after 45 min of illumination. At the other extreme of 35°C, 84% inhibition of photosynthetic activity was observed within 10 min of exposure to 2300 mol photons m^-2 s^-1. Between 20 and 30°C, the photoinhibitory effect was comparable. Photoinhibited P. cruentum cells recovered readily when transferred to low light (90 mol photons m^-2 s^-1) and darkness, and the specific rate of recovery was independent of the light intensity to which the cells were exposed, during the photoinhibitory treatment.Abbreviations Chlorophyll QL, specific light absorption rate Publication No. 28 of the Microalgal Biotechnology Laboratory