Strong Photoinhibition Research Articles

Inhibition of photosystems I and II activities in salt stress-exposed Fenugreek ( Trigonella foenum graecum)

Fenugreek ( Trigonella foenum graecum) seedlings were exposed to increasing NaCl concentrations in the growth medium to examine the effect of salt stress on the electron transport reactions of photosynthesis. Activities of both photosystem II (PSII), measured by chlorophyll fluorescence, and photosystem I (PSI), measured by P700 photooxidation, were decreased by salt stress. The inhibition proceeded in a two step manner. At the lower salt concentrations used and shorter exposition periods, electron transfer between the quinone acceptors of PSII, Q A and Q B, was strongly retarded as shown by an increased amplitude of the OJ phase of the OJIP chlorophyll fluorescence induction traces and slowed chlorophyll fluorescence relaxation kinetics following a single turn-over flash. The above indicated a disturbance of the Q B binding site likely associated with the first step of photoinhibition. In the second step, strong photoinhibition was observed as manifested by increased F 0 values, declined F v / F 0 and loss of photoactive P700.

Aclimatação de mudas de acariquara à alta irradiância

O objetivo deste trabalho foi determinar as mudanças fisiológicas que ocorrem em acariquara (Minquartia guianensis) durante o processo de aclimatação à alta irradiância, bem como a estratégia de aclimatação dessa espécie. Plantas mantidas em baixa irradiância foram transferidas para alta irradiância por 290 dias. Durante esse período, foi medida a relação entre fluorescência variável e máxima (Fv/Fm), em folhas desenvolvidas à sombra e, após a senescência prematura por foto-oxidação, em folhas aclimatadas ao sol. Ao final do experimento, foram determinadas as características fotossintéticas e anatômicas da folha. A exposição à alta irradiância causou, logo após a transferência, forte fotoinibição e foto-oxidação parcial da folhagem, mas não provocou a morte da planta. Folhas produzidas no ambiente ensolarado apresentaram valores de Fv/Fm similares aos do controle. A fotossíntese saturada por luz e a fotossíntese saturada por CO2 foram 90 e 50% maiores em plantas aclimatadas à alta irradiância. A velocidade máxima de carboxilação da rubisco e a taxa máxima de regeneração da ribulose bisfosfato seguiram a mesma tendência. Folhas produzidas ao sol apresentaram maior densidade estomática e maior espessura foliar. A produção de folhas novas é a principal estratégia de aclimatação da acariquara à alta irradiância.

Comparative effects of the quality and quantity of light and temperature on the growth of Planktothrix agardhii and P. rubescens1

The effects of temperature, light intensity, and quality on the growth of the cyanobacteria Planktothrix agardhii (Gomont) Anag. et Komárek and P. rubescens (Gomont) Anag. et Komárek were assessed in batch cultures. The relative competitiveness of the green‐pigmented P. agardhii and the red‐pigmented P. rubescens was evaluated in separate and mixed cultures, under different light intensities and qualities (green, red, and white), and at two different temperatures, chosen as representative of the natural conditions favoring the respective blooms of each species. In monocultures, the P. rubescens strain appeared to be particularly well adapted to low intensities of green light and displayed a strong photoinhibition under high irradiance levels. The P. agardhii strain appeared less specialized with regard to light quality and also less sensitive to photoinhibition at higher irradiances. In competition experiments, temperature (15°C vs. 25°C) was the most important parameter in determining relative fitness of the species and competitive success. At 15°C, P. rubescens appeared to be much more competitive than P. agardhii, while P. agardhii was more competitive at 25°C. Under low irradiance, however, the pigmentation of these strains was of primary importance in determining the outcomes of the competition experiments. On the basis of our experimental results and on field observations, we propose that the successful growth leading to the proliferation of these two differently pigmented strains may largely depend on the combined conditions of light and temperature. The two strains, being genetically close relatives, could therefore be considered as two ecotypes that are adapted to different light and temperature environments. Competition experiments showed that the combination of these parameters largely controls the success of one strain in comparison to the other.

Functional in situ evaluation of photosynthesis-protecting carotenoids in mutants of the cyanobacterium Synechocystis PCC6803

Cyanobacteria possess different carotenoids as scavengers of reactive oxygen species. In Synechocystis PCC6803, zeaxanthin, echinenone, β-carotene and myxoxanthophyll are synthesized. By disruption of the ketolase and hydroxylase genes, it was possible to obtain mutants devoid of either zeaxanthin, echinenone, or a combination of both carotenoids. With these mutants, their function in protecting photosynthetic electron transport under high light stress as well as chlorophyll and carotenoid degradation after initiation of singlet oxygen or radical formation was analyzed. Wild type Synechocystis is very well protected against high light-mediated photooxidation. Absence of echinenone affects photosynthetic electron transport to only a small extent. However, complete depletion of zeaxanthin together with a modification of myxoxanthophyll resulted in strong photoinhibition of overall photosynthetic electron transport as well as the photosystem II reaction. In the double mutant lacking both carotenoids the effects were additive. The light saturation curves of photosynthetic electron transport of the high light-treated mutants exhibited not only a lower saturation value but also smaller slopes. Using methylviologen or methylene blue as a radical or singlet oxygen generators, respectively, massive degradation of chlorophyll and carotenoids, indicative of photooxidative destruction of the photosynthetic apparatus, was observed, especially in the mutants devoid of zeaxanthin.

Characterization of a pigment-deficient mutant of sunflower (Helianthus annuus L.) with abnormal chloroplast biogenesis, reduced PS II activity and low endogenous level of abscisic acid

Nuclear mutation xantha1 (xan1) arose spontaneously in sunflower and it appeared to have many pleiotropic effects. Homozygous xan1/xan1 seedlings, in field conditions, die after depletion of cotyledonary reserves and can be only recovered by selfing normal heterozygous plants (XAN1/xan1). xan1 mutant showed aberrant development of chloroplasts, deficient pigment content and reduced CO2 assimilation rate. Also, xan1 mutation negatively affected endogenous ABA levels both in non-stressed and dehydrated leaves. Photodestruction of both chlorophyll a and β-carotene in xan1 leaves directly correlated with light intensity. The activation of the xanthophyll cycle under low light (165 PPFD) and the appreciable amounts of zeaxanthin and antheraxanthin retained by xan1 mutant after a 24 h dark period were clear signals of a strong photoinhibition state. The unusual patterns of thylakoid proteins composition detected in seedlings grown under 165 PPFD, by using both denaturing and native electrophoresis analyses, were also indicative of high sensitivity of the thylakoid polypeptide complexes to light stress. Moreover, immunoblot analysis of psbA gene product (D1 protein) indicated that even at 165 PPFD the structure of photosystem (PS) II reaction center was severely damaged. Interestingly, xan1 seedlings grown in dim light (3.5 PPFD) revealed a severely altered chloroplast ultrastructure, despite the lack of photooxidative effects on leaf pigments and the lack of alterations in the electrophoretic profiles of thylakoid proteins. Electron microscopy indicated that XAN1 product is required for normal organization and distribution of thylakoids in the stroma. Chlorophyll a fluorescence analysis pointed to xan1 incompetence to fully express and retain PS II activity, even in the absence of photooxidation.

Characterization of a pigment-deficient mutant of sunflower (Helianthus annuus L.) with abnormal chloroplast biogenesis, reduced PS II activity and low endogenous level of abscisic acid

Nuclear mutation xantha1 (xan1) arose spontaneously in sunflower and it appeared to have many pleiotropic effects. Homozygous xan1/xan1 seedlings, in field conditions, die after depletion of cotyledonary reserves and can be only recovered by selfing normal heterozygous plants (XAN1/xan1). xan1 mutant showed aberrant development of chloroplasts, deficient pigment content and reduced CO2 assimilation rate. Also, xan1 mutation negatively affected endogenous ABA levels both in non-stressed and dehydrated leaves. Photodestruction of both chlorophyll a and β-carotene in xan1 leaves directly correlated with light intensity. The activation of the xanthophyll cycle under low light (165 PPFD) and the appreciable amounts of zeaxanthin and antheraxanthin retained by xan1 mutant after a 24 h dark period were clear signals of a strong photoinhibition state. The unusual patterns of thylakoid proteins composition detected in seedlings grown under 165 PPFD, by using both denaturing and native electrophoresis analyses, were also indicative of high sensitivity of the thylakoid polypeptide complexes to light stress. Moreover, immunoblot analysis of psbA gene product (D1 protein) indicated that even at 165 PPFD the structure of photosystem (PS) II reaction center was severely damaged. Interestingly, xan1 seedlings grown in dim light (3.5 PPFD) revealed a severely altered chloroplast ultrastructure, despite the lack of photooxidative effects on leaf pigments and the lack of alterations in the electrophoretic profiles of thylakoid proteins. Electron microscopy indicated that XAN1 product is required for normal organization and distribution of thylakoids in the stroma. Chlorophyll a fluorescence analysis pointed to xan1 incompetence to fully express and retain PS II activity, even in the absence of photooxidation.

Cold‐induced photoinhibition and growth of seedling snow gum (Eucalyptus pauciflora) under differing temperature and radiation regimes in fragmented forests

Fluorescence characteristics and growth of seedling snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) during autumn and winter were related to variation in radiation and temperature regime in a fragmented forest. Seedlings were planted in four treatments along transects perpendicular to tree island edges to characterize plant responses to microclimates ranging from those of cleared areas to those beneath forest canopies. Three‐dimensional mapping of seedling leaf display, in combination with information retrieved from hemispherical photographs about shading from overstory canopies, were used to calculate the intercepted amounts of direct radiation energy for unit area of leaves on clear days (IDRE). IDRE was highest on the outside, most variable at the edges and lowest well inside the tree islands. Minimum temperature decreased with increasing view of the sky. Photoinhibition, measured as decrease in Fv/Fm, was correlated with spatial and seasonal differences in weekly minimum temperature and IDRE. Seedlings in the open and under the most canopy cover, with low variability in IDRE in a scale of weeks, exhibited less variability in photoinhibition than those growing along forest edges. Seedlings in the open tended to be most photoinhibited and grew the most. The combination of increased IDRE with reduced minimum temperatures resulted in persistent and strong photoinhibition as the season progressed. Results are discussed in relation to the potential for seedling establishment following forest fragmentation.

The Effect of Photoinhibition on Photosynthetic Oxygen Production in the Brown Alga Dictyota dichotoma

AbstractPhotoinhibition of photosynthesis in the brown alga, Dictyota dichotoma, was studied with a PAM fluorometer (Walz, Effeltrich, Germany) and a homemade oxygen measuring device. As a measure of fluorescence, Fv/Fm, and for the photosynthetic yield, ΔF/Fm', were used. Oxygen measurements show clearly that the observed degree, as well as the time course, of photoinhibition depends on the fluence rate of the light used to measure changes of the production rate. After photoinhibition of photosynthesis the depression of oxygen production caused by non‐saturating fluence rates was generally much more pronounced than that caused by saturating or nearly saturating fluence rates. At minimal photoinhibition the initial slope and the convexity of the fluence rate‐response curve of oxygen evolution decrease, whereas the level of light saturation decreases only after strong photoinhibition. Nevertheless, at different degrees of photoinhibition, changes in the degree of the upper bending of the fluence rate‐response curve of oxygen production are also linearly correlated to changes in the fluorescence ratios (Fv/Fm and ΔF/Fm'). The action spectrum of photoinhibition, calculated on the basis of changes of Fv/Fm, indicates that the reaction center of PS I is not involved in photoinhibition. The lower effectiveness of blue light in comparison to effects of green and red light may be due to chloroplast displacement, as in the so‐called strong light position, the light absorbed by the thalli in vivo is decreased.

Acclimation of photosystem II in a cyanobacterium and a eukaryotic green alga to high and fluctuating photosynthetic photon flux densities, simulating light regimes induced by mixing in lakes.

Photoacclimation of Scenedesmus protuberans Fritsch and Microcystis aeruginosa Kützing emend. Elenkin to high and fluctuating PPFD was studied in continuous cultures with computer-controlled variable light regimes. The aim of the work was to provide a better understanding of species-specific acclimation to high PPFD (as encountered by cyanobacteria in surface waterblooms), and of suppression of the growth of colony-forming cyanobacteria during periods of prolonged mixing in lakes. The dynamics of a set of variables was followed during the light period, including pigment composition, maximum rate, efficiency and minimum quantum requirement of photosynthesis, PS II cross-sections, and fluorescence variables. Both the green alga and the cyanobacterium displayed strong photo-inhibition of photosynthesis in the sinusoidal light regime, which simulated a natural light regime in the absence of mixing. Pmax , α, QR and the ratio of variable to maximum fluorescence declined, and the number of inactive PS II centres and PS IIβ centres increased towards midday. Introduction of oscillations in the diurnal light regime, simulating different intensities of wind-induced mixing in lakes, mitigated photo-inhibition. Microcystis showed a prompt non-photochemical quenching of fluorescence in all light regimes, even at low to moderate PPFD. The sustained presence of zeaxanthin in Microcystis possibly induced instant, thermal dissipation of excitation energy from the antenna. Microcystis also exhibited a more reluctant acclimation to fluctuating PPFD. Growth rate of Scenedesmus was higher in all light regimes. This implied that if (known) differences in loss processes were ignored, Scenedesmus would outcompete Microcystis in lakes. The results underlined the importance of buoyancy regulation in increasing the daily light dose of cyanobacteria (but at the same time preventing over-excitation), and ultimately in the success in Microcystis in stable lakes.

Recovery of photosynthesis and phtosystem II fluorescence in Chlamydomonas reinhardtii after exposure to three levels of high light

Recovery from 60 min of photoinhibitory treatment at photosynthetic photon flux densities of 500, 1400 and 2200 μMmol m−2 s− was followed in cells of the green alga Chlamydomonas reinhardtii grown at 125 μMmol m−2 s−1. These light treatments represent photoregulation, moderate photoinhibition and strong photoinhibition, respectively. Treatment in photoregulatory light resulted in an increased maximal rate of oxygen evolution (Pmax) and an increased quantum yield (Φ), but a 15% decrease in Fv/FM. Treatment at moderately photoinhibitory light resulted in a 30% decrease in Fv/FM and an approximately equal decrease in Φ. Recovery in dim light restored Fv/FM within 15 and 45 min after high light treatment at 500 and 1400 μMmol m−2 s−1, respectively. Convexity (Θ), a measure of the extent of co‐limitation between PS II turnover and whole‐chain electron transport, and Φ approached, but did not reach the control level during recovery after exposure to 1400 μMmol m−2 s−1, whereas Pmax increased above the control. Treatment at 2200 μMmol m−2 s−1 resulted in a strong reduction of the modeled parameters Φ, Θ and Pmax. Subsequent recovery was initially rapid but the rate decreased, and a complete recovery was not reached within 120 min. Based on the results, it is hypothesized that exposure to high light results in two phenomena. The first, expressed at all three light intensities, involves redistribution within the different aspects of PS II heterogeneity rather than a photoinhibitory destruction of PS II reaction centers. The second, most strongly expressed at 2200 μmol m−2 s−1, is a physical damage to PS II shown as an almost total loss of PS IIα and PS II QB‐reducing centers. Thus recovery displayed two phase, the first was rapid and the only visible phase in algae exposed to 500 and 1400 μmol m−2 s−1. The second phase was slow and visible only in the later part of recovery in cells exposed to 2200 μmol m−2 s−1.

Photoinhibition of photosynthesis under natural conditions in ivy (Hedera helix L.) growing in an understory of deciduous trees

We investigated to what extent south-exposed leaves (E-leaves) of the evergreen ivy (Hedera helix L.) growing in the shadow of two deciduous trees suffered from photoinhibition of photosynthesis when leaf-shedding started in autumn. Since air temperatures drop concomitantly with increase in light levels, changes in photosynthetic parameters (apparent quantum yield, Φ i and maximal photosynthetic capacity of O2 evolution, Pmax; chlorophyll-a fluorescence at room temperature) as well as pigment composition were compared with those in north-exposed leaves of the same clone (N-leaves; photosynthetic photon flux density PPFD< 100 μmol · m(-2) · s(-2)) and phenotypic sun leaves (S-leaves; PPFD up to 2000 μmol · m(-2) · s(-1)).In leaves exposed to drastic light changes during winter (E-leaves) strong photoinhibition of photosynthesis could be observed as soon as the incident PPFD increased in autumn. In contrast, in N-leaves the ratio of variable fluorescence to maximum fluorescence (FV/FMm) and Φ i did not decline appreciably prior to severe frosts (up to -12° C) in January. At this time, Φ i was reduced to a similar extent in all leaves, from about 0.073 μmol O2 · μmol(-1) photons before stress to about 0.020. Changes in Φ i were linearly correlated with changes in fv/fm (r = 0.955). The strong reduction in FV/FM on exposure to stress was caused by quenching in FM. The initial fluorescence (F0), however, was also quenched in all leaves. The diminished fluorescence yield was accompanied by an increase in zeaxanthin content. These effects indicate that winter stress in ivy primarily induces an increase in non-radiative energy-dissipation followed by "photoinhibitory damage" of PSII. Although a pronounced photooxidative bleaching of chloroplast pigments occurred in January (especially in E-leaves), photosynthetic parameters recovered completely in spring. Thus, the reduction in potential photosynthetic yield in winter may be up to three times greater in leaves subjected to increasing light levels than in leaves not exposed to a changing light environment.

Studies on the light-induced loss of the D1 protein in photosystem-II membrane fragments

PS II membrane fragments produced from higher plant thylakoids by Triton X-100 treatment exhibit strong photoinhibition and concomitant fast degradation of the D1 protein. Involvement of (molecular) oxygen is necessary for degradation of the D1 protein.The herbicides atrazine and diuron, but not ioxynil, partly protect the D1 protein against degradation. Binding of atrazine to the D1 protein is necessary to protect the D1 polypeptide, as shown with PS II membrane fragments from an atrazine-resistant biotype of Chenopodium album which are protected by diuron not by atrazine.

Fluorescence as a tool in photosynthesis research: application in studies of photoinhibition, cold acclimation and freezing stress

Chlorophyll fluorescence induction (at 20 °C and 77 K) and quenching were analysed in relation to effects of environmental stresses imposed by chilling in high light and by freezing and thawing of spinach (Spinacia oleraceaL.) leaves. The data indicate that cold acclimation of spinach plants, which leads to increased frost tolerance of the leaves, results in decreased susceptibility to photoinhibition of photosynthesis at chilling temperatures. When plants acclimated to 18 °C and 260-300 µmol quanta m-2s-1were exposed to higher light (550 µmol quanta m-2s-1) at 4 °C, they developed strong photoinhibition, as characterized by decreased quantum yield of O2evolution and decreased ratio of variable: maximum fluorescence (FV/FM) of photosystem II. The decrease in FV/FMresulted from a decline in FVand an increase in F0. The FV/FMratio was lowered to a significantly greater extent when induction was recorded at 20 °C, as compared with 77 K. The effects related to photoinhibition were fully reversible at 18 °C in dim light. Plants that had been cold-acclimated for 10 days exhibited slightly decreased quantum yield and lowered FV/FMratio. However, they did not show further photoinhibition on exposure to 550 µmol quanta m-2s-1at 4 °C. The reversible photoinhibition is discussed as a protective pathway serving for thermal dissipation of excessive light energy. It is hypothesized that such a mechanism prevents destruction of the photosynthetic apparatus, until other means of protection become effective during long-term acclimation to high light. Inhibition of photosynthetic carbon assimilation caused by freezing and thawing of leaves in the dark was closely correlated with inhibition of photochemical fluorescence quenching (qQ). As a sensitive response of the thylakoid membranes to freezing stress, the energy-dependent quenching, qE, was inhibited. Only more severe impact of freezing caused a significant decline in the FV/FMratio. It is concluded that measurements of fluorescence induction signals ( FV/FMratios) provide a sensitive tool with which to investigate photoinhibition, whereas freezing damage to the photosynthetic system can be detected more readily by the quenching coefficients qQand qEthan by FV/FMratios.

Photoinhibition at chilling temperature

The effects of moderate light at chilling temperature on the photosynthesis of unhardened (acclimated to +18° C) and hardened (cold-acclimated) spinach (Spinacea oleracea L.) leaves were studied by means of fluorescence-induction measurements at 20° C and 77K and by determination of quantum yield of O2 evolution. Exposure to 550 μmol photons·m(-2)·s(-1) at +4° C induced a strong photoinhibition in the unhardened leaves within a few hours. Photoinhibition manifested by a decline in quantum yield was characterized by an increase in initial fluorescence (F o) and a decrease in variable fluorescence (F v) and in the ratio of variable to maximum fluorescence (F V/F M), both at 77K and 20° C. The decline in quantum yield was more closely related to the decrease in the F V/F M ratio measured at 20° C, as compared with F V/F M at 77K. Quenching of the variable fluorescence of photosystem II was accompanied by a decline in photosystem-I fluorescence at 77K, indicating increased thermal de-excitation of pigments as the main consequence of the light treatment. All these changes detected in fluorescence parameters as well as in the quantum yield of O2 evolution were fully reversible within 1-3 h at a higher temperature in low light. The fast recovery led us to the view that this photoinhibition represents a regulatory mechanism protecting the photosynthetic apparatus from the adverse effects of excess light by increasing thermal energy dissipation. Long-term cold acclimation probably enforces other protective mechanisms, as the hardened leaves were insensitive to the same light treatment that induced strong inhibition of photosynthesis in unhardened leaves.

The 9-kDa Phosphoprotein Involved in Photoinhibition

Photosystem-II particles exhibit strong photoinhibition. Short-term illumination of photosystem- II particles with high-intensity light (5000 μE/m2 × s) leads to a typical change of the protein pattern on SDS-PAGE. Two proteins are mainly affected, namely the well-described 32-kDa herbicide-binding protein which probably is degraded [1] and, first published here, the 9-kDa phosphoprotein, whose function in the PS-II complex is still unknown. This protein is not degraded, but seems to be linked to other polypeptides of the PS-II complex. During light treatment new bands of 23, 41, 50 and 54 kDa appear in the protein pattern of SDS-PAGE. A monospecific antiserum was produced against the 9-kDa phosphoprotein to investigate its fate. After light treatment the antibodies reacted with new proteins of higher molecular weights, most pronounced with a 23-kDa and a 41-kDa peptide.

Écophysiologie comparée des Mysidacés Hemimysis speluncola ledoyer (cavernicole) et Leptomysis lingvura G.O. Sars (non-cavernicole). L'orientation à la lumière: Tests ponctuels

With a view to a better understanding of the factors affecting the diurnal way of life of the cavernicolous mysid Hemimysis speluncola Ledoyer some aspects of its physiology have been compared with a similar species, Leptomysis lingvura G.O. Sars which, although abundant at the same bathymétric level, is only rarely found in caves. This paper gives the reactions of each species when exposed briefly to a horizontal beam of white light (varying from 1000 to 10 −9 lux) and coloured light (action spectrum with six colours, the energy of the light being unchanged). The notable difference between the two species is essentially one of phototaxis. When dark-adapted Hemimysis speluncola exhibits a strong negative phototaxis above 10 −4 lux and a clear positive reaction at 10 −4 lux. Leptomysis lingvura shows only slight orientation. The differences are greater in animals adapted to daylight; Hemimysis remains strongly photonegative above 1 lux while Leptomysis tends to become clearly photopositive. The light stimulus threshold varies according to the intensity of the adaptation light; it is particularly low in Hemimysis (10 −9 lux) and higher in Leptomysis (10 −7 lux), for dark-adapted animals. The differences are distinctly less noticeable with respect to photo kinesis; in each species the speed of displacement is high, particularly in dim light (10 −6 to 10 −1 lux). In strong light photoinhibition is feeble or almost non-existent. The reactions to the various colours of the spectrum do not indicate differences between the species other than those recorded when studying the effects of various levels of white light. This first study reveals original characteristics of the cave-living species: the association with the levels of environmental daylight considered to prevail in the mysid's habitat of a marked negative phototaxis, a high kinesis and the absence of photoinhibition. These characteristic reactions are absent in the majority of mysids; diurnally mobile species such as L. lingvura show a reduced negative phototaxis while superficial benthic species show a strong photoinhibition. Hemimysis speluncola by its reactions to light allies itself, on the contrary, to species which burrow deeply in the sediment or hide under rocks during the day. The choice of caves as a habitat allows permanent mobility and seems to have been imposed on the species by a non-adaptation to the burrowing life or by a strong negative thigmotaxis.

Le contrôle de l'émergence et des nages nocturnes chez les péracarides des plages de méditerranée. Eurydice affinis Hansen (Isopoda), Gastrosaccus mediterraneus Bacescu, Gastrosaccus spinifer (goës) (Mysidacea)

Along the Mediterranean sandy shores, Eurydice affinis Hansen, Gastrosaccus mediterraneus Bacescu and Gastrosaccus spinifer (Goës) are present during the day in the sand of the beaches, either near the limit of the wave wash or in the superficial fringe of the infralittoral. In the evening there is a general emergence and the animals rejoin the planktonic populations of the open sea. All the three species when kept in aktographs under constant conditions and continuous darkness exhibit a circadian rhythm of emergence and swimming activity of nocturnal frequency. The rhythm is clear and in the two Gastrosaccinae persists for ten days at least; it has been observed over about fifty days in G. spinifer. In E. affinis, the rhythm is only on some occasions clear; one frequently observes a permanent burying in the sediment, in which case the swimming rhythm can be brought to light by a mechanical disturbance or the withdrawal of the sediment. A study of the changes in the photokinetic reactions in a horizontal beam of white light, with intensities varying from 4000 to 10 −6 lux, allow the synchronization of the rhythms to be ascribed to a slight photonegative tendency of the animals at intensities > 1000 lux and a photopositive tendency, often marked, at lower intensities. The variations in the intensity of the reactions in the three species allows an explanation of the autonomous rhythms. A comparison between the photokinetic reactions of these species and those of the infralittoral Peracarida, which migrate in the nocturnal plankton, shows great differences with respect to light intensity. In the latter, the phototaxis is strongly negative and frequently accompanied by a strong photo-inhibition while in the former the phototaxis is still often positive and the kinesis is high. The photopositive reaction allows the animals to rejoin their daytime habitat soon after dawn when the swimming activity decreases.

Influence of light and temperature on the photosynthetic rate of marine benthic diatoms

The influence of light and temperature on photosynthetic rate as measured by C14O2-fixation of marine benthic diatoms was investigated, using both intact sediment samples (Marshall et al., 1973) and suspensions of diatoms harvested by a lenstissue technique (Eaton and Moss, 1966). After C14-incubation, sediment samples were filtered, burned in a sample oxidizer, and their activity determined in a liquid scintillation counter. Photosynthetic rate of mixed field populations is saturated by a light intensity of approximately 10,000 lux; at still higher light intensities no photoinhibition was found. In contrast to the mixed field populations, unialgal cultures of the benthic diatom Amphiprora alata Kutz. exhibited strong photoinhibition at higher light intensities (10,000 to 60,000 lux). Within a range of 4° to 20°C, the photosynthetic rate increased about 10%/Co. No differences in photosynthetic pattern were observed between epipelic and epipsammic species.