Remote sensing of heterogeneity in photosynthetic efficiency, electron transport and dissipation of excess light in Populus deltoides stands under ambient and elevated CO2 concentrations, and in a tropical forest canopy, using a new laser-induced fluorescence transient device

Determining the spatial and temporal diversity of photosynthetic processes in forest canopies presents a challenge to the evaluation of biological feedbacks needed for improvement of carbon and climate models. Limited access with portable instrumentation, especially in the outer canopy, makes remote sensing of these processes a priority in experimental ecosystem and climate change research. Here, we describe the application of a new, active, chlorophyll fluorescence measurement system for remote sensing of light use efficiency, based on analysis of laser‐induced fluorescence transients (LIFT). We used mature stands of Populus grown at ambient (380 ppm) and elevated CO₂ (1220 ppm) in the enclosed agriforests of the Biosphere 2 Laboratory (B2L) to compare parameters of photosynthetic efficiency, photosynthetic electron transport, and dissipation of excess light measured by LIFT and by standard on‐the‐leaf saturating flash methods using a commercially available pulse‐modulated chlorophyll fluorescence instrument (Mini‐PAM). We also used LIFT to observe the diel courses of these parameters in leaves of two tropical forest dominants, Inga and Pterocarpus, growing in the enclosed model tropical forest of B2L. Midcanopy leaves of both trees showed the expected relationships among chlorophyll fluorescence‐derived photosynthetic parameters in response to sun exposure, but, unusually, both displayed an afternoon increase in nonphotochemical quenching in the shade, which was ascribed to reversible inhibition of photosynthesis at high leaf temperatures in the enclosed canopy. Inga generally showed higher rates of photosynthetic electron transport, but greater afternoon reduction in photosynthetic efficiency. The potential for estimation of the contribution of outer canopy photosynthesis to forest CO₂ assimilation, and assessment of its response to environmental stress using remote sensing devices such as LIFT, is briefly discussed.     

Determination of the primary charge separation rate in Photosystem II reaction centers at 15 K

We have measured the rate constant for the formation of the oxidized chlorophyll a electron donor (P680⁺) and the reduced electron acceptor pheophytin a ^– (Pheo a ^–) following excitation of isolated Photosystem II reaction centers (PS II RC) at 15 K. This PS II RC complex consists of D₁, D₂, and cytochrome b-559 proteins and was prepared by a procedure which stabilizes the protein complex. Transient absorption difference spectra were measured from 450–840 nm as a function of time with 500fs resolution following 610 nm laser excitation. The formation of P680⁺-Pheo a ^– is indicated by the appearance of a band due to P680⁺ at 820 nm and corresponding absorbance changes at 490, 515 and 546 nm due to the formation of Pheo a ^–. The appearance of the 490 nm and 820 nm bands is monoexponenital with =1.4±0.2 ps. Treatment of the PS II RC with sodium dithionite and methyl viologen followed by exposure to laser excitation results in accumulation of Pheo a ^–. Laser excitation of these prereduced RCs at 15 K results in formation of a transient absorption spectrum assigned to ^1*P680. We observe wavelength-dependent kinetics for the recovery of the transient bleach of the Q_y absorption bands of the pigments in both untreated and pre-reduced PS II RCs at 15K. This result is attributed to an energy transfer process within the PS II RC at low temperature that is not connected with charge separation.Abbreviations PS I Photosystem I - PS II Photosystem II - RC reaction center - P680 primary electron donor in Photosystem II - Chl a chlorophyll a - Pheo a pheophytin a     

Ecological studies of Chloroflexis,a gliding photosynthetic bacterium

Summary Chloroflexis, a gliding, filamentous, photosynthetic bacterium, is present in the stratified algal-bacterial mats which occur in the 50°–70°C temperature range of alkaline hot spring effluents. The organism is in association with the alga in the upper, algal layer, and also forms thick, orange mats beneath the algal layer. Natural populations of Chloroflexis from these mats demonstrated light-stimulated uptake of some ¹⁴C-labelled organic compounds. Photosynthetic ¹⁴CO₂ fixation by natural samples of Chloroflexis was investigated with respect to temperature, light intensity and mat depth. Bacterial photosynthesis was determined in samples in which algae were present by use of the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Bacterial photosynthesis was maximal at depths down to about 3 mm and then decreased rapidly to very low levels at greater depths. The greatest amounts of bacteriochlorophyll pigments were also concentrated in the top 3–4 mm of the mat. The optimum light intensity for bacterial photosynthesis (about 400 ft-c) was considerably lower than the normal summer light intensity at the surface of the mat (5000-8000 ft-c).The temperature optima for photosynthesis by the bacterial component of natural mat samples from several sites of different temperatures in a hot spring thermal gradient were determined. Temperature optima approximated the environmental temperatures, indicative of the occurrence of strains of Chloroflexis adapted to different temperatures. Although bacterial standing crop was greatest in the temperature range 50°–55°C, maximum photosynthetic efficiency was observed at about 45°C. Sulfide was stimulatory to photosynthetic ¹⁴CO₂ fixation by naturally occurring populations of Chloroflexis under field conditions. These data are consistent with the hypothesis that Chloroflexis may utilize sulfide as an electron donor for photosynthetic CO₂ reduction. However, it is also likely that Chloroflexis grows photoheterotrophically in these mats, obtaining organic compounds from algal excretory products.     

Exciton Annihilation in the Two Photosystems in Chloroplasts at 100°K

The fluorescence yield (F) of spinach chloroplasts at 100°K measured at 735 nm (photosystem I fluorescence—F 735) and at 685 nm (photosystem II fluorescence—F 685) has been determined with different modes of laser excitation. The modes of excitation included a single picosecond pulse, sequences of picosecond pulses (4, 22, and 300 pulses spaced 5 ns apart) and a single nonmode-locked 2-μs pulse (MP mode). The F 735/F 685 intensity ratios decrease from 1.62 to 0.61 when a single picosecond pulse (or low-power continuous helium-neon laser) is replaced by excitation with the 300-ps pulse train (PPT mode) or MP mode. In the PPT mode of excitation, the 735-nm fluorescence band is quenched by a factor of 45 as the intensity is increased from 10¹⁵ to 10¹⁸ photons/cm² per pulse train and the 685-nm fluorescence is quenched by a factor of 10. In the MP mode, the quenching factors are 25 and 7, respectively, in the same intensity range. Fluorescence quantum yield measurements with different picosecond pulse sequences indicate that relatively long-lived quenching species are operative, which survive from one picosecond pulse to another within the pulse train. The excitonic processes possible in the photosynthetic units are discussed in detail. The differences in the quenching factors between the MP and PPT modes of excitation are attributed to singlet-singlet annihilation, possible when picosecond pulses are utilized, but minimized in the MP mode of excitation. The long-lived quenchers are identified as triplets and/or bulk chlorophyll ions formed by singlet-singlet annihilation. The preferential quenching in photosystem I is attributed to triplet excitons. The influence of heating effects, photochemistry, bleaching, and two-photon processes is also considered and is shown to be negligible.     

Excitation dynamics of two spectral forms of the core complexes from photosynthetic bacterium Thermochromatium tepidum

The intact core antenna-reaction center (LH1-RC) core complex of thermophilic photosynthetic bacterium Thermochromatium (Tch.) tepidum is peculiar in its long-wavelength LH1-Q_y absorption (915 nm). We have attempted comparative studies on the excitation dynamics of bacteriochlorophyll (BChl) and carotenoid (Car) between the intact core complex and the EDTA-treated one with the Q_y absorption at 889 nm. For both spectral forms, the overall Car-to-BChl excitation energy transfer efficiency is determined to be ∼20%, which is considerably lower than the reported values, e.g., ∼35%, for other photosynthetic purple bacteria containing the same kind of Car (spirilloxanthin). The RC trapping time constants are found to be 50∼60 ps (170∼200 ps) for RC in open (closed) state irrespective to the spectral forms and the wavelengths of Q_y excitation. Despite the low-energy LH1-Q_y absorption, the RC trapping time are comparable to those reported for other photosynthetic bacteria with normal LH1-Q_y absorption at 880 nm. Selective excitation to Car results in distinct differences in the Q_y-bleaching dynamics between the two different spectral forms. This, together with the Car band-shift signals in response to Q_y excitation, reveals the presence of two major groups of BChls in the LH1 of Tch. tepidum with a spectral heterogeneity of ∼240 cm⁻¹, as well as an alteration in BChl-Car geometry in the 889-nm preparation with respect to the native one.     

High-performance liquid chromatographic determination of cocaine and its metabolites in serum microsamples with fluorimetric detection and its application to pharmacokinetics in rats

A sensitive, selective and simple HPLC method with fluorimetric detection is described for quantitating cocaine and its three metabolites in rat serum microsamples (50 μl). Chromatographic separation is achieved on a Hypersil BDS C₁₈ column (100×2.1 mm, 5 μm) with an isocratic mobile phase consisting of methanol–acetonitrile–25.8 mM sodium acetate buffer, pH 2.6, containing 1.0·10⁻⁴ M tetrabutylammonium phosphate (14:10:76, v/v/v). The detection limit (0.5 ng/ml) for all the compounds, using direct fluorometric detection operated at excitation and emission wavelengths of 230 and 315 nm, respectively, was approximately five-times lower than that of using a UV detector operated at 235 nm. The effects of ratio of 2-propanol to chloroform in extraction solvents on the recovery and precision for cocaine and its metabolites were systematically examined. The method was used to study the pharmacokinetics of cocaine after administration of intravenous 2 mg/kg and oral 20 mg/kg doses.     

Variation in some photosynthetic characteristics of microalgae cultured in outdoor thin-layered sloping reactors

In outdoor thin-layer sloping reactors algae are batch cultured and harvested at biomass concentrations of about 15 g (dw) I^-1 whereafter a portion is used as inoculum for the next cycle. Light saturation curves of the oxygen evolution (PII curves) of the algae were measured using diluted aliquots of suspension taken from the reactors. The maximum specific photosynthetic rates (P ^B _max) and the light intensity at the onset of saturated photosynthesis (I _k ) decreased whilst the maximum specific photosynthetic efficiency ( ^B ) increased with an increase in the biomass concentration, during the production cycle. These differences reflect transition from light- to dark-acclimated state of the algae that occurs as a result of an increase of the suspension concentration during the production cycle. During these experiments the thin-layered smooth sloping cultures (TLSS, culture depth 5–7 mm) had higher photosynthetic rates per volume than the thin-layered baffled sloping cultures (TLBS, culture depth 5–15 mm). This was ascribed to the higherP ^B _max values of the algae grown in the TLSS cultures, allowing them to utilise high incident irradiancies more effectively. However, the areal productivity of the TLBS was higher than the TLSS indicating a higher photosynthetic efficiency of the TLBS reactors. The specific productivity decreased rapidly with an increase in the biomass concentration, but the yield remained linear during the batch production cycle, even at high areal densities.     

Determination of idarubicin and idarubicinol in rat plasma using reversed-phase high-performance liquid chromatography and fluorescence detection

A reversed-phase high-performance liquid chromatographic method is described for the simultaneous determination of idarubicin and idarubicinol in rat plasma. Blood samples were analyzed from 16 rats which had received an intravascular dose of 2.25 mg kg⁻¹ idarubicin. After deproteinization with acetonitrile, the separation was performed with a LiChrospher 100 RP-18 column (5 μm), using fluorescence detection (excitation: 485 nm/emission: 542 nm). The mean recovery was 95.6% for idarubicin and 90.7% for idarubicinol, respectively. The detection limit was 0.25 ng ml⁻¹ using an injection volume of 50 μl. Daily relative standard deviation (RSD) was 3.2% (10 ng idarubicin/ml, n=10) and 4.4% (10 ng idarubicinol/ml, n=10).     

Photosynthetic electron transport in thylakoids from cotton cultivars (Gossypium sp.) differing in tolerance to prometryn

A method to determine photosynthetic electron transport in thylakoid membranes is described for Gossypium barbadense (cv. Pima S-7) and G. hirsutum (cv. DP 5415). These cultivars differed markedly in tolerance to prometryn, a PS II inhibitor. The rates of photosynthetic electron transport obtained were 245 mole oxygen mg^–1 chl h¹. Plant age and leaf size influenced the activity of the thylakoid preparations. Thylakoids from leaves of plants 24 to 37 d and 50–70 mm in diameter had the highest activities; thylakoids from cotyledons, fully expanded leaves and young leaves had low activity. Thylakoids from both species had similar photosynthetic activities and I₅₀'s for prometryn, atrazine and diuron. Thus, tolerance to prometryn was not due to differential binding at D1 protein.Abbreviations PSII photosystem II - DAP day after planting - DQ duroquinone - DBMIB dibromothymoquinone - DMBQ 2,5-dimethyl-p-benzoquinone - I₅₀ concentration to inhibit reaction by 50% - Q_A quinone A - Q_B quinone B     

Photosynthetic production of the filamentous cyanobacteriumSpirulina platensis in a cone-shaped helical tubular photobioreactor

The photosynthetic productivity of the filamentous cyanobacteriumSpirulina platensis was investigated in a cone-shaped helical tubular photobioreactor. A laboratory-scale photobioreactor was constructed with a 0.255-m² basal area and a conical shape (0.64 m high × 0.57 m top diameter). The photostage comprised transparent reinforced polyvinyl chloride (PVC) tubing with spirally wound, metal-wire reinforcing in the tubing wall (31 m in length and 1.6 cm internal diameter with 0.25 cm wall thickness; total volume = 6.23 l). The inner surface of the photostage (0.651 m²) was illuminated with compact fluorescent cool white lamps; the photosynthetically active radiation (400–700 nm) energy input into the photobioreactor was 1249 KJ day^–1 (12 h day/12 h night). The operation of an air-lift photobioreactor with CO₂-enriched air (4%) at a flow rate of 0.3 l min^–1 showed a maximum daily photosynthetic efficiency of 6.83% under batch-culture conditions. This corresponded to a production rate of 15.9 g dry biomass m^–2(basal area) day^–1 or 0.51 g dry biomass l medium^–1 day^–1.     

Coregulation of electron transport through PS I by Cyt b 6 f,excitation capture by P700 and acceptor side reduction. Time kinetics and electron transport requirement

Regulation of electron transport rate through Photosystem I (PS I) was investigated in intact sunflower leaves. The rate constant of electron donation via the cytochrome b ₆ f complex (k_q, s^–1) was obtained from the postillumination P700⁺ reduction rate, measured as the exponential decay of the light-dark difference (D830) of the 830 nm transmission signal. D830 corresponding to maximum oxidisable P700 (D830_m) was obtained by applying white light flashes of different intensity and extrapolating the plot of the quantum yield Y vs. D830 to the axis of abscissae (Y->0). Maximum quantum yield of PS I at completely reduced P700 (Y_m) was obtained by extrapolating the same plot to the axis of ordinates (D830->0). Regulation of k_q, D830_m and Y_m under rate-limiting CO₂ and O₂ concentrations applied after air (21% O₂, 310 ppm CO₂) was investigated. The amplitude of the downregulation of k_q (photosynthetic control) was maximal when electron transport rate (ETR) was limited to about 3 nmol cm^–2 s^–1 and decreased when ETR was higher or lower. Downregulation did not occur in the absence of CO₂ and O₂. These gases acted only as substrates of ribulosebisphosphate carboxylase-oxygenase, no high-affinity reaction of O₂ leading to enhanced photosynthetic control (e.g. Mehler reaction) was detected. After the transition, D830_m at first decreased and then increased again, showing that the reduction of the PS I acceptor side disappeared as a result of the downregulation of k_q. The variation of Y_m had two reasons, PS I acceptor side reduction and variable excitation capture efficiency by P700. It is concluded that electron transport through PS I is coregulated by the rate of plastoquinol oxidation at Cyt b ₆ f, excitation capture efficiency by P700, and by acceptor side reduction.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - D830 difference of the 830 nm signal from the dark level - ETR electron transport rate - PAD photon absorption density nmol cm^–2 s^–1 - PFD incident photon flux density, nmol cm^–2 s^–1 - PS I Photosystem I - PS II Photosystem II - PQH₂ plastoquinol - P700 Photosystem I donor pigment - Y quantum yield of PS I electron transport, rel. un.     

Differential response of leaf gas exchange to enhanced ultraviolet-B (UV-B) radiation in three species of herbaceous climbingplants

We measured diurnal changes in photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency in three species of herbaceous climbing plants (Luffa cylindrica, Trichosanthes kirilowii and Dioscorea opposita) exposed to two intensities of UV-B radiation: 3.0 μw cm^?2 (R1) and 8.0 μw cm^?2 UV-B (R2) radiation under ambient growth conditions. Responses differed per species and per treatment. In Luffa all values increased compared to the Control in both treatments, except for stomatal conductance in R2. In Trichosanthes photosynthetic rates and water use efficiency increased, while the transpiration rates decreased under both treatments, and stomatal conductance was lower in R1. In Dioscorea photosynthetic rates and water use efficiency decreased under both treatments, while the transpiration rates and stomatal conductance increased. The results suggested that to some extent increased UV-B radiation was beneficial to the growth of L. cylindrica and T. kirilowii, but detrimental to D. opposita.     

Excitation Energy Transfer between Pigments in Photosynthetic Cells

The excitation lifetimes of photosynthetic pigments and the times needed for energy transfer between pigments in various algae, were determined in vitro and in vivo. For this purpose, the time curves of fluorescence rise and decay were measured by means of Brody''s instrument (10), and compared with theoretical curves obtained by the method of “convolution of the first kind.”¹     

Analysis of Qualitative Contribution of Assimilatory and Non-Assimilatory De-Excitation Processes to Adaptation of Photosynthetic Apparatus of Barley Plants to High Irradiance

The adaptation of barley (Hordeum vulgare L. cv. Akcent) plants to low (LI, 50 µmol m^–2 s^–1) and high (HI, 1000 µmol m^–2 s^–1) growth irradiances was studied using the simultaneous measurements of the photosynthetic oxygen evolution and chlorophyll a (Chl a) fluorescence at room temperature. If measured under ambient CO₂ concentration, neither increase of the oxygen evolution rate (P) nor enhancement of non-radiative dissipation of the absorbed excitation energy within photosystem 2 (PS2) (determined as non-photochemical quenching of Chl a fluorescence, NPQ) were observed for HI plants compared with LI plants. Nevertheless, the HI plants exhibited a significantly higher proportion of Q_A in oxidised state (estimated from photochemical quenching of Chl a fluorescence, q_P), by 49–102 % at irradiances above 200 µmol m^–2 s^–1 and an about 1.5 fold increase of irradiance-saturated PS2 electron transport rate (ETR) as compared to LI plants. At high CO₂ concentration the degree of P stimulation was approximately three times higher for HI than for LI plants, and the irradiance-saturated P values at irradiances of 2 440 and 2 900 µmol m^–2 s^–1 were by 130 and 150 % higher for HI plants than for LI plants. We suggest that non-assimilatory electron transport dominates in the adaptation of the photosynthetic apparatus of barley grown at high irradiances under ambient CO₂ rather than an increased NPQ or an enhancement of irradiance-saturated photosynthesis.     

Response of Audouinella saviana (Meneghini) Woelkerling (Nemaliales,Rhodophyta) cultures to monochromatic light

Audouinella saviana (Meneghini) Woelkerling was cultured at a constant temperature (24 °C) and different irradiances (from 1 µmol to 30 µmol photons m^–2 s^–1) of blue (430–470 nm) and green (500–560 nm) light in order to study its adaptive response. Modifications in colour, morphology and ultrastructure of the thalli, together with changes in pigment composition and in the spectral properties of chlorophyll a and R-phycoerythrin, were observed both by means of light and electron microscopy (TEM, SEM) and spectrophotometric and spectrofluorimetric analyses. In this paper we report the adaptive response of the seaweed to blue and green radiation by focussing on the cell wall and on the photosynthetic apparatus, particularly on phycobilisomes in situ and on R-PE after extraction. PBSs were fully structured only under blue light at low irradiance whilst they were absent under green light, whatever the irradiance, in spite of the high R-PE content. This fact, together with the spectral changes of R-PE, suggests adaptation at a molecular level, presumably referable to changes in aggregation state.     

Production of Deuterated Zeaxanthin by <Emphasis Type="Italic">Flavobacterium Multivorum</Emphasis> and its Detection by Resonance Raman and Mass Spectrometric Methods

Flavobacterium multivorum, a zeaxanthin-producing organism, was grown aerobically in a medium prepared with deuterated water. Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) and resonance Raman spectroscopy (RRS) analysis revealed 75% replacement of hydrogen by deuterium atoms as indicated by the molecular mass cluster at around m/z 600. Deuterated zeaxanthin upon excitation with a 488 nm laser exhibited characteristic resonance Raman vibrational modes at 1161 and 1504 cm⁻¹ as compared to 1007, 1159 and 1525 cm^–1 for undeuterated zeaxanthin. HPLC/APCI-MS and HPLC/RRS were specific and sensitive with limits of detection of 2.5 pg and 50 ng, respectively.     

Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awasa,Ethiopia

The biomass and primary production of phytoplankton in Lake Awasa, Ethiopia was measured over a 14 month period, November 1983 to March 1985. The lake had a mean phytoplankton biomass of 34 mg chl a m^–3 (n = 14). The seasonal variation in phytoplankton biomass of the euphotic zone (mg chl a m^–2 h^–1) was muted with a CV (standard deviation/mean) of 31%. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light inhibition on all but overcast days. The maximum specific rates of photosynthesis or photosynthetic capacity (Ø_max) for the lake approached 19 mg O₂ (mg chl a)^–1 h^–1, with high values during periods of low phytoplankton biomass. Areal rates of photosynthesis ranged between 0.30 to 0.73 g O₂ m^–2 h^–1 and 3.3 to 7.8 g O₂ m^–2 d^–1. The efficiency of utilisation of PhAR incident on the lake surface varied from 2.4 to 4.1 mmol E^–1 with the highest efficiency observed corresponding to the lowest surface radiation. Calculated on a caloric basis, the efficiency ranged between 1.7 and 2.9%. The temporal pattern of primary production by phytoplankton showed limited variability (CV = 21 %).     

Ultrafast excited‐state dynamics in chlorosomes isolated from the photosynthetic filamentous green bacterium Chloroflexus aurantiacus

Bacteriochlorophyll (BChl) c pigments in the aggregated state are responsible for efficient light harvesting in chlorosomes of the filamentous anoxygenic photosynthetic bacterium, Chloroflexus (Cfx.) aurantiacus. Absorption of light creates excited states in the BChl c aggregates. After subpicosecond intrachlorosomal energy transfer, redistribution and relaxation, the excitation is transferred to the BChl a complexes and further to reaction centers on the picosecond time scale. In this work, the femtosecond excited state dynamics within BChl c oligomers of isolated Cfx. aurantiacus chlorosomes was studied by double difference pump‐probe spectroscopy at room temperature. Difference (A^light ? A^dark) spectra corresponding to excitation at 725 nm (blue side of the BChl c absorption band) were compared with those corresponding to excitation at 750 nm (red side of the BChl c absorption band). A very fast (time constant 70 ± 10 fs) rise kinetic component was found in the stimulated emission (SE) upon excitation at 725 nm. This component was absent at 750‐nm excitation. These data were explained by the dynamical red shift of the SE due to excited state relaxation. The nature and mechanisms of the ultrafast excited state dynamics in chlorosomal BChl c aggregates are discussed.     

Effects of UV Radiation on Photosynthesis,Antioxidant Capacity and the Accumulation of Bioactive Compounds in Gracilariopsis longissima,Hydropuntia cornea and Halopithys incurva (Rhodophyta)

The red macroalgae Hydropuntia cornea, Gracilariopsis longissima and Halopithys incurva were cultured for 14 d under laboratory conditions, in enriched seawater with a high nutrient content (N‐NH₄⁺ and P‐PO₄^3?) and two radiation regimes: PAR (400–700 nm) and PAB (280–700 nm). The UV radiation effects under high availability of nutrients on growth, photosynthetic pigments (chlorophyll a, carotenoids and phycobiliproteins), photosynthetic activity and biochemical composition were studied. Maximum quantum yield (F_v/F_m) was not significantly different among the PAR and PAB treatments during the experiment. However, the maximum electronic transport rate (ETR_max) increased over time, showing the highest values in PAR for H. incurva and H. cornea, whereas for G. longissima it was found in PAB. Photosynthetic efficiency (α_ETR) decreased over time in the first two species, but increased in G. longissima. Saturation irradiance (Ek_ETR) and maximum nonphotochemical quenching (NPQ_max) increased in PAB with time up to 80% and 30%, respectively, indicating a photosynthetic acclimatization like that of sun‐type algae. Five MAAs were identified in all species using high performance liquid chromatography (HPLC). The total content of MAAs increased over time, being 30% higher in H. incurva, 40% in G. longissima and 50% in H. cornea in PAB than in the PAR treatment. Finally, the antioxidant activity was also higher in the PAB treatment. All of the species presented an effective mechanism of photoprotection based on the accumulation of photoprotective compounds with antioxidant activity, as well as a high dissipation of excitation energy (high NPQ_max).     

The Effects of Food Concentration and Quality on the Feeding Rates of Three Size Classes of the Greenshell<Superscript>TM</Superscript> Mussel, <Emphasis Type="Italic">Perna canaliculus</Emphasis>

In order to calibrate carrying capacity models, investigations were conducted into the effects of food concentration and food quality on the feeding rates of small (25–50 mm), medium (60–85 mm) and large (90–115 mm) Greenshell mussels (Perna canaliculus). Experimental diets varying from 3.3 to 6.0 μg l⁻¹ chlorophyll a concentration and 12–25% organic content were fed to mussels housed in individual flow through chambers. Not surprisingly, this study found that the main factor affecting feeding rates is mussel size. Small mussels were observed to maintain a constant filtration rate of approximately 20 mg h⁻¹ irrespective of food concentration or quality, whereas mussels of greater than 60 mm length had more variable filtration rates between 30 and 80 mg h⁻¹. The filtration rates of these large mussels were also observed to increase positively with organic content, and showed no sign of levelling out, even at the highest organic content tested (25%). Highest rejection rates (50–70 mg h⁻¹) were observed when the organic content of the available seston was low, suggesting that P. canaliculus are able to selectively reject organic material, thereby organically enriching their diet. It appears that the organic content of the seston is the primary determinant of the net efficiency with which food is selected from the available seston by the mussel. The present study shows that P. canaliculus of all sizes are capable of adapting their feeding behaviour to compensate for changes in the food supply, which may occur over relatively short time periods, in the culture environment.