DAYLENGTH AND LIGHT RESPONSES IN GROWTH AND FERTILITY OF GLOSSOPHORA KUNTHII (PHAEOPHYTA,DICTYOTALES) FROM PACIFIC SOUTH AMERICA1

Excised ligulae of Glossophora kunthii (C. Ag.) J. Ag. were cultured in photoperiods of 4–24 h and photon fluence rates of 10–75 μmol.m^?2.s^?1. Daylength interacted with irradiance on the growth of the ligulae. Maximal growth of primary ligulae occurred in long-day regimens with high irradiances suggesting an effect of irradiance on photosynthesis and growth. In contrast, growth of secondary ligulae was greatest in short-day regimes. Differences were significant at the highest irradiance tested. Differentiation of tetrasporangia on the ligulae is a short-day photoperiodic response. Daylengths of 8.5 h or less induced a sharp increase in numbers of fertile ligulae and tetrasporangia attaining maturity. Interruptions of the dark period decreased the development of tetrasporangia; the number of interruptions had a cumulative inhibitory effect. Differentiation of reproductive structures was influenced by interactions of photoperiod and irradiance. Maximum numbers of tetrasporangia were formed at short-day regimes and low irradiances; differentiation was completely inhibited at long-day conditions and high irradiance.     

Effects of light history on primary productivity in a phytoplankton community dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii

1. The effects of instantaneous irradiance and short‐term light history on primary production were determined for samples from a subtropical water reservoir dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii. ¹⁴C‐bicarbonate uptake incubations were conducted on water samples from the reservoir, for irradiance (photosynthetically active radiation) ranging from 0 to 1654 μmol quanta m⁻² s⁻¹. Prior to the ¹⁴C incubations, cells were pre‐treated at irradiance levels ranging from 0 to 1006 μmol quanta m⁻² s⁻¹. 2. The average irradiance experienced by cells during the 2–2.5 h pre‐treatment incubations affected the productivity–irradiance (P–I) parameters: exposure to high light in pre‐treatment conditions caused a substantial decrease in maximum rate of primary production P_max and the photoinhibition parameter β when compared to cells pre‐treated in the dark. 3. While the data collected in this study were not sufficient to develop a full dynamic model of C. raciborskii productivity, P_max and β were modelled as a function of pre‐treatment irradiance, and these models were applied to predict the rate of primary production as a function of both instantaneous and historical irradiance. The results indicated that while cells with a history of exposure to high irradiance will be the most productive in high irradiance, production rates will be highest overall for dark‐acclimated cells in moderate irradiance. 4. Our results may explain why optically‐deep mixing favours C. raciborskii. If the mixing depth z_m exceeds the euphotic depth z_eu, cells will be dark‐acclimated, which will increase their rate of production when they are circulated through the euphotic zone. These results also predict that production rates will be higher during morning hours than for the same irradiance in the afternoon, which is consistent with other phytoplankton studies. 5. Since the rate of production of C. raciborskii‐dominated systems cannot be described by a single P–I curve, accurate estimates of production rates will require measurements over the daily light cycle.     

Light conditions affect the measurement of oceanic bacterial production via leucine uptake

The effect of irradiance in the range of 400 to 700 nm or photosynthetically active radiation (PAR) on bacterial heterotrophic production estimated by the incorporation of 3H-leucine (referred to herein as Leu) was investigated in the northwestern Mediterranean Sea and in a coastal North Atlantic site, with Leu uptake rates ranging over 3 orders of magnitude. We performed in situ incubations under natural irradiance levels of Mediterranean samples taken from five depths around solar noon and compared them to incubations in the dark. In two of the three stations large differences were found between light and dark uptake rates for the surface most samples, with dark values being on average 133 and 109% higher than in situ ones. Data obtained in coastal North Atlantic waters confirmed that dark enclosure may increase Leu uptake rates more than threefold. To explain these differences, on-board experiments of Leu uptake versus irradiance were performed with Mediterranean samples from depths of 5 and 40 m. Incubations under a gradient of 12 to 1,731 micromol of photons m(-2) x s(-1) evidenced a significant increase in incorporation rates with increasing PAR in most of the experiments, with dark-incubated samples departing from this pattern. These results were not attributed to inhibition of Leu uptake in the light but to enhanced bacterial response when transferred to dark conditions. The ratio of dark to light uptake rates increased as dissolved inorganic nitrogen concentrations decreased, suggesting that bacterial nutrient deficiency was overcome by some process occurring only in the dark bottles.     

Effects of light intensity and quality on the growth rate and photosynthetic pigment content of Spirulina platensis

The quantitative and qualitative effects of light on carotenoid production by Spirulina were studied. Maximum total carotenoid production was measured in cells grown under white light at an irradiance of 432 μmol photon m^?2 s^?1, the onset of light saturation for this organism as determined by growth rates. A true maximum may exist at irradiances above 1500 μmol photon m^?2 s^?1 under white light. Individual carotenoids responded differently to light conditions. Under white light, β-carotene and echinenone were most abundant at the lowest and highest irradiance levels tested. Myxoxanthophyll and lutein/zeaxanthin did not change over the same irradiance range. Under red and blue light, we found decreased values of myxoxanthophyll, while β-carotene increased and lutein/zeaxanthin and echinenone showed little change. In general, maximum carotenoid production requires optimization of the culture conditions that favor growth.     

STRATEGIES AND KINETICS OF PHOTOACCLIMATION IN THREE ANTARCTIC NANOPHYTOFLAGELLATES1

Three Antarctic nanophytoflagellates (two cryptophyte species and a Pyramimonas sp.) were compared for their capacity to phiotoacclimate and for their kinetic responses in changing photic environments. Division rate, cell size cellular fluorescence, and chlorophyll a content were measured steady and transient states of semi-continuous cultures maintain at 1.0° C. Of all parameters tested, cell size was most affected by irradiance. Acclimation kinetics were modeled using a first-order equation. Rates of change in cell size following shifts in irradiance were comparable with rates of change in chemical composition reported for temperate algae. Response rates of cellular in vivo red and orange fluorescence were lower. In many cases, however, responses could not be described by the first-order kinetic model. Division rates remained high for approximately 3 days following a shift down in irradiance, after which new division rates were established. The nanoflagellates studied here appear to respond to small irradiance perturbations at low rates. However, they may fail to adapt and abrupt changes in photon flux density (PFD). When shade-adapted (25 μmol, m^?2, m^?2, s^?1) cells were exposed to high PFD (400 μmol, m^?2, s^?1) for 1–3 days, cell were incapable of readapting division rate and pigment content to the initial irradiance condition (25 μmol, m^?2, s^?1) for about 1 month following the shift-down step. The ecological role of the kinetics of photoacclimation in nanophytoflagellate growth performance in Antarctic ecosystems is discussed.     

Light-path length and population density in photoacclimation of Nannochloropsis sp. (Eustigmatophyceae)

Photoacclimation in the marine eustigmatophyte Nannochlropsis sp., used extensively as a food chaincomponent in aquaculture, was studied both in thelaboratory and outdoors. Cell-chlorophyll andcarotenoids were used as markers to assessphotoacclimation to strong light, as well as todecreasing growth irradiance due to cellproliferation. Focusing on practical aspects involvedin mass cultivation, three different approaches wereused as follows: (a) cultures initially exposed to lowlight (150 mol photon m^-2 s^-1) thentransferred to strong light (1000 to 3000 molphoton m^-2 s^-1); (b) initially low celldensity cultures grown in reactors of differentlight-paths, exposed to strong PFD, in the laboratoryand outdoors; (c) initially low or high cell densitycultures exposed to strong light. As has already beenestablished in many reports, cell-chlorophyllrepresented a sensitive parameter in assessing cellresponse to changes in the intensity of the lightsource as well as to modifications in the light regimeto which the cells were exposed. Cell-chlorophyllconcentration sharply decreased initially upontransferring the culture from low PFD cell^-1 tohigh PFD cell^-1 due to either culture dilution(i.e. decrease in cell density and mutual shading) orto an increase in PFD. After some 7 days ofphotoacclimating to 2000 and 3000 mol photonm^-2 s^-1, chlorophyll a content began to riseat a much faster rate than cell number, which alsoincreased in response to the higher irradiance.Cell-chlorophyll in the culture exposed to 2000mol photon m^-2 s^-1 increased afteracclimation earlier and at a faster rate than in theculture exposed to 3000 mol photon m^-2s^-1, indicating the later irradiance affected astronger stress. The length of the reactor's lightpath exerted a decisive effect on cell response tostrong light through its influence on the light regimein the culture. Upon a sharp increase in PFD,carotenoids in the 1-cm reactor increased in muchhigher rate than chlorophyll, compared with the 3-cmlight path reactors. This marked difference in cellresponse to a shift-up in light was attributed to thevast variations in the light regime associated withdifferences in the length of the light path and areal density. Growth oflow cell density cultures ceased temporarily upontransfer to strong light, in contrast with high celldensity cultures transferred to strong light, whichcontinued growth without a lag.     

Tissue culture of Ecklonia radiata (Phaeophyceae,Laminariales): effects on growth of light,organic carbon source and vitamins

The effects of light quality and irradiance, and supply of organic carbon and vitamins on the growth of two forms of Ecklonia radiata in tissue culture were examined. A callus of unpigmented cells developed over the cut surface of newly excised explants of stipe. This growth was best in the dark but stopped after 10 weeks. Pigmented, mainly filamentous clumps of cells developed from explants after several weeks in culture. These required light for growth, with growth being enhanced by increasing photon flux density up to 30 μmol photon m^-2 s^-1, with the active spectral component being red light (> 600 nm). The addition to the medium of a range of organic carbon sources or vitamins did not stimulate growth of either culture type in the dark. author for correspondence     

Seasonal changes in CO2 and H2O gas exchange of young European beech (Fagus sylvatica L.)

Summary The CO₂ and H₂O gas exchange of young beech trees (Fagus sylvatica L.) were measured over a growing season. Of particular interest was the adaptation of gas exchange to the low level of photon flux density in the understorey of the old beech. The recorded diurnal courses were subdivided into several classes of irradiance. The most frequent class was from only 30–40 E * m^-2 * s^-1. Even at the highest irradiance values, no light saturation in assimilation occurred. The light compensation point lies below 3 E * m^-2 * s^-1, because net dark respiration values are very low. Calculated from the initial slope of the light response curves a mean value of 0.02 mol CO₂ * mol photons^-1 shows a very efficient use of light be the young trees. At the optimal phase of assimilation, the relationship between the daily sum of irradiance and net photosynthesis is highly significantly correlated. Under the local climatic situation, the stomatal opening primarily depends on irradiance. In response to a change in irradiance, stomatal opening also changes rapidly. Therefore, there is only a loose relationship between transpiration rate and vapour pressure saturation deficit. Towards autumn, the transpiration coefficient (E/A-ratio, estimated under light saturation) increases strongly because net photosynthesis decreases simultaneously.     

Suboptimal light conditions negatively affect the heterotrophy of Planktothrix rubescens but are beneficial for accompanying Limnohabitans spp

We examined the effect of light on the heterotrophic activity of the filamentous cyanobacterium Planktothrix rubescens and on its relationship with the accompanying bacteria. In situ leucine uptake by bacteria and cyanobacteria was determined in a subalpine mesotrophic lake, and natural assemblages from the zone of maximal P. rubescens abundances were incubated for 2 days at contrasting light regimes (ambient, 100× increased, dark). Planktothrix rubescens from the photic zone of the lake incorporated substantially more leucine, but some heterotrophic activity was maintained in filaments from the hypolimnion. Exposure of cyanobacteria to increased irradiance or darkness resulted in significantly lower leucine incorporation than at ambient light conditions. Highest abundances and leucine uptake of Betaproteobacteria from the genus Limnohabitans were found in the accompanying microflora at suboptimal irradiance levels for P. rubescens or in dark incubations. Therefore, two Limnohabitans strains (representing different species) were co-cultured with axenic P. rubescens at different light conditions. The abundances and leucine incorporation rates of both strains most strongly increased at elevated irradiance levels, in parallel to a decrease of photosynthetic pigment fluorescence and the fragmentation of cyanobacterial filaments. Our results suggest that Limnohabitans spp. in lakes might profit from the presence of physiologically stressed P. rubescens.     

Modelling photosynthetic photon flux density and maximum potential gross photosynthesis

Irradiance data software developed by the NREL Solar Radiation Laboratory (Simple Model of Atmospheric Radiative Transfer of Sunshine, SMARTS) has been used for modelling photosynthesis. Spectra and total irradiance were expressed in terms of quanta [mol m⁻² s⁻¹, photosynthetic photon flux density, PPFD (400–700 nm)]. Using the SMARTS software it is possible to (1) calculate the solar spectrum for a planar surface for any given solar elevation angle, allowing for the attenuating effects of the atmosphere on extraterrestrial irradiance at each wavelength in the 400–700 nm range and for the thickness of atmosphere the light must pass through during the course of a day, (2) calculate PPFD vs. solar time for any latitude and date and (3) estimate total daily irradiance for any latitude and date and hence calculate the total photon irradiance for a whole year or for a growing season. Models of photosynthetic activity vs. PPFD are discussed. Gross photosynthesis (P _g) vs. photosynthetic photon flux density (PPFD) (P _g vs. I) characteristics of single leaves compared to that of a canopy of leaves are different. It is shown that that the optimum irradiance for a leaf (I_opt) is the half-saturation irradiance for a battery of leaves in series. A C₃ plant, with leaves having an optimum photosynthetic rate at 700 μmol m⁻² s⁻¹ PPFD, was used as a realistic worked example. The model gives good estimates of gross photosynthesis (P _g) for a given date and latitude. Seasonal and annual estimates of P _g can be made. Taking cloudiness into account, the model predicts maximum P _g rates of about 10 g(C) m⁻² d⁻¹, which is close to the maximum reported P _g experimental measurements.     

FTIR spectra of algal species can be used as physiological fingerprints to assess their actual growth potential

Fourier transform infrared (FTIR) spectra were measured from cells of Microcystis aeruginosa and Protoceratium reticulatum, whose growth rates were manipulated by the availability of nutrients or light. As expected, the macromolecular composition changed in response to the treatments. These changes were species‐specific and depended on the type of perturbation applied to the growth regime. Microcystis aeruginosa showed an increase in the carbohydrate‐to‐protein ratio with decreased growth rates, under nutrient limitation, whereas light limitation induced a decrease of the carbohydrate‐to‐protein ratio with decreasing proliferation rates. The macromolecular pools of P. reticulatum showed a higher degree of compositional homeostasis. Only when the lowest light irradiance and nutrient availability were supplied, an increase of the carbohydrate‐to‐protein FTIR absorbance ratio was observed. A species‐specific partial least squares (PLS) model was developed using the whole FTIR spectra. This model afforded a very high correlation between the predicted and the measured growth rates, regardless of the growth conditions. On the contrary, the prediction based on absorption band ratios generally used in FTIR studies would strongly depend on growth conditions. This new computational method could constitute a substantial improvement in the early warning systems of algal blooms and, in general, for the study of algal growth, e.g. in biotechnology. Furthermore, these results confirm the suitability of FTIR spectroscopy as a tool to map complex biological processes like growth under different environmental conditions.     

Unusual allometry between in situ growth of freshwater phytoplankton under static and fluctuating light environments: possible implications for dominance

The effects of fluctuating light fields on the growth of phytoplanktonare not well understood and conclusions in the literature havebeen equivocal. Most studies have examined responses such asproductivity and chlorophyll a content (laboratory culture andfield tests) or growth rates (laboratory culture tests). Inthis study we examined the in situ growth rates of differenttypes of phytoplankton within two natural populations. Comparisonswere made between populations grown in a static environment(suspended in a fixed position in the water column) and an equivalentpopulation moving through the water column simulating the mixingof entrained phytoplankton. Growth under fluctuating light fieldsin this experiment only significantly (P < 0.05) increasedthe growth of the diatom Skeletonema and decreased the growthof Anabaena circinalis, Microcystis aeruginosa and Scenedesmussp. All other phytoplankton, including the genera Nitzschia,Fragilaria and Dactylococcopsis, did not have growth rates thatwere significantly different between static and fluctuatinglight treatments. A general pattern where diatoms grew best,followed by chlorophytes with the toxicogenic cyanophytes M.aeruginosa and A. circinalis growing least well, was distinguishedunder fluctuating irradiance. This seems consistent with thecommon occurrence of these groups of phytoplankton in the naturalenvironment. The cyanophytes Dactylococcopsis and Aphanothecedid not follow this pattern, with the former growing betterunder fluctuating light and the latter exhibiting an unusualgrowth pattern where growth was higher under lower light intensities.     

Light Conditions Affect the Measurement of Oceanic Bacterial Production via Leucine Uptake

The effect of irradiance in the range of 400 to 700 nm or photosynthetically active radiation (PAR) on bacterial heterotrophic production estimated by the incorporation of ³H-leucine (referred to herein as Leu) was investigated in the northwestern Mediterranean Sea and in a coastal North Atlantic site, with Leu uptake rates ranging over 3 orders of magnitude. We performed in situ incubations under natural irradiance levels of Mediterranean samples taken from five depths around solar noon and compared them to incubations in the dark. In two of the three stations large differences were found between light and dark uptake rates for the surfacemost samples, with dark values being on average 133 and 109% higher than in situ ones. Data obtained in coastal North Atlantic waters confirmed that dark enclosure may increase Leu uptake rates more than threefold. To explain these differences, on-board experiments of Leu uptake versus irradiance were performed with Mediterranean samples from depths of 5 and 40 m. Incubations under a gradient of 12 to 1,731 μmol of photons m⁻² s⁻¹ evidenced a significant increase in incorporation rates with increasing PAR in most of the experiments, with dark-incubated samples departing from this pattern. These results were not attributed to inhibition of Leu uptake in the light but to enhanced bacterial response when transferred to dark conditions. The ratio of dark to light uptake rates increased as dissolved inorganic nitrogen concentrations decreased, suggesting that bacterial nutrient deficiency was overcome by some process occurring only in the dark bottles.     

Buoyancy regulation in phosphate-limited cultures of Microcystis aeruginosa

Buoyancy regulation was studied in P-limited continuous cultures of Microcystis aeruginosa grown on light-dark cycles of 8–16 h. Gas-vesicle content did not vary systematically over a range of dilution rates form 0.004 to 0.015 h⁻¹. A reduction in irradiance did not cause a significant change in gas-vesicle content. The proportion of floating cells decreased during the photoperiod and increased during the dark period. At three dilution rates, parallel cultures were grown at growth-saturating irradiance and at a lower irradiance. The cultures at low irradiance had a higher proportion of floating cells and a smaller decrease in buoyancy during the light period. The buoyancy losses were not due to destruction of gas vesicles but, rather, to the accumulation of heavy substances. However, measured increases in polysaccharide ballast accounted for only 60% of the required ballast. The molecule(s) which comprised the remainder of the ballast are unknown. Upon relief of phosphate limitation, P-limited cultures increased their buoyancy when incubated in the dark or light. Buoyancy increases in the dark were correlated with a decrease in polysaccharide content, whereas there was an increase in gas vesicle content in the light.     

Net assimilation rate and shoot area development in birch (Betula pendula Roth.) at different steady-state values of nutrition and photon flux density

Summary Small birch plants (Betula pendula Roth.) were grown in a climate chamber at different, exponentially increasing rates of nitrogen supply and at different photon flux densities. This resulted in treatments with relative growth rate equal to the relative rate of increase in nitrogen supply and with different equilibrium values of plant nitrogen concentration. Nitrogen productivity (rate of dry matter increase per plant nitrogen) was largely independent of nitrogen supply and was greater at higher photon flux density. Leaf weight ratio, average specific leaf area (and thus leaf area ratio) were all greater at better nitrogen supply and at lower values of photon flux density. The dependencies were such that the ratio of total projected leaf area to plant nitrogen at a given photon flux density was similar at all rates of nitrogen supply. The ratio was greater at lower values of photon flux density. At a given value of photon flux density, net assimilation rate and net photosynthetic rate per shoot area (measured at the growth climate) were only slightly greater at better rates of nitrogen supply. Values were greater at higher photon flux densities. Acclimation of the total leaf area to plant nitrogen ratio and of net assimilation rate was such that nitrogen productivity was largely saturated with respect to photon flux density at values greater than 230 mol m^-2 s^-1. At higher photon flux densities, any potential gain in nitrogen productivity associated with higher net assimilation rates was apparently offset by lower ratios of total leaf area to plant nitrogen.     

Photoacclimation of Scenedesmus protuberans (Chlorophyceae) to fluctuating irradiances simulating vertical mixing

Photoacclimation of Scenedesmus protuberans Fritsch to fluctuatingirradiances, simulating vertical mixing, was studied in light-limitedcontinuous cultures. The algae were exposed to a simple sinusoidallight regime simulating diumal irradiance, and two fluctuatingregimes in which light fluctuations resulting from four andeight vertical circulations of the algae through the water columnwere imposed on the sinusoidal light regime. The total dailylight dose (TDLD) was kept constant. Maximum photosynthesisincreased, and cellular chlorophyll content and photosyntheticunit size decreased in response to light fluctuations. The efficiencyof photosynthesis decreased during the light period in the sinusoidallight regime, whereas it remained at the same level in the fluctuatinglight regimes. The results suggest that prolonged exposure tointermediate irradiances lowers the photosynthetic efficiencyof S.protuberans more than short exposure to high irradiancesalternated by lowlight periods, and that this species is ableto optimize its photosynthesis in fluctuating light.     

CHANGES IN PHOTOSYNTHESIS IN RESPONSE TO COMBINED IRRADIANCE AND TEMPERATURE STRESS IN CYANOBACTERIAL SURFACE WATERBLOOMS1

Buoyant cyanobacteria, previously mixed throughout the water column, float to the lake surface and form a surface waterbloom when mixing subsides. At the surface, the cells are exposed to full sunlight, and this abrupt change in photon irradiance may induce photoinhibition; at the same time, temperature rises as well. This study investigated the damaging effects of this increase in temperature as well as the ecologically more relevant combination of both an increased temperature and a high photon irradiance. Analysis of surface blooms with oxygen microelectrodes showed that integrated oxygen contents that are dependent on the balance of photosynthetic oxygen evolution and respiratory oxygen uptake decreased when temperature was raised above the lake temperature. Gross rates of photosynthesis were unaffected by temperatures up to of 35°C; hence, a moderate increase in temperature mainly stimulated oxygen uptake. Preincubation of cells of the cyanobacterium Anabaena flos-aquae (Lyngb.) de Brébisson at temperatures up to 35°C did not affect the subsequent measurement of rates of net photosynthesis. Another 5°C rise in temperature severely damaged the photosynthetic apparatus. Failure to restore net rates of photosynthesis was coupled to a strong quenching of the ratio of variable to maximum fluorescence, F_v/F_m, that was the result of a rise in F_o. A combination of high temperature and high photon irradiance was more damaging than high temperature alone. In contrast, low photon irradiances offered substantial protection against heat injury of the photosynthetic apparatus. I conclude from this study that because cyanobacteria usually are acclimated to low average irradiance prior to bloom formation, there is a reasonable risk of chronic photoinhibition. The increase in temperature will enhance the photodamage of cells in the top layer of the bloom. Low photon irradiances in subsurface layers will offer protection against heat injury. If the high temperatures extend to the deepest, dark layers of the bloom, damage in those layers is likely to occur.     

Effects of light intensity and temperature on the photosynthetic irradiance response curves and chlorophyll fluorescence in three picocyanobacterial strains of Synechococcus

Chrococcoid cyanobacteria of the genus Synechococcus are the important component of marine and freshwater ecosystems. Picocyanobacteria comprise even 80% of total cyanobacterial biomass and contribute to 50% of total primary cyanobacterial bloom production. Chlorophyll (Chl) fluorescence and photosynthetic light response (P-I) curves are commonly used to characterize photoacclimation of Synechococcus strains. Three brackish, picocyanobacterial strains of Synechococcus (BA-132, BA-124, BA-120) were studied. They were grown under 4 irradiances [10, 55, 100, and 145 μmol(photon) m^?2 s^?1] and at 3 temperatures (15, 22.5, and 30°C). Photosynthetic rate was measured by Clark oxygen electrode, whereas the Chl fluorescence was measured using Pulse Amplitude Modulation fluorometer. Based on P-I, two mechanisms of photoacclimation were recognized in Synechococcus. The maximum value of maximum rate of photosynthesis (P _max) expressed per biomass unit at 10 μmol(photon) m^?2 s^?1 indicated a change in the number of photosynthetic units (PSU). The constant values of initial slope of photosynthetic light response curve (α) and the maximum value of P _max expressed per Chl unit at 145 μmol(photon) m^?2 s^?1 indicated another mechanism, i.e. a change in PSU size. These two mechanisms caused changes in photosynthetic rate and its parameters (compensation point, α, saturation irradiance, dark respiration, P _max) upon the influence of different irradiance and temperature. High irradiance had a negative effect on fluorescence parameters, such as the maximum quantum yield and effective quantum yield of PSII photochemistry (φ_PSII), but it was higher in case of φ_PSII.     

EFFECTS OF PHOTON FLUENCE RATE AND LIGHT SPECTRUM COMPOSITION ON GROWTH,PHOTOSYNTHESIS AND PIGMENT RELATIONS IN GRACILARIA SP.1

The optimal photon fluence rate for growth of tha llus tips of Gracilaria sp. was low (about 100 μE·^–2·¹); higher photon fluence rates inhibited growth. Both phycoerythrin (PE) and chlorophyll (chl) contents decreased with increasing photon fluence rates (up to 100 μE·–m^–2s^–1) in a fashion inverse to the growth response. Chl/PE ratios varied directly as the growth response over a larger photon fluence rate range. The peak chl/PE ratios were obtained at a photon fluence rate optimal for growth, suggesting that this parameter may be used to estimate in situ growth rates. A low compensation point (about 7 μE·^–2s^–1) was observed for low light (15 μE·^–2s^–1) grown plants. This compensation point was also obtained for growth in the long–term (5–6 weeks) experiments. Plants grown at 60 and 140 μE·^–2s^–1 showed higher light compensation and saturation points, suggesting that the variations in pigment composition found between the different treatments determine the photosynthetic responses at sub–optimal photon fluence rates. Photosynthetic rates at light saturation were the same, on a biomass basis, for plants grown at the various photon fluence rates. Thus, the photosynthetic dark reactions were not influenced by previous light regimes. It is suggested that maximal photosynthetic rates expressed on a biomass basis better reflect the potential productivity at tight saturation than if expressed on a pigment basis. Gracilaria sp. grew better under non–filtered fluorescent and greenish than under reddish and blue–enriched light of equal and sub–optimal photon, fluence rate. However, the pigment relations of the algae did not change in a direction complementary to the light composition at which they grew. This, together with the relatively higher photosynthetic rates under reddish and blueish light for plants previously grown under reddish and blueish light, suggests that adaptations to variouslight spectra are based on mechanisms different from complementary chromatic adaptation of the pigments.