PHYSIOLOGICAL RESPONSES OF ANABAENA VARIABILIS (CYANOPHYCEAE) TO INSTANTANEOUS EXPOSURE TO VARIOUS COMBINATIONS OF LIGHT INTENSITY AND TEMPERATURE1

Light intensity and temperature interactions have a complex effect on the physiological process rates of the filamentous bluegreen alga Anabaena variabilis Kütz. The optimum temperature for photosynthesis increased with increasing light intensity from 10°C at 42 μE·m^?2·s^?1 to 35°C at 562 μE·m^?2·s^?1. The light saturation parameter, I_K, increased with increasing temperatures. The maximum photosynthetic rate (2.0 g C·g dry wt.^?1·d^?1) occurred at 35°C and 564 μE·m^?2·s^?1. At 15°C, the maximum rate was 1.25 g C·g dry wt.^?1·d^?1 at 332 μE·m^?2·s^?1. The dark respiration rate increased exponentially with temperature. Under favorable conditions of light intensity and temperature the percent of extracellular release of dissolved organic carbon was less than 5% of the total C fixed. This release increased to nearly 40% under combinations of low light intensity and high temperature. A mathematical model was developed to simulate the interaction of light intensity and temperature on photosynthetic rate. The interactive effects were represented by making the light-saturation parameters a function of temperature.     

CRITICAL IRRADIANCE LEVELS AND THE INTERACTIVE EFFECTS OF QUANTUM IRRADIANCE AND DOSE ON GAMETOGENESIS IN THE GIANT KELP,MACROCYSTIS PYRIFERA1

Gametophytes of Macrocystis pyrifera (L.) C. Ag. were cultured under a series of quantum irradiances in three photoperiod regimes. The quantum irradiances in each photoperiod were adjusted to provide equal daily irradiation dosages between photoperiods which allowed a critical examination of the interactions between quantum irradiance and quantum dose in determining gametophyte fertility. The lowest quantum irradiance which stimulated gametogenesis in more than 50% of the female gametophytes was 5 μE·m^?2·s^?1. The saturating irradiance was ca. 10 μE·m^?2·s^?1 at photoperiods of 12 h or greater. In terms of daily quantum dose, the lowest dose at which greater than 50% gametogenesis occurred was 0.2 E·m^?2·d^?1. However, this critical quantum dose was higher (0.4 E·m^?2·d^?1) when instantaneous irradiances were less than 5 μE·m^?2·s^?1. The saturation quantum dose was also affected by the rate at which the quantum dose was received and varied from 0.4 to 0.8 E·m^?2·d^?1. Gametophytes in all three photoperiods reached 100% fertility at quantum irradiances above 5 μE·m^?2·s^?1. Photoperiod effects were small and could be accounted for by quantum dosage effects.     

LIGHT AND TEMPERATURE AS FACTORS REGULATING SEASONAL GROWTH AND DISTRIBUTION OF ULOTHRIX ZONATA (ULVOPHYCEAE)1

Ulothrix zonata (Weber and Mohr) Kütz. is an unbranched filamentous green alga found in rocky littoral areas of many northern lakes. Field observations of its seasonal and spatial distribution indicated that it should have a low temperature and a high irradiance optimum for net photosynthesis, and at temperatures above 10°C it should show an increasingly unfavorable energy balance. Measurements of net photosynthesis and respiration were made at 56 combinations of light and temperature. Optimum conditions were 5°C and 1100 μE·m^?2·s^?1 at which net photosynthesis was 16.8 mg O₂·g^?1·h^?1. As temperature increased above 5° C optimum irradiance decreased to 125 μE·m^?2·s^?1 at 30°C. Respiration rates increased with both temperature and prior irradiance. Light-enhanced respiration rates were significantly greater than dark respiration rates following irradiance exposures of 125 μE·m^?2·s^?1 or greater. Polynomials were fitted to the data to generate response surfaces. Polynomial equations represent statistical models which can accurately predict photosynthesis and respiration for inclusion in ecosystem models.     

Effects of diurnal fluctuations in light intensity on the efficiency of growth of Skeletonema costatum (Grev.) Cleve (Bacillariophyceae) in a cyclostat

The effects of diurnal variations in light intensity on the biomass characteristics and the efficiency of daily growth of Skeletonema costatum (Grev.) Cleve were evaluated. The relative importance of changes in carbon specific rates of respiration and organic release to the efficiency of growth was determined. Light intensity was either constant at 130 μE · m^?2 · s^?1 during the light period or fluctuated throughout the light period from 500 to 10 μE · m^?2 · s^?1 at rates of either 1 or 12 cycles · day^?1. Total daily light was equivalent for all light regimes at 5.6 E · m^?2 · day^?1.Daily rates of growth remained comparable at ≈ 1 · day^?1 under constant and fluctuating light regimes. Cell size as daily mean carbon · cell^?1, nitrogen · cell^?1 and cellular volume was decreased under diurnally varying light whereas daily mean chlorophyll a · cell^?1 was unaffected.Rates of respiration, organic release and gross production were elevated several fold under diurnally varying light in comparison to constant light. Net growth efficiency decreased from 0.69 under constant light to values of 0.50 and 0.38 under 1 and 12 cycles · day^?1, respectively. Decreased efficiency of growth under diurnally fluctuating light resulted mostly from greater respiratory activity while organic release remained < 10% of gross production. Increased rates of gross production reflected enhancement in the efficiency of carbon fixation with fluctuating light.     

PRODUCTIVITY OF THE FILAMENTOUS ALGA PITHOPHORA OEDOGONIA (CHLOROPHYTA) IN SURREY LAKE,INDIANA1

Biomass, akinete numbers, net photosynthesis, and respiration of Pithophora oedogonia were monitored over two growing seasons in shallow Surrey Lake, Indiana. Low rates of photosynthesis occurred from late fall to early spring and increased to maximum levels in late spring to summer (29–39 mgO₂·g^?1 dry wt·h^?1). Areal biomass increased following the rise in photosynthesis and peaked in autumn (163–206g dry wt·m^?2). Photosynthetic rates were directly correlated with temperature, nitrogen, and phosphorus over the entire annual cycle and during the growing season. Differences in photosynthetic activity and biomass between the two growing seasons (1980 and 1981) were apparently related to higher, early spring temperatures and higher levels of NO₃-N and PO₄-P in 1981. Laboratory investigations of temperature and light effects on Pithophora photosynthesis and respiration indicated that these processes were severely inhibited below 15°C. The highest P_max value occurred at 35°C (0.602 μmol O₂·mg^?1 chl a·min^?1). Rates of dark respiration did not increase above 25°C thus contributing to a favorable balance of photosynthetic production to respiratory utilization at high temperatures. Light was most efficiently utilized at 15°C as indicated by minimum values of I_k(47 μE·m^?2·s^?1) and I_c (6 μE·m^?2·s^?1). Comparison of P. oedogonia and Cladophora glomerata indicated that the former was more tolerant of temperatures above 30°C. Pithophora's tolerance of high temperature and efficient use of low light intensity appear to be adaptive to conditions found within the dense, floating algal mats and the shallow littoral areas inhabited by this filamentous alga.     

THF INFLUENCE OF IRRADIANCE ON THF BIOCHEMICAL COMPOSITION OF THE PRYMNESIOPHYTE ISOCHRYSIS SP. (CLONE T-ISO)1

The effects of irradiance on the biochemical composition of the prymnesiophyte microalga, Isochrysis sp. (Parke; clone T-ISO), a popular species for mariculture, were examined. Cultures were grown under a 12:12 h light: dark (L:D) regime at five irradiances ranging from 50 to 1000 μE·m ²·s^?1 and harvested at late-logarithmic phase for analysis of biochemical composition. Gross composition varied aver the range of irradiances. The highest levels of protein were present in cells from cultures grown at 100 and 250 μE·m ³·s¹, and minimum levels of carbohydrate and lipid occurred at 50 μE·m^?2·s^?1. Because the cell dry weight was reduced at lower irradiances, different trends were evident when results were expressed as percentage of dry weights. Protein percentages were highest at Wand 100 μE·m^?2·s^?1 and carbohydrate at 100 μE·m^?2·s^?1. The composition of amino acids did not differ over the range of irradiances. Glutamate and aspartate were always present in high proportions (9.0–13.5%); histidine. methionine, tryptophan, cystine, and hydroxy-proline were minor constituents (0.0–2.6%). Glucose was the predominant sugar in all cultures, ranging from 23.0% (50 μE·m^?2·s^?1) to 45.0% (100 μE·m^?2·s^?1) of total polysaccharide. No correlation was found between the proportion of any of the sugars and irradiance. The proportions of the lipid class components and fatty acids showed little change with irradiance. The main fatty acids were 14:0, 16:0, 16:1(n-7), 18:1(n-9), 18:3(n-3). 18:4(n-3), 18:5(n-3), and 22:6(n-3). Proportions of 22: 6(n-3) increased, whereas l8:3(n-3). 18:3(n-6). and 18:4(n-3) decreased, with increasing irradiance. Pigment concentrations were highest in cultures grown at 50 μE·m^?2·s^?1, except for fucoxanthin and diadinoxanthin (100 μE·m^?2·s^?1). The concentrations of accessory pigments correlated with chlorophyll a, which decreased in concentration with increasing irradiance. On the basts of biochemical composition, an irradiance of 100 μE·m^?1·s^?1 (12:12 h L:D cycle)for the culture of Isochrysis sp. (clone T-ISO) may provide optimal nutritional value for maricultured animals, although feeding trials are now necessary to substantiate this.     

IMPORTANCE OF PHYSICAL VARIABLES ON THE SEASONAL DYNAMICS OF EPILITHIC ALGAE IN A HIGHLY SHADED CANYON STREAM1

The seasonal abundance of epilithic algae was correlated with major physico-chemical parameters in a first-order, heavily shaded stream in northern Arizona. Diatoms made up over 85%, by numerical abundance, of the epilithon community Light energy, water temperature, and stream discharge were most highly correlated with seasonal abundance of epilithic diatom taxa when analyzed with stepwise multiple regression. None of the chemical variables measured in the study (NO₃-N, O-PO₄, SiO₂, including PH) was found to be significantly correlated with the seasonal community structure of epilithic diatoms. Total diatom cell densities showed a significant negative correlation to stream bed light energy. Likewise, total diatom cell densities along a transect in the stream bed showed a negative correlation to current velocity during those months when base flow was low and stable, and current velocity was ≤25 cm·sec-¹. Most diatom taxa had highest cell densities at temperatures < 16°C and at daily mean stream bed light levels < 400 μE·m^?2·s^?1. Highest cell densities of green algae occurred at temperatures between 6–16°C and at daily mean stream bed light levels of > 400 μE·m^?2·s^?1. Blue-green algae (cyanobacteria) grew best at the highest recorded water temperatures and daily mean stream bed light energy (16–20°C and 900–1200 μE·m^?2·s^?1). Abrupt increases in NO₃-N coincided with a brief pulse of Nostoc pruniforme colonies during June, and leaf drop from Alnus oblongifolia during October.     

LIGHT,TEMPERATURE AND PHOTOPERIOD AS FACTORS CONTROLLING REPRODUCTION IN ULOTHRIX ZONATA (ULVOPHYCEAE)1

In many lakes in the northern United States and Canada the filamentous green alga Ulothrix zonata (Weber and Mohr) Kütz grows abundantly in early spring in shallow waters. Asexual reproduction occurs by formation of quadriflagellate zoospores which disrupt, the integrity of the cells upon release causing the filament to disintegrate. Study of the effects of 100 different combinations of irradiance, temperature and photoperiod revealed that zoospore formation is favored by high temperatures near 20°C, high light levels of 520 μE·m^?2·s^?1 and photoperiods of either short day (8:16 h light-dark) or long day cycles (16:8 h light-dark). Zoospore formation is minimal under conditions of low temperature (5°C), low irradiance (32.5 μE·m^?2·s^?1) and neutral day-lengths (12:12 h light-dark). These observations explain the decline in U. zonata biomass when water temperatures rise above 10° C. The combined effect of rising water temperatures and increasing daylengths causes progressively more filaments to switch from vegetable growth to zoospore production resulting in an increasing loss of biomass.     

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.     

EFFECTS OF LIGHT AND TEMPERATURE ON GROWTH,NITRATE UPTAKE,AND TOXIN PRODUCTION OF TWO TROPICAL DINOFLAGELLATES: ALEXANDRIUM TAMIYAVANICHII AND ALEXANDRIUM MINUTUM (DINOPHYCEAE)1

The two tropical estuarine dinoflagellates, Alexandrium tamiyavanichii Balech and A. minutum Halim, were used to determine the ecophysiological adaptations in relation to their temperate counterparts. These species are the two main causative organisms responsible for the incidence of paralytic shellfish poisoning (PSP) in Southeast Asia. The effects of light (10, 40, 60, and 100 μmol photons·m^?2·s^?1) and temperature (15, 20, and 25°C) on the growth, nitrate assimilation, and PST production of these species were investigated in clonal batch cultures over the growth cycle. The growth rates of A. tamiyavanichii and A. minutum increased with increasing temperature and irradiance. The growth of A. tamiyavanichii was depressed at lower temperature (20°C) and irradiance (40 μmol photons·m^?2·s^?1). Both species showed no net growth at 10 μmol photons·m^?2·s^?1 and a temperature of 15°C, although cells remained alive. Cellular toxin quotas (Q_t) of A. tamiyavanichii and A. minutum varied in the range of 60–180 and 10–42 fmol PST·cell^?1, respectively. Toxin production rate, R_tox, increased with elevated light at both 20 and 25°C, with a pronounced effect observed at exponential phase in both species (A. tamiyavanichii, r²=0.95; A. minutum, r²=0.96). Toxin production rate also increased significantly with elevated temperature (P<0.05) for both species examined. We suggest that the ecotypic variations in growth adaptations and toxin production of these Malaysian strains may reveal a unique physiological adaptation of tropical Alexandrium species.     

PHOTOSYNTHESIS-IRRADIANCE RELATIONSHIPS IN SEA ICE MICROALGAE FROM McMURDO SOUND,ANTARCTICA1

Sea ice microalgae in McMurdo Sound, Antarctica were examined for photosynthesis-irradiance relationships and for the extent and time course of their photoadaptation to a reduction in in situ irradiance. Algae were collected from the bottom centimeter of coarse-grained congelation ice in an area free of natural snow cover. Photosynthetic rate was determined in short term (1 h) incubations at ?2° C over a range of irradiance from 0 to 286 μE·m^?2·s^?1. Assimilation numbers were consistently below 0.1 mg C·mg chl a^?1·h^?1. The I_k's³ averaged only 7 μE·m^?2·s^?1, and photosynthesis was inhibited at irradiances above 25 μE·m^?2·s^?1. Photosynthetic parameters of the ice algal community were examined over a nine day period following the addition of 4 cm of surface snow while a control area remained snow-free. A reduction of 40% in P_m^B relative to the control occurred after two days of snow cover; α, β, I_k, and I_m were not significantly altered. Low assimilation numbers and constant standing crop size, however, suggested that the algal bloom may have already reached stationary growth phase, possibly minimizing their photoadaptive response.     

EFFECTS OF VARIATIONS IN LIGHT INTENSITY ON THE EFFICIENCY OF GROWTH OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE) IN A CYCLOSTAT1

The relative importance of respiration and organic carbon release to the efficiency of carbon specific growth of Skeletonema costatum (Grev.) Clave was evaluated over a light range from 1500–15 μE · m^?2· s^?1. Net growth efficiency ranged from 0.45–0.69 with a maximum at 130 μE · m^?2· s^?1. Respiration was 93% or more of the variations in growth efficiency. Organic carbon release ranged from 0–7% of gross production and increased with light intensity. Carbon specific particulate production was a hyperbolic function of incident light intensity and was related exponentially to particulate carbon production per unit chlorophyll a. Full sunlight conditions, 1500 μE · m^?2· s^?1, did not induce photoinhibition of gross production. Variations in the efficiency of growth of S. costatum were minimized over a wide range of light intensities mainly because of variations in cellular pigments which permitted the efficient utilization of available light energy, and a reduction in the losses of carbon which increases the growth rate, possibly as a consequence of the recycling of respired carbon within the cell.     

SHADE ADAPTED BENTHIC DIATOMS BENEATH ANTARCTIC SEA ICE1

A dense community of shade adapted microalgae dominated by the diatom Trachyneis aspera is associated with a siliceous sponge spicule mat in McMurdo Sound, Antarctica. Diatoms at a depth of 20 to 30 m were found attached to spicule surfaces and in the interstitial water between spicules. Ambient irradiance was less than 0.6 μE · m^?2· s^?1 due to light attenuation by surface snow, sea ice, ice algae, and the water column. Photosynthesis-irradiance relationships determined by the uptake of NaH¹⁴CO₃ revealed that benthic diatoms beneath annual sea ice were light-saturated at only 11 μE·m^?2·s^?1, putting them among the most shade adapted microalgae reported. Unlike most shade adapted microalgae, however, they were not photoinhibited even at irradiances of 300 μE·m^?2·s^?1. Although in situ primary production by benthic diatoms was low, it may provide a source of fixed carbon to the abundant benthic invertebrates when phytoplankton or ice algal carbon is unavailable.     

PHYSIOLOGICAL VARIATION AND ELECTROPHORETIC BANDING PATTERNS OF GENETICALLY DIFFERENT SEASONAL POPULATIONS OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1

Clones of Skeletonema costatum (Grev.) Cl. isolated from Narragansett Bay, R.I., during different seasons were grouped according to their electrophoretic banding patterns. The growth rates, pg chlorophyll · cell^?1, carbon uptake · cell^?1· h^?1, and carbon uptake · pg chl^?1· h^?1 were measured at 20°C, in a 14:10 h L:D cycle at 180 μE · m^?2· s^?1. Statistically significant sources of variation were found among groups of clones in growth rate, pg chl · cell^?1, and carbon uptake · pg chl^?1· h^?1. It was concluded that there is a significant relationship between the physiological characteristics of clones isolated from populations in different seasons and patterns of genetic variation inferred from the electrophoretic studies. However, genetic diversity detected by banding patterns tends to underestimate the total genetic diversity in natural populations. The groups of clones most common in summer bloom populations had significantly higher growth rates, lower values of pg chl · cell^?1, and higher rates of carbon uptake · pg chl^?1· h^?1 at 20°C than did the group of clones most common in winter bloom populations. However, differences among groups in these parameters at 20°C alone cannot account for the seasonal cycling of genetically variable populations of Skeletonema in Narragansett Bay. The range of growth rates among clones of this species is 0.1–5.0 divisions · d^?1 under a single set of temperature and light conditions. Chlorophyll concentrations range from 0.2–1.7 pg chl · cell^?1 and carbon uptake · pg chl^?1· h^?1 varies by a factor of 7 among clones. The range of physiological variation in this species means that it is difficult to use laboratory studies of single clones to analyze the responses of natural populations of Skeletonema.     

TEMPERATURE AND IRRADIANCE EFFECTS ON GROWTH OF PITHOPHORA OEDOGONIA (CHLOROPHYCEAE) AND SPIROGYRA SP. (CHAROPHYCEAE)1

Growth responses of Pithophora oedogonia (Mont.) Wittr. and Spirogyra sp. to nine combinations of temperature (15°, 25°, and 35°C) and photon flux rate (50, 100, and 500 μmol·m^?2·s^?1) were determined using a three-factorial design. Maximum growth rates were measured at 35°C and 500 pmol·m^?2·s^?1 for P. oedogonia (0.247 d^?1) and 25°C and 500 μmol·m^?2·s^?1 for Spirogyra sp. (0.224 d^?1). Growth rates of P. oedogonia were strongly inhibited at 15°C (average decrease= 89%of maximum rate), indicating that this species is warm stenothermal. Growth rates of Spirogyra sp. were only moderately inhibited at 15° and 35°C (average decrease = 36 and 30%, respectively), suggesting that this species is eurythermal over the temperature range employed. Photon flux rate had a greater influence on growth of Spirogyra sp. (31% reduction at 50 pmol·m^?2·s^?1 and 25°C) than it did on growth of P. oedogonia (16% reduction at 50 μmol·m^?2·s^?1 and 35°C). Spirogyra sp. also exhibited much greater adjustments to its content of chlorophyll a (0.22–3.34 μg·mg fwt^?1) than did P. oedogonia (1.35–3.08 μg·mg fwt^?1). The chlorophyll a content of Spirogyra sp. increased in response to both reductions in photon flux rate and high temperatures (35°C). Observed species differences are discussed with respect to in situ patterns of seasonal abundance in Surrey Lake, Indiana, the effect of algal mat anatomy on the internal light environment, and the process of acclimation to changes in temperature and irradiance conditions.     

EFFECTS OF SUBSTRATE RELIEF ON THE DISTRIBUTION OF PERIPHYTON IN LABORATORY STREAMS. II. INTERACTIONS WITH IRRADIANCE1

Laboratory streams were used in a 42-day experiment designed to investigate how the spatial and temporal distribution of lotic periphyton created by current flow over cobble-size substrates is a affected by irradiance. The streams contained 22.5 × 22.5 × 4 cm substrate blocks and were exposed to either 385, 90 or 20 μE·m^?2·s^?1. We monitored periphyton succession in fast current regimes on top of blocks and in slower current regimes on surfaces recessed between blocks. The absolute differences in AFDW algal biomass between top and recessed substrates were significantly affected by irradiance and time. At the end of the experiment, biomass in streams exposed to 385 μE·m^?2·s^?1. was approximately 2 and 8 times greater than in streams exposed to 90 and 20 μE·m^?2·s^?1, respectively. Differences in biomass were greater between irradiance levels than between top and recessed substrates within an irradiance level. Irradiance also had a greater effect than current regime on the taxonomic composition of assemblages. Oscillatoria agardhii Gomont and Navicula minima Grun. characterized assemblages at 20 μE·m^?2·s^?1, whereas Fragilaria vaucheriae (Kütz.), Nitzschia oregona Sov., Navicula arvensis Hust. and Stigeoclonium tenue (Ag.) Kütz. were more abundant at the two higher irradiances. Detrended correspondence analysis indicated that the rate of succession was relatively high for assemblages at high irradiance and in the slow current regimes between blocks. The results suggested that in natural streams, periphyton patches produced by large differences in irradiance should have a greater effect on periphyton heterogeneity than substrate-induced patches. Moreover, the heterogeneity of algal patches produced by hydrologic differences over a substrate is constrained by irradiance level.     

PHOTOINHIBITION OF MECHANICALLY STIMULABLE BIOLUMINESCENCE IN THE AUTOTROPHIC DINOFLAGELLATE CERATIUM FUSUS (PYRROPHYTA)1

Ceratium fusus (Ehrenb.) Dujardin was exposed to light of different wavelengths and photon flux densities (PFDs) to examine their effects on mechanically stimulable bioluminescence (MSL). Photoinhibition of MSL was proportional to the logarithm of PFD. Exposure to I μmol photons·m^?2s^?1 of broadband blue light (ca. 400–500 nm) produced near-complete photoinhibition (≥90% reduction in MSL) with a threshold at ca. 0.01 μmol photons·m^?2·s^?1. The threshold of photoinhibition was ca. an order of magnitude greater for both broadband green (ca. 500–580 nm) and red light (ca. 660–700 nm). Exposure to narrow spectral bands (ca. 10 nm half bandwidth) from 400 and 700 nm at a PFD of 0.1 μmol photons·m^?2·s^?1 produced a maximal response of photoinhibition in the blue wavelengths (peak ca. 490 nm). A photoinhibition response (≥ 10%) in the green (ca. 500–540 nm) and red wavelengths (ca. 680 nm) occurred only at higher PFDs (1 and 10 μmol photons·m^?2·s^?1). The spectral response is similar to that reported for Gonyaulax polyedra Stein and Pyrocystis lunula Schütt and unlike that of Alexandrium tamarense (Lebour) Balech et Tangen. The dinoflagellate's own bioluminescence is two orders of magnitude too low to result in self-photoinhibition. The quantitative relationships developed in the laboratory predict photoinhibition of bioluminescence in populations of C. fusus in the North Atlantic Ocean.     

OXYGEN MICROSENSOR STUDIES ON ZOOXANTHELLATE CLIONAID SPONGES FROM THE COSTA BRAVA,MEDITERRANEAN SEA1

Photosynthetic properties of two symbiotic demosponges were compared using Clark‐type oxygen microsensors. The putatively distinct sponge species, Cliona viridis (Schmidt, 1862) and Cliona nigricans (Schmidt, 1862) were discriminated by their mean megasclere lengths of 296 and 387 μm, respectively. Photosynthetic behavior was used to generate additional taxonomic information. Sponge–dinoflagellate symbioses were well adapted to low light due to the hosts' endolithic lifestyle. Both sponges reached light compensation and saturation at similar light levels with means close to 10 and 30 μmol photons·m^?2·s^?1, respectively. The gross photosynthetic activity was closely related to symbiont cell density in the sponge surface tissue. Mean symbiont densities, chl a content, and gross photosynthesis were about six times higher in C. viridis than in C. nigricans, with respective values of 3000 and 440 symbiont·mm^?2, 1.3 and 0.2 μg chl a·g^?1, and 5.4 and 1.0 μmol O₂·cm^?3·s^?1 gross photosynthesis. Net photosynthesis and respiration could not be calculated accurately from the oxygen gradients, because significant gas exchange occurs through the pumping activity. Thus, assumptions of diffusional oxygen exchange via the surface do not hold for sponges. Combined data of this study indicate that the metabolic activity of C. viridis depends on photosynthetic activity of its symbionts, whereas C. nigricans appears to have a higher pumping intensity and is more actively filter feeding. The difference in photosynthetic activities is not caused by different light adaptations but provides new evidence against the conspecifity of C. viridis and C. nigricans.     

INFLUENCE OF IRRADIANCE ON THE FATTY ACID COMPOSITION OF PHYTOPLANKTON1

Eight species of marine phytoplankton commonly used in aquaculture were grown under a range of photon flux densities (PEDs) and analyzed for their fatty acid (FA) composition. Fatty and composition changed considerably at different PFDs although no consistent correlation between the relative proportion of a single FA and μ or chl a · cell^?1 was apparent. Within an individual species the percentage of certain fatty acids covaried with PFDs, growth rate and/or chl a · cell^?1. The light conditions which produced the greatest proportion of the essential fatty acids was species specific. Eicosapentaenoic acid. 20:5ω3 increased from 6.1% to 15.5% of the total fatty acids of Chaetoceros simplex Ostenfield grown at PFDs which decreased from 225 μE · m^?2· s^?1 to 6 μE · m^?2· s^?1, respectively. Most species had their greatest proportion of 20: 5ω3 at low levels of irradiance. Conversely, docosahexaenoic acid, 22:6ω3, decreased from 9.7% to 3.6% of the total fatty acids in Pavlova lutheri Droop as PFD decreased. The percentage of 22:6ω3 generally decreased with decreasing irradiances. In all diatoms the percentage of 16:0 was significantly correlated with PFD, and in three of five diatoms, with growth rate (μ). Results suggest that fatty acid composition is a highly dynamic component of cellular physiology, which responds significantly to variation in PFD.     

EFFECTS OF IRRADIANCE AND GRAZING ON LOTIC ALGAL ASSEMBLAGES1

A laboratory experiment was conducted for 75 days to examine how irradiance levels and grazing influence algal biomass and community structure. Twelve laboratory streams were used for experimental analyses, with four channels exposed to one of three irradiance levels (15, 100, or 400 μE·m^?2·s^?1). Three of the four stream at each light level were stocked with the snail Juga silicula (250·m^?2), leaving one stream at each light level without snails. Grazed stream exposed to low light levels developed low amounts of algal biomass (<2 g AFDW·m^?2) and were dominated by adnately attached diatoms. Mean algal biomass increased over time in the grazed streams exposed to intermediate light; by day 75, these streams were characterized by moderate algal biomasses (30-40 g AFDW·m^?2) and filamentous chlorophytes. Algal assemblages in high light, grazed channels had high levels of biomass at day 43 (70 g AFDW·m^?2) that declined to 30 g AFDW·m^?2at day 75 and were dominated by chlorophytes. Among ungrazed streams, algal biomass at day 75 was relatively low in the low light streams (<7g AFDW·m^?2) and was dominated by adnately attached diatoms. Ungrazed streams exposed to intermediate and high light levels had moderate biomasses (23 and 19 g AFDW·m^?2, respectively) and were dominated by chlorophytes and large diatoms. Grazing appeared both to delay and alter successional trajectories of algal assemblages, with alterations most noticeable during early seral stages at intermediate and high light levels. Grazing had the least effect on successional trajectories at low light.