Changes in photosynthetic pigment concentration in seaweeds as a function of water depth

We conducted a study of the relationship between changes in photosynthetic pigment content and water depth in Great Harbor near Woods Hole, Massachusetts, USA, on the green algae Ulva lactuca and Codium fragile and the red algae Porphyra umbilicalis and Chondrus crispus. A calibrated underwater photometer equipped with spectral band filters measured light attenuation by the water column. The depth required for a 10-fold diminution of photon flux was 3.6, 5.3, 6.0 and 6.0 m for red, blue, yellow and green light, respectively. Seaweeds were attached to vertically buoyed lines and left to adapt for 7 days; then, with their positions reversed, they were allowed to readapt for 7 days. All species showed greater photosynthetic pigment content with increased depth. Further, the ratio of phycobiliproteins and chlorophyll b to chlorophyll a increased with depth. Changes in pigment content were reversible and occurred in the absence of cell division. There was a net loss of pigments near the surface (high irradiance), and subsequent synthesis when seaweeds were transferred to a position deep in the water column (low irradiance). In contrast, seaweeds which were found in intertidal habitats changed only their pigment concentration, and not pigment ratio, a phenomena analogous to higher plant sun and shade adaptation. Therefore, seaweeds modify their photon-gathering photosynthetic antennae to ambient light fields in the water column by both intensity adaptation and complementary chromatic adaptation.     

Seasonal variations in salt-march macroalgae photosynthesis. II. Fucus vesiculosus and Ulva lactuca

Photosynthesis in whole plants of the salt-marsh algae Fucus vesiculosus and Ulva lactuca was evaluated by ¹⁴C-uptake under a variety of light intensities at approximately mately monthly intervals during a 15-month study. Photosynthetic capacity in both species was closely related to seasonal irradiation patterns and changes in field biomass. Maximum photosynthesis occurred in the spring and summer months. Photosynthesis on a dry weight basis was higher in U. lactuca, while photosynthesis on a chlorophyll a basis was equal in both species. Photosynthetic capacity was inversely related to pigment content. Maximum chlorophyll a concentrations occurred during the winter. Frond profile studies in F. vesiculosus indicated that apices always exhibited greatest photosynthetic capacity. Uptake of ¹⁴C into ethanol-soluble and insoluble fractions was different in each species. F. vesiculosus showed greater activity in the ethanol-soluble fraction while U. lactuca exhibited greater activity in ethanol-insoluble fractions.This research was supported by Research Grant AG-375 from the National Science Foundation and, in part, by the State University of New York Research Foundation and the Energy Research and Development Administration (ERDA).     

Survival,metabolism and growth of Ulva lactuca under winter conditions: a laboratory study of bottlenecks in the life cycle

Photosynthesis and growth in low light and survival under simulated winter conditions were studied in the freefloating green alga Ulva lactuca L., collected in Roskilde Fjord, Denmark during late autumn and maintained in stock in natural water. It adapts efficiently to low light by increasing chlorophyll concentration and light absorption and continues to grow at the lowest irradiance tested, 0.6 E m^-2 s^-1. This irradiance corresponds to minimum light requirements of deep-living marine macroalgae and phytoplankton growing under ice. The photosynthetic efficiency per unit of incident light is five-fold higher for U. lactuca grown at 1.7 E PAR m^-2 s^-1 as compared with 56.3 E m^-2 s^-1, and the efficiency per unit of light absorbed is twice as high. The maximum photosynthetic efficiency (0.051 mol C E^-1 absorbed) is similar to values for shade-adapted marine phytoplankton. U. lactuca is able to survive for two months in the dark and to resume growth immediately when transferred to light. Exposure to anoxia and sulphide gradually reduces vitality, but does not affect survival over two months. Rigorous deep freezing is detrimental to survival of U. lactuca, while field samples show that more gradual, natural freezing is not. U. lactuca is not easily fitted into one of the traditional strategy concepts. U. lactuca is a very plastic species that combines rapid growth during favourable periods (opportunism) with high survival capacity in the same type of tissue during stress periods (persistence). U. lactuca occupies a niche as a free-floating form in sedimentary coastal areas that are devoid of attached algae.     

Inter-polyp genetic and physiological characterisation of <Emphasis Type="Italic">Symbiodinium</Emphasis> in an <Emphasis Type="Italic">Acropora valida</Emphasis> colony

Corals harbouring genetically mixed communities of endosymbiotic algae (Symbiodinium) often show distribution patterns in accordance with differences in light climate across an individual colony. However, the physiology of these genetically characterised communities is not well understood. Single stranded conformation polymorphism (SSCP) and real time quantitative polymerase chain reaction (qPCR) analyses were used to examine the genetic diversity of the Symbiodinium community in hospite across an individual colony of Acropora valida at the spatial scale of single polyps. The physiological characteristics of the polyps were examined prior to sampling with a combined O₂ microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabling simultaneous measurements of O₂ concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface as a function of increasing irradiances. Both sun- and shade-adapted polyps were found to harbour either Symbiodinium clade C types alone or clades A and C simultaneously. Polyps were grouped in two categories according to (1) their orientation towardps light, or (2) their symbiont community composition. Physiological differences were not detected between sun- and shade-adapted polyps, but O₂ concentration at 1,100 μmol photons m⁻² s⁻¹ was higher in polyps that harboured both clades A and C symbionts than in polyps that harboured clade C only. These results suggest that the acclimatisation of zooxanthellae of individual polyps of an A. valida colony to ambient light levels may not be the only determinant of the photosynthetic capacity of zooxanthellae. Here, we found that photosynthetic capacity is also likely to have a strong genetic basis and differs between genetically distinct Symbiodinium types.     

Growth rate and carbon affinity ofUlva lactuca under controlled levels of carbon,pH and oxygen

We examined the growth rate (µ) ofUlva lactuca L. (collected from Roskilde Fjord, Denmark in 1987) at different levels of dissolved inorganic carbon (DIC), pH and oxygen in two culture facilities. Growth was faster in Facility A (µ _max ca 0.3 d^–1) than in B (µ _max ca 0.2 d^–1), probably because of more efficient stirring and higher light intensity. The growth-DIC response curve exhibited low half-saturation constant (K _1/2) values (0.35 mM DIC in A, 0.55 mM in B) and growth rates close toµ _max at natural seawater concentration of 2 mM DIC. Growth rate showed a low sensitivity to oxygen over a wide range of DIC and oxygen concentrations. Collectively, the results demonstrated an efficient mechanism for DIC use, unaffected by acclimatization to DIC concentrations between 0.2 and 3 mM. The growth rate decreased little between pH 7.5 and 9 at 2 mM DIC, but steeply above pH 9 approaching zero just above pH 10. The decline of growth at high pH may result from direct pH effects on cell pH, reduced HCO ₃ ^- availability and impaired operation of the carbon uptake process. The growth responses ofU. lactuca to DIC, pH and oxygen resembled those observed in previous short-term photosynthetic experiments. This similarity is probably due to the fast growth ofU. lactuca which means that photosynthetic products are rapidly converted into cell growth. Based on the culture experiments we argue that field plants ofU. lactuca not exposed to stagnant water and DIC depletion are likely to be limited in growth by environmental factors other than DIC (e.g. light and nutrients). Dense mats ofU. lactuca, however, may show reduced growth as a result of DIC depletion, high pH and self-shading.     

Intra-colonial variability in light acclimation of zooxanthellae in coral tissues of Pocillopora damicornis

We investigated heterogeneity of light acclimation of photosynthesis in sun- and shade-adapted coenosarc and polyp tissues of Pocillopora damicornis. The zooxanthellar community within P. damicornis colonies at Heron Island is genetically uniform, yet they showed a large degree of plasticity in their photo-physiological acclimation linked to light microclimates characterised by fibre-optic microprobes. Microscale scalar irradiance measurements showed higher absorption in polyp than coenosarc tissues and higher absorption in the more densely pigmented shade-adapted polyps than in sun-adapted polyps. The combination of an O₂ microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabled parallel measurements of O₂ concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface under steady-state conditions as a function of increasing irradiances. Lower O₂ levels at the tissue surface and higher compensation irradiance indicated a higher respiration activity in sun-adapted polyp tissue as compared to shade-adapted polyps. Shade-adapted coenosarc and polyp tissues exhibited lower maxima of relative electron transport rates (rETR_max) (84±15 and 41±10, respectively) than sun-adapted coenosarc and polyp tissues (136±14 and 77±13, respectively). Shade-adapted tissues showed stronger decrease of rETR at high scalar irradiances as compared to sun-adapted tissues. The relationship between the relative PSII electron transport and the rate of gross photosynthesis, as well as O₂ concentration, was non-linear in sun-adapted tissues over the entire irradiance range, whereas for shade-adapted tissues the relationship became non-linear at medium to high scalar irradiances >200 μmol photons m⁻² s⁻¹. This suggests that rETR measurements should be used with caution in corals as a proxy for photosynthesis rates. The apparently high rates of photosynthesis (oxygen evolution rates) suggest that there must be a considerable electron transport rate through the photosystems that is not observed by the rETR measurements. This may be accounted for by vertical heterogeneity of zooxanthellae in the tissue and the operation of an alternative electron pathway such as cyclic electron flow around PSII.     

Kinetic measurements of metal accumulation in two marine macroalgae

 We measured the uptake kinetics of four metals (Cd, Cr, Se and Zn) in two marine macroalgae (the green alga Ulva lactuca and the red alga Gracilaria blodgettii). Metal uptake generally displayed a linear pattern with increasing exposure time. With the exception of Cr, which exhibited comparable uptake rate constants at different concentrations, uptake rate constants of Cd, Se and Zn decreased with increasing metal concentration, indicating that the seaweeds had a higher relative uptake at lower metal concentration. Uptake of Cd and Zn was higher in U. lactuca than in G. blodgettii, whereas uptake of Cr and Se was comparable between the two species. Only Cd and Zn uptake in U. lactuca was significantly inhibited by dark exposure. A decrease in salinity from 28 to 10‰ enhanced the uptake of Cd, Cr, Se and Zn in U. lactuca 1.9-, 3.0-, 3.6-, and 1.9-fold, respectively. In G. blodgettii, Cd uptake increased twofold when salinity was decreased from 28 to 10‰, whereas uptake of Cr and Zn was not significantly affected by salinity change. The calculated depuration rate constants of metals in U. lactuca were 0.01 d⁻¹ for Cd, 0.05 to 0.08 d⁻¹ for Cr, 0.14 to 0.16 d⁻¹ for Se, and 0.12 to 0.15 d⁻¹ for Zn, and were relatively independent of the metal body burden in the algae. The predicted bioconcentration factor was 3 × 10⁴ for Cd, 2 × 10³ for Cr, 40 to 150 for Se, and 1 to 2 × 10⁴ for Zn in U. lactuca. Our kinetic study suggested that U. lactuca would be a good biomonitor of Cr and Zn contamination in coastal waters. Received: 14 September 1998 / Accepted: 29 May 1999     

Nutrient enrichment of seagrass beds in a rhode island coastal lagoon

Seagrass and algal beds showed a variety of reponses when the water column was treated with low level additions of ammonium, nitrate and phosphate. The nutrients were added separately to 3 uniform seagrass beds of a temperature coastal lagoon during 1979 and 1980. (1) Ammonium caused the production of dense mats of free-floating green algae Enteromorpha plumosa and Ulva lactuca. It also stimulated growth in both the leaf and root-rhizome fractions of Zostera marina. This growth response in Z. marina was greater in the area where current reached 12 cm · s^-1 than in the area with little or no current. The concentration of nitrogen in the tissue did not change. In contrast, where current was lacking, Z. marina growth increase with ammonium was small, but the concentration of nitrogen in the tissue doubled over that in control plots. The growth of Ruppia maritima was inversely related to the growth of green algae in the same plots. The red alga Gracilaria tikvahiae did not grow better in ammonium, but its tissue reddened. (2) Nitrate additions enhanced the growth of the green seaweeds Enteromorpha spp. and U. lactuca, but not Z. marina or R. maritima. G. tikvahiae, when fertilized in isolation from other plants, showed a marginal response to this nutrient, and the tissue always reddened. (3) Phosphate enhanced growth in Z. marina and R. maritima exposed to moderate current. G. tikvahiae growing alone showed a small growth response to phosphate. The phosphate made no difference in the growth of the green seaweeds. (4) None of the nutrient supplements noticeably altered the species composition of either epiphytic or planktonic algae associated with the beds, although we did detect small increases in their numbers. The rapid and dense growth of green algae in nitrogen-enriched water probably limited growth of adjacent seagrasses and red algae. Because these seaweeds did not use the phosphate, it became available to other plant components. The overall floral response to nutrient addition in seagrass communities depends, therefore, upon the particular nutrient supplied, the ability of alternate species in the area to compete for that nutrient and the velocity of current in the specific area.     

Questions on the mechanism of temperature adaptation in marine phytoplankton

The rate of light-saturated photosynthesis in 3 marine algae [Phaeodactylum tricornutum Bohlin, Nitzschia closterium (Ehrenberg) Smith and Dunaliella tertiolecta Butcher] varies during growth in batch culture. The photosynthetic rare declines most rapidly during growth at the higher temperatures. Because of these changes in photosynthesis rate, the previously reported enhanced photosynthetic abilities caused by growth at lower temperatures (generally interpreted as evidence for higher enzyme levels) can only be observed when measurements are made late in the exponential phase or after the onset of the stationary phase of growth. When allowance is made for the earlier peak of photosynthetic ability in cultures growing at higher temperatures, there is no evidence for adaptation to lower temperatures being caused by increased levels of the enzymes required for carbon-dioxide fixation. When the changes due to growth in batch culture are taken into account, certain effects of temperature can be recognized. the dry weight: chlorophyll ratio of all 3 algae increases with decreasing growth temperatures. For P. tricornutum and N. closterium, growth at lower temperatures reduces the cellular content of chlorophyll a, but has little effect on the chlorophyll content of D. tertiolecta. The dry weight: cell-number ratio of D. tertiolecta and P. tricornutum increases with lower growth temperatures, but growth temperature has little effect on the cell mass of N. closterium. Growth of the 3 algae at lower temperatures does not increase their ability to photosynthesize at these lower temperatures. Rather, it reduces their ability to assimilate carbon dioxide at the higher temperatures.     

Photobiology of endolithic microorganisms in living coral skeletons: 1. Pigmentation, spectral reflectance and variable chlorophyll fluorescence analysis of endoliths in the massive corals Cyphastrea serailia, Porites lutea and Goniastrea australensis

We used microscopy, reflectance spectroscopy, pigment analysis, and photosynthesis-irradiance curves measured with variable fluorescence techniques to characterise the endolithic communities of phototrophic microorganisms in the skeleton of three massive corals from a shallow reef flat. Microscopic observations and reflectance spectra showed the presence of up to four distinct bands of photosynthetic microorganisms at different depths within the coral skeleton. Endolithic communities closer to the coral surface exhibited higher photosynthetic electron transport rates and a green zone dominated by Ostreobium quekettii nearest the surface had the greatest chlorophyll pigment concentration. However, Ostreobium was also present and photosynthetically active in the colourless band between the coral tissue and the green band. The spectral properties and pigment density of the endolithic bands were also found to closely correlate to photosynthetic rates as assessed by fluorometry. All endolithic communities were extremely shade-adapted, and photosynthesis was saturated at irradiances <7 μmol photons m⁻²s⁻¹.     

Effects of photoacclimation on the light niche of corals: a process-based approach

The ecology of photosynthetic organisms is influenced by the need to adjust the photosynthetic apparatus to variable light environments (photoacclimation). In this study, we quantified different components of the photoacclimation process for a reef-building coral (Turbinaria mesenterina, Lamarck, 1816): including, variation in absorption cross-section, size of photosynthetic units, turnover time, chlorophyll content, and colony respiration. We used these calibrations to characterize this species’ light niche, and to determine the sensitivity of the niche boundaries to different processes of photoacclimation. Results showed that the breadth of the light niche was most sensitive to the size of the photosynthetic unit, absorption cross-section, and rates of respiration. Habitats with the highest light availability did not lead to maximal energy acquisition. This was because, although corals acclimated to high light have high rates of photosynthesis per unit chlorophyll, their chlorophyll content was strongly reduced. This suggests that potential energetic benefits that could be achieved through increased light harvesting (i.e., increased chlorophyll content) in high-light habitats are outweighed by costs associated with photoprotection. Such costs appear to place an upper bound on the habitat distributions of coral species. Our approach reveals how the photophysiological processes involved in photoacclimation interact to determine the light niche.     

Photosynthesis,photorespiration, and dark respiration in eight species of algae

The rates of photosynthesis and dark respiration for 7 marine algae and 1 fresh-water alga were measured and compared. The dinoflagellates Glenodinium sp. and zooxanthellae have high dark respiration rates relative to photosynthetic rates, which may decrease their net growth rates. Photorespiration in the 8 algal species was studied by examining the effects of the concentration of oxygen on the rates of photosynthesis, on the incorporation of ¹⁴CO₂ into the photorespiratory pathway intermediates glycine and serine, and on the postillumination burst of carbon dioxide production and oxygen consumption. A combination of these results indicates that all the algae tested can photorespire, but that Glenodinium sp., Thalassiosira pseudonana, and zooxanthellae either have a photorespiratory pathway different from that proposed for freshwater algae (Tolbert, 1974), or an additional pathway for glycolate metabolism.     

Nitrate reductase and glutamine synthetase activity,internal N pools,and growth of<Emphasis Type="Italic"> Ulva lactuca</Emphasis>: responses to long and short-term N supply

Fast-growing macroalgae, including Ulva lactuca Linnaeus, respond rapidly to changes in nutrient conditions, particularly to short-term N supply. This ability to rapidly take up and assimilate N contributes to the increasing occurrence of macroalgal blooms in heavily N loaded coastal ecosystems. To determine whether long-term nutrient histories affect short-term responses in activity of N-assimilating enzymes, including nitrate reductase (NRA) and glutamine synthetase activity (GSA), internal N storage, and macroalgal growth, we conducted an in situ nitrate fertilization experiment between 7 and 22 July 2004, with fronds of U. lactuca collected from estuaries with high and low N loads in Waquoit Bay, Cape Cod, Massachusetts, USA (N 41° and W 70°). Initial NRA, GSA, % N, δ¹⁵N, and growth of U. lactuca fronds were higher in the site where nitrate was in high supply. Differences in NRA persisted even after short-term experimental enrichment. Differences in internal N pools, δ¹⁵N, and growth, in contrast, mirrored the changes of nutrient supply. The rate of turnover of the internal N content of U. lactuca was quite short (<2 d), and turnover of enzyme activity may have been even shorter. N isotopic fractionation by U. lactuca appeared to be of small magnitude, unlike the case of phytoplankton, and similar to that of vascular plants. δ¹⁵N was a better indicator of short-term response to external and internal nutrient supplies in U. lactuca than enzyme activity or N content, and may reliably detect rapid changes in N availability, source, and uptake and assimilation processes.     

Effects of light quality on chlorophyll and biliprotein accumulation in seaweeds

The control of chlorophyll and biliprotein synthesis by different qualities of continuous light in four seaweeds, collected off the coast of Málaga (Spain) in the winter of 1986 and 1987, was investigated over the short term. In the green algaUlva rigida C. Aghardh, and in the red algaeCorallina elongata Ellis et Soland andPlocamium cartilagineum (L.), chlorophyll synthesis was induced by blue light. However, in the red algaPorphyra umbilicalis (L.), chlorophyll accumulation was primarily induced by red light. Phycoerythrin synthesis inCorallina elongata, Plocamium cartilagineum andPorphyra umbilicalis was stimulated primarily by green light, whilst phycocyanin synthesis was induced by red. The effect of light quality on the synthesis of photosynthethic pigments was rapid. The induction of chlorophyll synthesis of after 6 h in light was 11 to 20% higher than in the dark control. The induction of biliprotein synthesis was two times greater than that of chlorophyll. The results are discussed in terms of chromatic adaptation and as adaptative responses to the underwater light environment. The possible role of phytochrome, and of a blue-light photoreceptor, in the control of chlorophyll and biliprotein synthesis is discussed.Please address all correspondence and requests for reprint to Dr López-Figueroa at his present address: Botanisches Institut der Universität München, Menzingerstraße 67, D-8000 München 19, FRG     

Fish foraging periodicity correlates with daily changes of diet quality

We tested the adaptive feeding hypothesis of Taborsky and Limberger (1980; Pubbl. Staz. zool. Napoli (I: Mar. Ecol.) 1: 143–153) by comparing two populations ofParablennius sanguinolentus (observed ca. 20 km north of Trieste, Italy, and near Calvi, France, during August and September 1989), each of which fed on different algae: turf and sea lettuce (Ulva lactuca). Turf energy peaked in the afternoon,U. lactuca energy around noon. In both fish populations, feeding paralleled the energy content of their food source, which is strong evidence for the adaptive feeding hypothesis. Turf feeders took an average of 2 to 7 times more bites in the afternoon than at other times of the day, whereasU. lactuca feeders bit 1.4 to 15 times more often around noon. We checked for chemical components of algae, but our data did not suffice to reduce the diel pattern of energy content to the short-term distribution of either starch, protein or ash.     

Light responses of a scleractinian coral (Plerogyra sinuosa)

During daytime Plerogyra sinuosa Dana displays globular expandable tentacles (bubbles) which foster the photosynthetic ability of the coral. Adaptational responses of this coral to different depths (5–25 m) and light conditions were investigated by photosynthetic pigment analysis, insitu measurements of oxygen production, transplantation and shading experiments. Pigment concentrations per unit tissue dry weight were variable, but unrelated to depth. Pigment concentrations per zooxanthellae cell remained constant and bubble size increased with depth. Light intensity at 25 m was 20 to 25% of the 5-m value, but daily integrated rates of photosynthesis were 65% of the 5-m rates, indicating a higher light utilization efficiency in deeper corals. Coral heads transplanted from 25 to 5 m died within 20 d if not protected against UV-radiation, but corals transplanted from 5 to 25 m acclimatized to the new light condition. Photosynthetic oxygen production and bubble size increased in shaded, sun-adapted corals within 60 min and decreased in sun-exposed, shade-adapted corals. The variable bubble size is interpreted as an adaptational mechanism to optimize light exposure of zooxanthellae.     

Effects of irradiance and ultraviolet radiation on photoadaptation in the zooxanthellae of Aiptasia pallida: primary production,photoinhibition, and enzymic defenses against oxygen toxicity

Cnidarians which contain symbiotic algae are constantly faced with the challenges of a changing photic regime and a hyperoxic environment. Zooxanthellae (Symbiodinium sp.) from the sea anemone Aiptasia pallida (Verrill), collected and cultured at Bermuda Biological Station in 1986, exhibit a suite of compensatory responses to changes in irradiance, ultraviolet radiation (UV), and to the toxicity resulting from their interaction with photosynthetically produced oxygen. Superoxide dismutase (SOD) and catalase inactivate superoxide radicals (O₂ ^-) and hydrogen peroxide (H₂O₂), which are mediators of oxygen toxicity, show an increase in specific activity with irradiance and in response to UV, both in cultured zooxanthellae (CZ) and freshly isolated zooxanthellae (FIZ) from acclimated anemones. CZ and FIZ exposed to environmentally realistic UV levels show a 30 to 40% increase in SOD activities compared with zooxanthellae exposed to similar irradiances without UV. CZ consistently show higher activities of both SOD and catalase compared to FIZ. Both CZ and FIZ exhibit changes in chlorophyll content and in the relationship between photosynthesis and irradiance which suggest photoadaptive changes in CO₂-fixing enzymes, the photosynthetic-electron transport system, or in photosynthetic unit size (PSU). UV has a greater effect on the photosynthetic capacity (P _max) of FIZ when compared to CZ acclimated at an equivalent irradiance with or without a UV component. UV also enhances the photoinhibition observed at high irradiance in both CZ and FIZ. Differences in enzyme activity between CZ and FIZ suggest an important role for the host in the protection of zooxanthellae against the direct effects of environmentally realistic UV while the photosynthetic performance of zooxanthellae in situ may not be as well protected.     

Occurrence and biological role of light-induced chromatophore displacements in seaweeds

The occurrence of light-induced chromatophore displacements and concomitant transmittance changes in marine algae was investigated by microscope and photometrically with an automated recording microphotometer system; 16 brown, 6 green and 20 red algae were studied. In most of the brown algae, both phaeoplast displacements and transmittance changes were found. In some red algae which are frequently exposed to direct sun light during emergence at low tide, light-induced transmittance changes were measured, but they could not unequivocally be correlated with changes in the position of rhodoplasts. Among green algae, only Ulva lactuca shows chloroplast displacements which, however, follow circadian rhythms and are consequently not light-induced in the strict sense. The dose-response curves of light-induced chromatophore displacements were measured in the following Fucus and Laminaria species: F. spiralis, F. vesiculosus, F. serratus, L. digitata, L. saccharina and L. hyperborea. While in Fucus species correlations between light-induced transmittance changes and zonation of the intertidal area seem to exist, no significant differences have been found in the Laminaria species. The physiological role and ecological importance of light-induced chromatophore displacements for seaweeds living in the intertidal belt are discussed.     

A “CO2 supply” mechanism in zooxanthellate cnidarians: role of carbonic anhydrase

Carbonic anhydrase (CA, EC 4.2.1.1) activity was detected in 22 species of tropical cnidarians which contain endosymbiotic dinoflagellates (=zooxanthellae). CA activity was 2 to 3 times higher in animal tissue than in algae and ca. 29 times higher in zooxanthellate than azooxanthellate species. It was also higher in the zooxanthellate tentacle tissue than in the azooxanthellate column tissue of the anemone Condylactis gigantea. CA was therefore significantly related to the presence of endosymbiotic algae. Further results indicated that CA functions in the photosynthetic carbon metabolism of zooxanthellate cnidarians as evidenced by (1) low CA activity in shade-adapted and deep water colonies compared to the more productive shallow water, light-adapted colonies of the coral Stylophora pistillata, and (2) the 56 to 85% reduction in photosynthetic carbon assimilation by zooxanthellae in situ in the presence of Diamox, an inhibitor of CA. Although CA has been proposed to function in calcification, its association with zooxanthellae and photosynthetic activity in both calcifying and non-calcifying associations suggests a role in photosynthetic metabolism of algal/cnidarian symbioses. It is proposed that CA acts as a CO₂ supply mechanism by releasing CO₂ from bicarbonate, and enabling zooxanthellae to maintain high rates of photosynthesis in their intracellular environment.     

Availability of ammonium influences photosynthesis and the accumulation of mycosporine-like amino acids in two<Emphasis Type="Italic"> Porphyra</Emphasis> species (Bangiales,Rhodophyta)

The effect of ammonium concentration on photosynthetic activity estimated as in vivo chlorophyll fluorescence, i.e. maximal quantum yield (F_v/F_m) and electron transport rate (ETR) and on the accumulation of mycosporine-like amino acids (MAAs), chlorophyll a (chl a), biliproteins (BP) and soluble proteins (SP) in the red algae Porphyra leucosticta Thuret in Le Jolis collected from Lagos (Málaga, Spain) and Porphyra umbilicalis (Linnaeus) J. Agardh from Helgoland (Germany) was evaluated. Discs of both species were incubated with three ammonium concentrations (0, 100 and 300 µM) under artificial PAR and UV radiation for 7 days. Photosynthetic activity decreased under the culture conditions due to UV radiation and ammonium availability. The decrease of both F_v/F_m and maximal ETR was related to ammonium supply, i.e. the lowest decrease occurred in algae growing with the highest concentration of ammonium. In both species, after 7 days of culture, the content of chl a, BP and SP was higher under 300 µM than that under 0 and 100 µM ammonium. In both species, the content of MAAs was increased under 300 µM ammonium compared to the initial value, whereas a decrease under 0 and 100 µM ammonium was observed only in P. leucosticta. The content of MAAs in P. umbilicalis did not present significant differences compared to the initial value, probably because of the high initial content of MAAs. In both Porphyra species, four MAAs were identified: shinorine, porphyra-334, palythine and asterina-330. However, P. leucosticta modified its MAA pattern during the incubation time, reaching the same percentages found for P. umbilicalis, which did not show any change during the experimental period. P. leucosticta exhibited a decrease in BP/SP and BP/chl a ratios through the incubation time and an increase in MAAs/BP. The ratio MAAs/chl a did not show any variation with time or treatment, as was also true for all ratios in P. umbilicalis. In summary, ammonium supply diminished the decrease of F_v/F_m, increased the content of photosynthetic pigments (chlorophyll and biliprotein) and soluble protein, and stimulated of the accumulation of MAAs in the red algae P. leucosticta and P. umbilicalis.Communicated by O. Kinne, Oldendorf/Luhe