Influence of carbon supply on the stimulation of light-saturated photosynthesis by blue light in Laminaria saccharina: implications for the mechanism of carbon acquisition in higher brown algae

In saturating irradiances of red light, photosynthesis of Laminaria saccharina (L.) Lamouroux was stimulated by low irradiances of continuous blue light only when the supply of dissolved inorganic carbon (DIC) was limiting. The degree of this stimulation was inversely proportional to the logarithm of the concentration of free CO₂, whether this was adjusted by varying the total DIC or the pH at a given DIC concentration. The final pH reached in a closed system was higher in blue light than in red light. Both acetazolamide and ethoxyzolamide suppressed the responses to blue light almost completely, but reduced photosynthesis in red light by only 30%. Buffering the pH of the seawater also suppressed the stimulation of photosynthesis by blue light without affecting the photosynthetic rate in red light. The transient stimulation of O₂ evolution by a blue light pulse was not accompanied by a corresponding increase in CO₂ consumption. These observations could be explained if, in analogy to the mechanism proposed for Ectocarpus (Schmid, Mills & Dring 1996, Plant Cell and Environment 19,373–382, this issue, accompanying paper), photosynthesis was supported by a blue-light-activated release of CO₂ from an internal store. We suggest that the store is located in the vacuoles of the cortical tissue of the blades. The main photosynthetic tissue, however, is in the overlying meristoderm, and blue-light-activated mobilization of the store could stimulate O₂ evolution only if periplasmic carbonic anhydrase was available to facilitate CO₂ uptake from the cortex.     

Influence of carbon supply on the circadian rhythmicity of photosynthesis and its stimulation by blue light in Ectocarpus siliculosus: clues to the mechanism of inorganic carbon acquisition in lower brown algae

Stimulation or light-saturated rates of photosynthesis in Ectocarpus siliculosus (Dillwyn) Lyngb. by blue light was eliminated by increasing dissolved inorganic carbon (DIC) or by lowering pH in natural seawater. The amplitude of the circadian rhythm of photosynthesis was also diminished under these conditions, and the pH compensation points in a closed system were higher in the presence of blue light and during the circadian day. These observations suggest that blue light and the circadian clock regulate the activity of a carbon acquisition system in these plants. The inhibitor of external carbonic anhydrase, acetazolamide, reduced overall rates of photosynthesis by only about 30%, but ethoxyzolamide suppressed the circadian rhythm of photosynthesis almost completely and markedly reduced the duration of responses to blue light pulses. Similar patterns were obtained when photosynthesis was measured in strongly limiting DIC concentrations (0–0.5 mol m^?3). Since blue light stimulated photosynthesis under these conditions of strong carbon limitation, we suggest that blue light activates the release of CO₂ from an internal CO₂ store. We propose a metabolic pathway with similarities to that of CAM plants. Non-photosynthetic fixation leads to the accumulation of a storage metabolite. The circadian clock and blue light control the mobilization of CO₂ at the site of decarboxylation of this metabolite. In the presence of continuous blue light the pathway is proposed to cycle and act as a pump for CO₂ into the chloroplasts. This hypothesis helps to explain a number of previously reported peculiarities of brown algal photosynthesis.     

Effect of external pH on inorganic carbon assimilation in unicellular marine green algae

Carbonic anhydrase (CA) induction has been studied in three marine green algae under acidic (pH 4.5) or alkaline (pH 8.0) conditions. An inhibition of the induction of the external CA in acidic conditions, similar to that observed in some freshwater green algae, could be observed in only Chlorella saccharophila. In the two other species, Chlorococcum littorale and Stichococcus bacillaris, no significant difference in CA induction was found under two pH conditions. The exact function of the external CA of C. saccharophila remains unclear, since cells grown under acidic conditions (under which this enzyme is repressed) possess the same abilities to use inorganic carbon (Ci) as alkaline‐grown cells. Internal pH values were not modified by the pH of the medium used to cultivate C. saccharophila. Regardless of the growth conditions, activities related to carbon fixation, that is, photosynthetic oxygen evolution, Ci uptake and assimilation were enhanced when the measurements were performed at acidic pH. This indicates that this marine alga is able to use CO₂ more efficiently than HCO₃^?. No evidence could be found for a specific Ci uptake and assimilation system in the acid‐grown cells.     

The mitochondrial genome of the brown alga Laminaria digitata: a comparative analysis

We report here the complete sequence of the mitochondrial genome of the brown alga Laminaria digitata (Hudson) J.V. Lamouroux. L. digitata mtDNA is a circular molecule of 38,007?bp (64.9% A?+?T), encoding 63 genes and 3 ORFs and with only 6.7% of non-coding sequences. Based on gene content and order, its overall organization is very similar to that of the mitochondrial genome of Pylaiella littoralis, another brown alga belonging to a different sublineage of the Phaeophyceae. In particular, the two genomes share unusual features, which hence could be unique to brown algae among the heterokont lineage, namely the presence of a rn5 gene, a short nad11 gene, a cox2 gene with a large in-frame insertion and α-proteobacterial-like promoter sequences. On the other hand, L. digitata lacks the sequences which are thought to have been transmitted horizontally to the P. littoralis genome, that is, the group-II introns in the rnl and cox1 genes, and it features only traces of an ancestral T7-like RNA polymerase. Distance phylogenetic trees inferred from concatenated mitochondrial genes confirm that speciation of brown algae occurred recently compared to other heterokonts.     

Direct determination of alginate content in brown algae by near infra-red (NIR) spectroscopy

The methods used to quantify total alginate in brown algal tissue are time-consuming and may also be misleading, so faster and simpler methods for measuring alginate content would be beneficial in a variety of applications. This study reports on the use of near infra-red (NIR) analysis to monitor the alginate content of Laminaria hyperborea stipe during biodegradation. NIR reflectance spectra were recorded for 78 different freeze-dried samples of its stipe. The samples were collected during several biological degradation experiments and the total alginate content varied from 2.2 to 40.8% Na-alginate (w/w), determined by established methods based on ion exchange. Data analysis was performed using multivariate calibration methods in order to relate the spectral data to the alginate content. PLS2 analysis revealed some dependence on material type, probably reflecting differences in polyphenol content. In the end, a PLS1 model with 9 components was selected. The calculated model was validated both with internal data and with an external test set. Internal full cross validation explained 96.6% of the variance in alginate content. The external validation showed that the PLS1 model was able to predict the alginate concentration with a root mean square prediction accuracy of 2.1%. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sterols of the senegalese brown alga Padina vickersiae

Contrary to all the brown algae already studied for sterol content the Senegalese Padina vickersiae contains a great amount of cholesterol, chiefly in the later stages of growth. The seasonal variation of the sterols is studied and compared to other Phaeophyceae belonging to the genus Cystoseira.     

Bacterial communities of some brown and red algae from Peter the Great Bay, the Sea of Japan

The structure of microbial communities of brown algae, red algae, and of the red alga Gracilaria verrucosa, healthy and affected with thallus rot, were comparatively investigated; 61 strains of heterotrophic bacteria were isolated and characterized. Most of them were identified to the genus level, some Vibrio spp., to the species level according to their phenotypic properties and the fatty acid composition of cellular lipids. The composition of the microflora of two species of brown algae was different. In Chordaria flagelliphormis, Pseudomonas spp. prevailed, and in Desmarestia viridis, Bacillus spp. The composition of the microflora of two red algae, G. verrucosa and Camphylaephora hyphaeoides, differed mainly in the ratio of prevailing groups of bacteria. The most abundant were bacteria of the CFB cluster and pseudoalteromonads. In addition, the following bacteria were found on the surface of the algae: Sulfitobacter spp., Halomonas spp., Acinetobacter sp., Planococcus sp., Arthrobacter sp., and Agromyces sp. From tissues of the affected G. verrucosa, only vibrios were isolated, both agarolytic and nonagarolytic. The existence of specific bacterial communities characteristic of different species of algae is suggested and the relation of Vibrio sp. to the pathological process in the tissues of G. verrucosa is supposed.     

The interactive effects of light and inorganic carbon on aquatic plant growth

Submerged aquatic macrophytes grow across a wide, often coupled, range of light and inorganic carbon availabilities, and each single factor influences photosynthesis and acclimation. Here we examine the interactive effects of light and inorganic carbon on the growth of Elodea canadensis and Callitriche cophocarpa. The plants were grown in the laboratory at a range of light intensities (0–108 μmol m⁻²s⁻¹) and four inorganic carbon regimes in a crossed factorial design. Plant growth rates, measured over 3–4 weeks of incubation, increased in response to increasing light intensity and inorganic carbon availability, and significant interactive effects were observed. The light-use efficiency for growth at low light increased 2-fold for Callitriche and 6-fold for Elodea between the lowest and highest inorganic carbon concentrations applied. Also, the growth rate at the highest light intensity increased with inorganic carbon availability, but the relative increase was smaller than at low light. Both species acclimated to the light and carbon regime such that the chlorophyll content declined at low and high light intensities and the initial slopes of the photosynthetic CO₂ and HCO₃⁻ response curves declined at high levels of CO₂. Callitriche responded less markedly than Elodea to changing inorganic carbon availability during growth, and the initial slope of the photosynthetic HCO₃⁻ response curve, in particular, was greatly reduced (>90%) in Elodea by high CO₂. It is suggested that the coupled responses of aquatic macrophytes to light and inorganic carbon influence their ability to develop dense stands at high light in shallow water and to extend to greater depths in waters rich in inorganic carbon.     

High rhenium enrichment in brown algae: a biological sink of rhenium in the sea?

Twenty-one species of seaweed from the California coast were analyzed for rhenium. For the first time, high enrichment (thousandfolds) of rhenium relative to seawater was found in brown algae, but not in green or red algae. Brown algae was suggested as a biological sink of rhenium in the sea and the analogous behavior of technetium to rhenium was found in marine algae. Preliminary incubation experiments with a common brown alga (Pelvetia fastigiata) showed that algal surface is not a major accumulating locus of rhenium.     

Photosynthesis of Ectocarpus siliculosus in red light and after pulses of blue light at high pH – evidence for bicarbonate uptake

Pulses of blue light cause stimulation of red light saturated photosynthesis in Ectocarpus siliculosus, because blue light activates the operation of a pathway for inorganic carbon (C_i) acquisition by inducing the mobilization of CO₂ from an intermediate metabolite. In the absence of exogenous C_i, photosynthetic rates roughly equal those of CO₂ release by respiration. In seawater of pH 9·5 (2·3 mol m^–3 total C_i, but concentrations of free CO₂ below 0·2 mmol m^–3), photosynthesis was clearly above these rates, although they were only ≈ 30% of those in normal seawater (≈ pH 8). The degree and the time course of the stimulations of photosynthesis by pulses of blue light were unaltered at high pH. Essentially the same characteristics were found after buffering or in the presence of acetazolamide, an inhibitor of extracellular carbonic anhydrase activity. Therefore, it is concluded that Ectocarpus is able to directly take up HCO₃^– in addition to CO₂ (uptake of CO₃^2– cannot be excluded). The dependence of photosynthesis on C_i at pH 9·5 was biphasic, with C_i below 0·2 mol m^–3 having no effect at all. In C_i-free seawater, the shapes of the stimulations after blue light pulses differed for pH 6, pH 8 and pH 9·5. At low pH, only the fast peak (maximum ≈ 5 min after blue light) was detected, whereas at high pH mainly the slow peak (maximum ≈ 20 min after blue light) was observed. At the intermediate pH 8, both peaks were present. As inhibition of total carbonic anhydrase by ethoxyzolamide brought out the fast peak of the stimulations at pH 9·5 it is concluded that the fast component was due to a transient disequilibrium of an intracellular pool of C_i which, after blue light, was fed by CO₂ released from the postulated storage intermediate.     

The organic component of the cell wall of the marine diatom Melosira nummuloides (Dillw.) C. Ag

Organic material which may be several layers thick has been found beneath the siliceous cell wall of the marine centric diatom Melosira nummuloides (Dillw.) C. Ag. This supports the findings of Liebisch (1928, 1929). The organic component, thought to be polysaccharide, is deposited outside the plasmalemma after the siliceous component has been laid down. It exists as two distinct entities, one associated with each valve.     

Photosynthetic light-response curves and photoinhibition of the deep-water laminaria abyssalis and the intertidal laminaria digitata (phaeophyceae)

Photosynthetic light‐response curves of the deep‐water Laminaria abyssalis Oliveira and of the intertidal L. digitata Lamoroux were determined and related to photoinhibition phenomena as monitored by oxygen evolution and photosystem II efficiency (F_V/F_M). L. abyssalis has half the pigment content, number of cells and plastids, and photosynthetic capacity per unit area compared with L. digitata. L. abyssalis showed a higher in vivo Chl a absorption coefficient and higher photosynthetic efficiency on a Chl a basis, although the two algae showed somewhat similar light‐response curves on a Chl a basis. Both species showed similar Chl a/Chl c and Chl a/fucoxanthin ratios, and similar dark respiration rates and light compensation points. In addition, they also showed similar convexities in their light‐response curves and no differences in their light saturation of F_V/F_M. Room temperature chlorophyll fluorescence induction measurements of fronds incubated in 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) suggest that both species may have a similar PSII absorption cross section. Thus, L. abyssalis appears to optimize its light absorption at very low light intensities, not by increasing the pigment content, but by absorbing light more efficiently. However, L. abyssalis was more sensitive to photoinhibition than L. digitata and showed no recovery of F_V/F_M and O₂ evolution after a photoinhibitory treatment, even with a subsequent exposure to 24 h of dim light. L. digitata, on the other hand, recovered its photosynthetic capacity within 6 h under dim light. These results suggest that photosynthetic light‐induction curves based on Chl a are not a good indicator of either the photosynthetic capacity or the sensitivity to photoinhibition when macroalgae of different species are being compared. Based on their light‐response and photoinhibition characteristics, we suggest that L. abyssalis, a deep‐water oceanic macroalgae, is an atypical shade alga whereas L. digitata has the properties of a sun alga.     

Thermal acclimation and photoacclimation of photosynthesis in the brown alga Laminaria saccharina

Thermal acclimation and photoacclimation of photosynthesis were compared in Laminaria saccharina sporophytes grown at temperatures of 5 and 17 °C and irradiances of 15 and 150μmol photons m^?2 s^?1. When measured at a standard temperature (17°C), rates of light-saturated photosynthesis (P_max) were higher in 5 °C-grown algae (c. 3.0 μmol O₂ m^?2 s^?1) than in 17 °C-grown algae (c. 0.9 μmol O₂ m_-2 s_-1). Concentrations of Rubisco were also 3-fold higher (per unit protein) in 5 °C-grown algae than in algae grown at 17 °C. Light-limited photosynthesis responded similarly to high temperature and low light Photon yields (α) were higher in algae grown at high temperature (regardless of light), and at 5 °C in low light, than in algae grown at 5 °C in high light Differences in a were correlated with light absorption; both groups of 17 °C algae and 5 °C low-light algae absorbed c. 75% of incident light, whereas 5 °C high-light algae absorbed c. 55%. Increased absorption was correlated with increases in pigment content PSII reaction centre densities and the fucoxanthin-Chl ale protein complex (FCP). Changes in a were also attributed, in part, to changes in the maximum photon yield of photosynthesis (0_max). PSI reaction centre densities were unaffected by growth temperature, but the areal concentration of PSI in low-light-grown algae was twice that of high-light-grown algae (c. 160.0 versus 80.0 nmol m^?2). We suggest that complex metabolic regulation allows L, saccharina to optimize photosynthesis over the wide range of temperatures and light levels encountered in nature.     

Time-course of uptake of dissolved inorganic carbon through willow roots in light and in darkness

Uptake of dissolved inorganic carbon (DIC) from a nutrient solution by willow roots was measured in light and darkness and the distribution in the plant of DIC taken up by the roots was determined. It was also studied whether the transport system could be activated by preincubation with dissolved inorganic carbon.
Willow plants ( Salix cv. Aquatica gigantea) grown in hydroponic culture media were preincubated for 2 days with or without 0.74 mM NaHCO₃. After preincubation, either unlabelled or [¹⁴C]-labelled NaHCO₃ was injected into the media and after 1, 5, 10 and 24 h either in light or in darkness the plants were harvested in pieces into liquid nitrogen, lyophilized and burned in a combustion chamber.
¹⁴C was transported through the roots to the shoots and leaves both in light and in darkness, although incorporation of ¹⁴C in darkness was only half of that in light at the end of the 24-h feeding period. Both in light and in darkness the amount of ¹⁴C increased in all parts of willow plants with time. In light the rate of labelling was highest into cuttings and shoots. In darkness more than half of the total label was detected in cuttings of both the non-activated and the activated treatments.
In the shoots the middle part was most strongly labelled after 5 and 10 h, but after 24 h ¹⁴C moved towards the base of the shoot. In the leaves at all feeding times most radioactivity was incorporated into the young, fully open leaves on the upper part of the shoots. Preincubation of plants with unlabelled NaHCO³ in growth media had no clear effect on the rate of DIC uptake either in light or in darkness.     

A stable carbon isotope study of dissolved inorganic carbon cycling in a softwater lake

The dissolved inorganic carbon (DIC) cycle in a softwater lake was studied using natural variations of the stable isotopes of carbon,¹²C and¹³C. During summer stratification there was a progressive decrease in epilimnion DIC concentration with a concomitant increase in ¹³C_DIC), due to preferential uptake of¹²C by phytoplankton and a change in the dominant CO₂ source from inflow andin situ oxidation to invasion from the atmosphere. There was an increase in hypolimnion DIC concentration throughout summer with a concomitant general decrease in ¹³C_DIC from oxidation of the isotopically light particulate organic carbon that sank down through the thermocline from the epilimnion.Mass balance calculations of DI¹²C and DI¹³C in the epilimnion for the summer (June 23–September 25) yield a mean rate of net conversion of DIC to organic carbon (C_org) of 430 ± 150 moles d^-1 (6.5 ± 1.8 m moles m^-2 d^-1. Net CO₂ invasion from the atmosphere was 420 ± 120 moles d^-1 (6.2 ± 1.8 m moles m^-2 d^-1) with an exchange coefficient of 0.6 ± 0.3m d^-1. These results imply that at least for the summer months the phytoplankton obtained about 90% of their carbon from atmosphere CO₂. About 50% of CO₂ invasion and conversion to C_org for the summer occurred during a two week interval in mid-summer.DIC concentration increased in the hypolimnion at a rate of 350 ± 70 moles DIC d^-1 during summer stratification. The amount of DIC added to the hypolimnion was equivalent to 75 ± 20% of net conversion of DIC to C_org in the euphotic zone over spring and summer implying rapid degradation of POC in the hypolimnion. The ¹³C of DIC added to the deep water (-22.) was too heavy to have been derived from oxidation of particulate organic carbon alone. About 20% of the added DIC must have diffused from hypolimnetic sediments where relatively heavy CO₂ (-7) was produced by a combination of POC oxidation and as a by-product of methanogenesis.     

THE INVOLVEMENT OF A PLASMA MEMBRANE H+‐ATPase IN THE BLUE‐LIGHT ENHANCEMENT OF PHOTOSYNTHESIS IN LAMINARIA DIGITATA (PHAEOPHYTA)1

Many brown algae, including the kelp Laminaria digitata (Huds.) Lamour., exhibit enhanced photosynthesis when they are given a small amount of blue‐light in addition to a background of saturating red light. This blue light effect is correlated with an increased uptake of carbon. In the present study, we tested the hypothesis that blue light acts by increasing the activity of a plasma membrane H ⁺ ‐ATPase, thereby promoting an active carbon uptake across the plasma membrane. Photosynthetic carbon uptake was studied in pH‐drift experiments under illumination with red and blue light and using different inhibitors. Vanadate, an inhibitor of plasma membrane H ⁺ ‐ATPases, had a minor inhibitory effect on carbon uptake rates under saturating red light conditions, but inhibited the blue‐light enhancement by approximately 60%. An inhibitor of external carbonic anhydrase, acetazolamide, decreased the carbon uptake in both red light and in red plus blue light by 48% and 68%, respectively. These results suggest that photosynthetic carbon uptake depends on an external carbonic anhydrase under both red and red plus blue light conditions, and that blue light induces an increased activity of a P‐type H ⁺ ‐ATPase in the plasma membrane. The proton buffer Tris, which has a buffering capacity similar to vanadate in seawater, had no inhibitory effect on carbon uptake rates neither in red light nor in red plus blue light, showing that the inhibitory effect of vanadate is not caused by its effect as a buffer. The blue‐light enhancement was also abolished by a protein kinase inhibitor (H‐7), suggesting that the transduction of the blue‐light signal involves a protein kinase, which activates the plasma membrane H ⁺ ‐ATPase by phosphorylation.