Decomposition of urea associated with photosynthesis of phytoplankton in coastal waters

Decomposition of urea in seawater was studied in Mikawa Bay, a shallow eutrophic bay on the southern coast of central Japan. The urea concentration in seawater ranged from 1.3 to 5.9 μg-at. N/1 and comprised 12 to 40% of the dissolved organic nitrogen. Using ¹⁴C labelled urea, the rate of CO₂ liberation from urea and the incorporation rate of urea carbon into the particulate organic matter were determined. For the surface samples, high rates of CO₂ liberation from urea as well as the incorporation of urea carbon into the particulate organic matter were observed in the light, while much lower rates were obtained in the dark. Incubation experiments with exposure to different light intensities revealed that the rate of CO₂ liberation from urea and the incorporation of urea carbon into particulate organic matter changed with light intensity, showing a pattern similar to that of photosynthesis. The highest liberation and incorporation rates were observed at 12,000 lux. Incubation in light and in dark produced marked decreases and increases, respectively, in urea and ammonia, while no appreciable changes were observed for nitrate and nitrite. It is suggested that urea decomposition associated with photosynthetic activity of phytoplankton is one of the major processes of urea decomposition, and that it plays a significant role in the nitrogen supply for phytoplankton in coastal waters.     

Light-shade adaptation by the oceanic dinoflagellates Pyrocystis noctiluca and P. fusiformis

Species-specific rates of photosynthetic carbon uptake (P), chlorophyll a content and P versus irradiance (P-I), have been measured for cells of Pyrocystis noctiluca and P. fusiformis isolated from natural populations collected in the euphotic zone within and below the surface mixed layer in the Sargasso Sea. These same measurements and the assay for ribulose bis-phosphate carboxylase (RuBP-Case), have been made for cultures of P. noctiluca in a 12 h L: 12 h D photoperiod at 9 different constant or at changing light intensities. In nature chl a cell^-1 was constant throughout the euphotic zone. The photosynthetic capacity (P_max), of cells captured below the surface mixed layer was lower by a factor of 10 compared with cells collected from the surface mixed layer. The P_max for P. noctiluca collected and incubated within the surface mixed layer was the same as for cell cultures grown under high light, non nutrient-limiting conditions, suggesting that photosynthesis in the natural system was not nutrient limited. In laboratory cultures under constant low light intensities, chl a cell^-1 increased by a factor of 5 while both P_max and RuBPCase activity decreased by a factor of ca 4 compared with high light intensities. In changing light intensities both P_max and RuBPCase activities were decreased by factors of 4 during low light intervals while chl a cell^-1 approached a constant intermediate value. The change in chl a cell^-1 in response to prolonged exposure to constant low light intensities was first order with a rate constant of 0.33 d^-1. For all irradiance conditions in culture, the P-I dependence could be described by the simple Michaelis-Menten formula. The ratio of P_max to K_I, (the light intensity where P=P_max/2) was a constant with a Coefficient of Variation of 12%: The constancy of this ratio, the parallel changes in RuBPCase activity with P_max and the constant chl a cell^-1 in the Sargasso Sea imply that for P. noctiluca and presumably P. fusiformis in nature, a dark enzymatic step rather than changes in photosynthetic pigment concentrations may regulate the photosynthetic capacity in the changing photic environment.Contribution no. 1141 from McCollum-Pratt Institute and Department of Biology, The Johns Hopkins University. Supported by DOE contract no. EY 76S20 3278, NSF no. OCE 76-02571 and ONR no. N300014-81-C-0062     

Influence of light and temperature on the growth rate of estuarine benthic diatoms in culture

Four species of estuarine benthic diatoms: Amphiprora c. f. paludosa W. Smith, Nitzschia c. f. dissipata (Kützing) Grunow, Navicula arenaria Donkin, and Nitzschia sigma (Kützing) W. Smith were grown in unialgal cultures. The growth rates of the diatoms were determined as the rate of increase of the chlorophyll a content of the cultures. The diatoms were cultured at different combinations of temperture, daylength, and quantum irradiance. The highest growth rates of Navicula arenaria occurred at 16° to 20°C; the other 3 species had their optimum at 25°C or higher. The small-celled species had higher growth rates at their optimum temperature, but at lower temperatures the growth rates of all 4 species became very similar. The minimum daily quantum irradiance that could effect light-saturated growth at 12° and 20°C ranged from 2.5 to 5.0 E.m^-2.day^-1. At 12°C, two species had their highest growth rates under an 8 h daily photoperiod. At 20°C, the three species tested all had highest growth rates under 16 h daily photoperiod. The growth response of the benthic diatoms is comparable to that of several cultures of planktonic diatoms, as described in the literature. The influence of temperature and quantum irradiance on the diatoms in the present investigation was comparable to the influence of temperature and light intensity on the ¹⁴C-fixation of marine benthic diatoms (Colijn and van Buurt, 1975).     

Photoinhibition of photosynthesis in a sun and a shade species of the red algal genus Porphyra

Gametophytes of two species of Porphyra collected around San Juan Island, Washington in 1986 and acclimated to low light conditions in culture showed different resistances to photoinhibition of photosynthesis. The intertidal species P. perforata J. Agardh exhibited photoinhibition at one-third the rate exhibited by the subtidal species P. nereocystis Anderson following treatments at 2000 mol photons m^-2 s^-1 under conditions of full hydration and optimal temperature. The greater resistance of P. perforata to photoinhibition could not be attributed to reduced photosynthetic pigment concentration, higher photosynthetic capacity, avoidance of light by chloroplast movement or to enhanced rates of photorespiration. Total carotenoid concentrations were similar in the two species. It is probable that the mechanisms of this resistance are operating at the level of the thylakoid membranes. Resistance to photoinhibition represents an adaptation of photosynthesis in P. perforata which may contribute to its persistance in the extreme environment of its intertidal habitat.     

Effect of light and temperature on the population dynamics of two toxic bloom forming Cyanobacteria – Microcystis ichthyoblabe and Anabaena aphanizomenoides

The effect of light and temperature on the growth of Microcystis ichthyoblabe and Anabaena aphanizomenoides, isolated from the subtropical Oued Mellah lake, Morocco (33°30′N–07°20′W), were investigated in batch culture. Growth rates at 66 light–temperature combinations were determined and fitted with different mathematical models. The results show that the two Cyanobacteria grow at all light intensities and temperatures, except at 10 °C for A. aphanizomenoides, where the growth was strongly limited. The μ_max of M. ichthyoblabe increased with temperature from 0.56 d^?1 at 10 °C to 1.32 d^?1 at 35 °C. At all tested temperatures, a relative photoinhibition within the studied range of irradiance was observed and the photosensitivity was thermodependent. For Anabaena, the obtained μ_max ranged between 0.07 d^?1 at 10 °C and 1.46 d^?1 at 35 °C, and a weak photoinhibition was observed at 15 °C. The positive correlation between μ_max and I_opt (r²≥0.93) indicates a close interaction between light and temperature on the cyanobacteria growth. The results obtained in this work suggest that the growth of these two species is possible under low light and low temperature.     

Growth and competition of the marine diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. II. Light limitation

The marine diatoms Phaeodactylum tricornutum (Bohlin) and Thalassiosira pseudonana (Hasle and Heimdal) were grown under both continous illumination and a 14 h light: 10 h dark cycle at light intensities ranging from 1.53×10^-4 to 2.95×10^-1 ly min^-1. Under both photoperiods, T. pseudonana exhibited higher division rates than P. tricornutum at high light intensities, but the reverse was true at all light intensities <3×10^-3 ly min^-1. Comparison of these results with available data on light-limited growth of other planktonic algae suggests that P. tricornutum may be unusually efficient at maintaining its cell division rate at low light intensity. This efficiency may contribute substantially to its success in turbid, nutrient-enriched mass algal culture systems, the only environments in which it is known to attain great numbers.Contribution No. 4086 from the Woods Hole Oceanographic Institution.     

Experiments on resuspension of natural microphytobenthos populations

Laboratory experiments on resuspension of benthic diatoms were performed in 1988 on intact cores of sediment obtained from two transects in the Ems estuary (Dutch Wadden Sea, NW Europe). Diatoms were most abundant in sediment from sheltered stations. In the experiments, which were performed in a small carousel system, the percentage of resuspended diatoms increased concomitantly with angular velocity of the agitating paddles. Maximum microphytobenthos resuspension in terms of cell numbers was up to 45% of the total population present in the 0.5-cm top layer, and up to 11% in terms of chlorophylla. Maximum sediment resuspension was 6%. The results show that the presence of a diatom film (numerous cells on the sediment surface) increases sediment stability, thereby suppressing resuspension of sediment and diatoms. However, if the sediment contains many fine particles and much detritus, sediment stability decreases, leading to increased resuspension.These results support the contention that low detritus content and large numbers stabilize the surface layer of tidal flats, whereas high detritus content and fewer cells (<2 × 10⁵ cm^–2) do not. The stabilization is thought to be dependent on a combination of differences in sediment consolidation and in bottom roughness caused by the presence or absence of diatom films.     

Dynamique du phytoplancton et matière organique particulaire dans la zone euphotique de l'Atlantique Equatorial

At two fixed stations in the Equatorial Atlantic Ocean (0°–4° W), the physical, chemical and biological properties of the euphotic layer were determined for 14 d (Station A: 5–18 February, 1979) and 13 d (Station B: 20 October–7 November, 1979), respectively. The stability of the water column allowed comparison of 3 different “systems”: (i) a well-illuminated and nitrate-depleted mixed layer; (ii) a chlorophyll maximum layer (chl a _max) in the thermocline which is poorly illuminated (6.3% of surface irradiance); (iii) a well-illuminated but nitrate-rich (>0.9 μg-at l^-1) mixed layer. In each layer the particulate organic carbon (COP), nitrogen (NOP) and phosphorus (POP) contents were measured and compared with the phytoplankton biomass. In the chlorophyll maximum layer, the phytoplankton biomass contributed significantly to the total particulate organic matter (between 55 and 75%). In the nitrate-depleted mixed layer, the results varied according to whether the regression technique [COP=f(chl a)] was used, or the chl a synthesis during the incubation of the samples. With the former technique, the phytoplankton carbon (C _p) content appeared minimal, because the y intercept, computed using all the data of the water column, was probably overestimated for this layer. POP would be more associated with living protoplasm than with carbon and nitrogen in the three layers. In the chlorophyll a maximum layer it constitutes a valuable detritus-free biomass measurement, since 80% of the POP consist of phytoplankton phosphorus. The assimilation numbers (NA=μg C μg chl a ^-1 h^-1) were high in all three layers, but the highest values were recorded in the nitrate-depleted mixed layer (NA=15 μg C μg chl a ^-1 h^-1). In the chlorophyll maximum layer, light would be a limiting factor during incubation: between 10²⁵ and 8.10²⁴ quanta m^-2 d^-1 NA and light are positively correlated independant of nitrate concentration. The growth rates of phytoplankton (μ) were estimated and compared to the maximum expected growth rate. Our main conclusion was that despite very low biomass and nutrient content, the mixed layer was in a highly dynamic state, as evidenced by high rates of phytoplankton growth and short nutrient turnover times (1 d or less for PO-P₄ in the mixed layer versus 3 d in the thermocline). The presence of nitrate in the water column allows the development of a higher phytoplankton biomass but does not increase growth rate.     

Dark survival of autotrophic,planktonic marine diatoms

A general ecological problem is considered: how long can a photoautotrophic microalga, incapable of producing a resting spore (stage), retain its viability in the dark following removal from the euphotic zone? Nine coastal diatoms, including some capable of producing resting spores, were kept in the dark for 90 days at 15°C, and their growth (viability) checked at periodic intervals upon reillumination. Seven of the 9 diatoms retained their viability for 90 days; generation time of illuminated cells then ranged from 2.5 to 10 days. Skeletonema costatum survived only 7 weeks of darkness. Based on the present and published observations, dark survival of this species is inversely related to temperature; it survives at least 24 weeks at 2°C, and from 1 to 4 weeks at 20°C. None of the species was observed to grow in the dark. The effects of temperature and light on dark survival, and of darkness on the chemical composition and photosynthesis following reillumination as reported in the scattered literature are evaluated. Together with the present observations, it is suggested that dark survival of photoautotrophic microalgae: (1) varies between species; (2) may be temperature dependent in some species, as in S. costatum; (3) may be prolonged by periodic illumination at subcompensation intensities for photosynthesis, as shown in Dunaliella tertiolecta. The potential ecological significance of these findings is also considered, should these in vitro results apply to natural populations.     

Products of photosynthesis in natural populations of marine phytoplankton from the Gulf of Maine

We have measured the photosynthetic assimilation of ¹⁴C-carbon dioxide into (1) ethanol-soluble, (2) hot-trichloroacetic acid (TCA)-soluble (polysaccharide), and (3) protein fractions of natural populations of marine phytoplankton. Diurnal studies showed a continuing incorporation of carbon-14 into the protein fraction during hours of darkness. This was accompanied by a concomitant decrease in the proportion assimilated into polysaccharide. When incorporation was measured under constant experimental conditions, the pattern of photosynthesis did not vary from one time of day to another. At one station approximately 12 km south of Boothbay Harbor, the proportion of carbon entering protein showed marked seasonal changes. During the winter, approximately 10 to 20% of the fixed carbon was incorporated into protein. During the summer the value increased to 22 to 35%. Between these times, a transient high value of 37 to 47% of the fixed carbon entering protein coincided with the spring bloom. The increases in proportion incorporated into protein were largely paralleled by equivalent decreases in the polysaccharide fraction. The proportion of carbon incorporated into protein during photosynthesis also increased markedly at reduced light intensities. This increase occurred both when populations were incubated in neutral-density filters and when incubated at increasing depths in the photic zone. There was little consistent and significant difference between the neutral-density filters and depth in the water column, suggesting a minimal role for light quality. The extent of the increased relative rate of protein synthesis at the lower light intensities depended on the nutritional state of the phytoplankton. For example, summer populations from water containing low concentrations of inorganic nutrients responded less dramatically to reduced light intensities than did populations from nutrient-rich waters.     

Modeling kelp forest distribution and biomass along temperate rocky coastlines

We address the global deficit of data describing kelp forest distribution, relative covers and biomass by testing the ability of species distribution models to predict these attributes at locations where data are currently limited. We integrated biological ground truth data with high-resolution environmental datasets to develop generalized additive models that accurately predict the structure of Laminaria forests within the Bay of Morlaix (48°42′42″N, 3°55′40″W). Forest distribution and proportional covers were predicted using water depth, light availability, wave exposure and sediment dynamics. The biomass of individual kelp species was modeled by supplementing these same variables with measures of seafloor slope and benthic position. Biomass predictions for Laminaria digitata and Laminaria hyperborea contrast the physiological tolerances of these species to light and wave exposure gradients. As a direct management output, we produced high-resolution maps (25 m² grids) that closely match independent field data and provide vital information for marine spatial planning.     

Photoadaption in marine phytoplankton: Response of the photosynthetic unit

Some species of phytoplankton adapt to low light intensities by increasing the size of the photosynthetic unit (PSU), which is the ratio of light-harvesting pigments to P700 (reaction-center chlorophyll of Photosystem I). PSU size was determined for 7 species of marine phytoplankton grown at 2 light intensities: high (300 E m^-2 s^-1) and low (4 E m^-2 s^-1); PSU size was also determined for 3 species grown at only high light intensity. PSU size varied among species grown at high light from 380 for Dunaliella euchlora to 915 for Chaetoceros danicus. For most species grown at low light intensity, PSU size increased, while the percentage increase varied among species from 13 to 130%. No change in PSU size was observed for D. euchlora. Photosynthetic efficiency per chlorophyll a (determined from the initial slope of a curve relating photosynthetic rate to light intensity) varied inversely with PSU size. In contrast, photosynthetic efficiency per P700 was enhanced at larger PSU sizes. Therefore, phytoplankton species with intrinsically large PSU sizes probably respond more readily to the rapid fluctuations in light intensity that such organisms experience in the mixed layer.Contribution No. 1180 from the Department of Oceanography, University of Washington, Seattle, Washington, USA     

Cyst formation of the noxious red tide flagellate Chattonella marina (Raphidophyceae) in culture

Cyst formation of the noxious red tide flagellate Chattonella marina (Raphidophyceae), cultured from a cyst in sediments of Suo-nada, Japan in 1986, was investigated by culture experiments. Nitrogen limitation was effective in inducing cyst formation. After incubation in N-limited medium at 25°C with 10000 lux on a 14 h light, 10 h dark photo-cycle, cultures were subjected to six different light intensities (0 10000 lux) in order to determine effects of light intensity on cyst formation. Cysts were formed under light intensities of 1000 lux or below, but not at 10000 lux. Cysts formed in culture displayed morphological characteristics quite similar to those natural cysts observed in sediments collected from the Inland Sea of Japan. Observations on nucleus, using diaminidinophenylindole (DAPI)-staining and epifluorescence microscopy, indicated that the majority of cysts formed in culture were uninucleate, a few were binucleate. Germination of cysts was observed under adequate conditions for germination (22°C, 3500 lux with a 14 h light, 10 h dark photo-cycle) even after storage at 11°C in the dark for more than 4 mo.     

Capability of dynamic photoinhibition in Arctic macroalgae is related to their depth distribution

The capability of several macroalgal species to protect photosynthesis from excessive irradiance by dynamic photoinhibition was investigated relative to their depth distribution in summer 1995 in the Kongsfjord (79°N; 12°E, Ny Ålesund, Spitsbergen, Norway). Photoinhibition of photosynthesis was induced by exposure of algae from different water depths to a high photon fluence rate of 500?μmol?m^?2?s^?1 for 2?h. Changes in optimal quantum yield (F _v/F _m) were measured during the inhibition phase. Recovery of photosynthesis was subsequently induced by dim white light (10?μmol?m^?2?s^?1) and observed as changes in the variable fluorescence. With a newly developed mathematical model different parameters of the response kinetics of inhibition and recovery were calculated and related to the depth distribution of each algal species. It is shown that two components with slow and fast reaction kinetics, respectively, are involved in photoinhibition and recovery of photosynthesis. Their possible molecular bases are discussed. The half-life time (τ) of the inhibition and recovery phases, i.e. the time necessary to reach half maximal response, is clearly related to the depth distribution of the investigated species. Algae collected close to the water surface show a fast reaction of both photoinhibition and recovery and, hence, have a low τ. With increasing depth the reactions become slower and τ increases. τ was highest in deep water algae. Further analysis of the reaction kinetics in Laminaria saccharina shows that the relative proportion of the two kinetics involved change with the collection depth. In contrast, a significant difference in the reaction rates of both kinetics was not observed.     

Seasonal variations in salt-marsh macroalgae photosynthesis. I. Ascophyllum nodosum ecad scorpioides

Photosynthesis in whole plants of the intertidal alga Ascophyllum nodosum ecad scorpiodes was evaluated by measuring ¹⁴C-uptake under a variety of light intensities and approximately monthly intervals during a 15 month study. Photosynthetic rates were determined in terms of dry weight, pigment content and uptake into ethanolsoluble and insoluble fractions. The specimens, naturally acclimated to in situ light intensities and temperatures, exhibited photosynthetic responses to light intensity which differed with time of year. Maximum photosynthetic potential occurred during the spring months and minimum potential occurred during late summer and winter months. Variations in photosynthetic potential were closely related to seasonal changes in field biomass. Both photosynthetic potential and biomass were inversely related to growth patterns of the salt-marsh phanerogam Spartina alternilora.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).     

Pools of chlorophyll and live planktonic diatoms in aphotic marine sediments

Chlorophyll a and numbers of live pelagic diatoms were recorded from sediment depth profiles at 11 stations in the oligotrophic Øresund, Denmark, in late-June. Extraction efficiency of chlorophyll a analysed fluorometrically did not differ significantly between paired samples of frozen-thawed and fresh sediment. The depth profiles of chlorophyll a could be explained by a diagenetic model involving two different chlorophyll pools: one reactive pool declining exponentially with core depth, and one non-reactive pool, of about 1 µg Chl ml^-1 wet sediment, being constant with depth. The number of live diatoms, quantified by the dilution-extinction method, and expressed in terms of most probable number (MPN), declined from an average of about 300,000 g^-1 in the surface sediment to zero values at a depth of 13 cm. The number of live cells was significantly correlated with the sediment chlorophyll a, and the profiles of live cells as well as reactive chlorophyll followed the same exponential decline with core depth, suggesting that the main source of chlorophyll in the sediment was live pelagic diatoms. Taxonomic composition of diatoms in the sediment, dominated by the pelagic genera Chaetoceros, Thalassiosira and Skeletonema, matched the species composition in the water column 3 months earlier during the spring bloom. Regular recordings of the phytoplankton community in the water column showed that only these specific bloom species could be the source of the sediment content of diatoms and chlorophyll a. Further, the ratios between live cells and chlorophyll a were similar in the sediment and in the spring bloom. A conservative estimate of depth-integrated pools of diatoms in the sediment suggested that about 44% of the total phytoplankton biomass during the spring bloom was still present as live cells in the sediment after 3 months. This indicates that the spring bloom input to the sediment is not degraded immediately by the benthic fauna.     

Implications of geometric plasticity for maximizing photosynthesis in branching corals

Reef-building corals are an example of plastic photosynthetic organisms that occupy environments of high spatiotemporal variations in incident irradiance. Many phototrophs use a range of photoacclimatory mechanisms to optimize light levels reaching the photosynthetic units within the cells. In this study, we set out to determine whether phenotypic plasticity in branching corals across light habitats optimizes potential light utilization and photosynthesis. In order to do this, we mapped incident light levels across coral surfaces in branching corals and measured the photosynthetic capacity across various within-colony surfaces. Based on the field data and modelled frequency distribution of within-colony surface light levels, our results show that branching corals are substantially self-shaded at both 5 and 18 m, and the modal light level for the within-colony surface is 50 μmol photons m^?2 s^?1. Light profiles across different locations showed that the lowest attenuation at both depths was found on the inner surface of the outermost branches, while the most self-shading surface was on the bottom side of these branches. In contrast, vertically extended branches in the central part of the colony showed no differences between the sides of branches. The photosynthetic activity at these coral surfaces confirmed that the outermost branches had the greatest change in sun- and shade-adapted surfaces; the inner surfaces had a 50 % greater relative maximum electron transport rate compared to the outer side of the outermost branches. This was further confirmed by sensitivity analysis, showing that branch position was the most influential parameter in estimating whole-colony relative electron transport rate (rETR). As a whole, shallow colonies have double the photosynthetic capacity compared to deep colonies. In terms of phenotypic plasticity potentially optimizing photosynthetic capacity, we found that at 18 m, the present coral colony morphology increased the whole-colony rETR, while at 5 m, the colony morphology decreased potential light utilization and photosynthetic output. This result of potential energy acquisition being underutilized in shallow, highly lit waters due to the shallow type morphology present may represent a trade-off between optimizing light capture and reducing light damage, as this type morphology can perhaps decrease long-term costs of and effect of photoinhibition. This may be an important strategy as opposed to adopting a type morphology, which results in an overall higher energetic acquisition. Conversely, it could also be that maximizing light utilization and potential photosynthetic output is more important in low-light habitats for Acropora humilis.     

Effects of light intensity on aging of the dinoflagellate Gonyaulax polyedra

Gonyaulax polyedra Stein grown in increasingly nutrientlimited batch culture undergoes the following changes (collectively termed aging): there is a decline in the intracellular concentrations of carbon, nitrogen and photosynthetic pigments; nitrate reductase activity decreases; rates of respiration and photosynthesis fall; and cell division virtually ceases (accompanied in bright light by a decrease in the volume of individual cells). The effect of light intensity on these aging events was tested by growing cells in either bright or dim light. The bright light (330 E m^-2 s^-1) was enough to saturate photosynthesis and the dim light (80 E m^-2 s^-1) was low enough to induce significant shade adaptation of photosynthesis without lowering growth rate. At both light intensities, a decline in carbon and nitrogen content preceded or accompanied all other monitored changes, and the sequence of aging events was similar. However the onset of the decline in intracellular nutrients and photosynthetic rate in low-light cells was delayed by a least one cell division time (i.e., to twice the cell density) in comparison to cells under bright light. At both light levels, pigment-protein complexes of the photosynthetic apparatus began to break down after intracellular carbon and nitrogen had been depleted to a critically low level. The beginning of the drop in pigmentation signalled the end of log-phase growth. It is suggested that the greater pigmentation of low-light cells may represent a larger nutrient supply than found in bright-light cells and could increase the survival time of nutrient-stressed populations.     

Gene flow among population of a teleost (painted greenling,Oxylebius pictus) from Puget Sound to southern California

Net photosynthetic oxygen evolution in Amphiroa anceps (Lamarck) Decaisne is inhibited at high oxygen concentrations. Photosynthesis is highest between pH 6.5 and 7.5. At pH 9 to 10 there is still a significant photosynthetic rate, suggesting that this alga can use HCO _- ³ as a substrate for photosynthesis. At pH 7.0 to 8.5, the photosynthetic rate saturates at a total inorganic carbon concentration (C_i) greater than 3 mM. At pH 8.5 and 8.8, calcification rate continues to increase with increasing concentration of C_i. Between pH 7 and 9, the calcification rate in the light in A. foliacea Lamouroux is proportional to the photosynthetic rate, whereas at higher pH where the photosynthetic rate is very low, the calcification rate is stimulated by the higher concentration of CO _2- ³ ion. At all pH values examined, the calcification rate of living plants in the dark and of dead plants is directly proportional to the CO _2- ³ ion concentration, suggesting little metabolic involvement in calcification processes in the dark, whereas calcification in live A. foliacea in the light is influenced both by the photosynthetic rate and the CO _2- ³ ion concentration in the medium.     

Corallina and Ellisolandia (Corallinales,Rhodophyta) photophysiology over daylight tidal emersion: interactions with irradiance,temperature and carbonate chemistry

The photophysiology of three geniculate coralline algal species (Corallina officinalis, C. caespitosa and Ellisolandia elongata) was determined in intertidal rock pools in the south-west UK at Combe Martin (51°12′31N 4°2′19W) and Heybrook Bay (50°31′66N 4°11′41W), at the start, middle and end of summer (September 1 and 2) and winter (February 9 and 10) daylight tidal emersion periods, in relation to prevailing irradiance, temperature and carbonate chemistry conditions. Algal photophysiology was assessed from rapid light curves performed using pulse amplitude modulation fluorometry. Corallina and Ellisolandia experienced significant fluctuations in irradiance, temperature and carbonate chemistry over seasonal and tidal cycles. Rock pool carbonate chemistry was predictable (R ² = 0.82, P < 0.0001) by photodose (summed irradiance) plus water temperature, but not significantly related to photophysiology. In contrast, Corallina and Ellisolandia relative maximum electron transfer rate showed a significant negative relationship (R ² = 0.65, P < 0.0001) with irradiance plus water temperature. At a seasonal resolution, photoacclimation to maximize both light harvesting during winter months and photoprotection during summer months was observed for all species. Dynamic photoinhibition was apparent over both summer and winter tidal emersion, in relation to irradiance fluctuations. More effective photoinhibition was apparent during summer months, with greater sensitivity to irradiance and slower recovery in F _v/F _m, observed during winter. With sustained high irradiance over tidal emersion, the establishment of high pH/low inorganic carbon conditions may impact photochemistry. This study represents the first assessment of C. officinalis, C. caespitosa and E. elongata photophysiology underpinned by clear species concepts and highlights their ability to adapt to the dramatically fluctuating conditions experienced in intertidal rock pools.