Phototrophic microendoliths bloom during coral “white syndrome”

Following rapid lesion progression of white syndrome in tabular Acropora spp., the white bare skeleton gradually changes to green, a result of endolithic algae blooms (primarily Ostreobium spp.). Endolithic algal biomass and chlorophyll concentration were found to be an order of magnitude higher in the green zone compared with healthy appearing parts of each colony. Chl b to Chl a ratio increased from 1:1.6 in the healthy area to 1:2 and 1:3.5 in the white exposed skeleton and green zones, respectively. These observations together with pulse amplitude modulated (PAM) fluorometry suggest photoacclimation of the endoliths in the green zone. Histopathological microscopy revealed that the endolithic algal filaments penetrate the coral tissue. This study highlights the interaction of endolithic algae with both the skeleton and host tissue. This may have a critical role in the processes that accompany the post-disease state in reef-building corals.     

Functional diversity of microboring Ostreobium algae isolated from corals

The filamentous chlorophyte Ostreobium sp. dominates shallow marine carbonate microboring communities, and is one of the major agents of reef bioerosion. While its large genetic diversity has emerged, its physiology remains little known, with unexplored relationship between genotypes and phenotypes (endolithic versus free-living growth forms). Here, we isolated nine strains affiliated to two lineages of Ostreobium (>8% sequence divergence of the plastid gene rbcL), one of which was assigned to the family Odoaceae, from the fast-growing coral host Pocillopora acuta Lamarck 1816. Free-living isolates maintained their bioerosive potential, colonizing pre-bleached coral carbonate skeletons. We compared phenotypes, highlighting shifts in pigment and fatty acid compositions, carbon to nitrogen ratios and stable isotope compositions (δ¹³C and δ¹⁵N). Our data show a pattern of higher chlorophyll b and lower arachidonic acid (20:4ω6) content in endolithic versus free-living Ostreobium. Photosynthetic carbon fixation and nitrate uptake, quantified via 8 h pulse-labeling with ¹³C-bicarbonate and ¹⁵N-nitrate, showed lower isotopic enrichment in endolithic compared to free-living filaments. Our results highlight the functional plasticity of Ostreobium phenotypes. The isotope tracer approach opens the way to further study the biogeochemical cycling and trophic ecology of these cryptic algae at coral holobiont and reef scales.     

Fine-scale mapping of physicochemical and microbial landscapes of the coral skeleton

The coral skeleton harbours a diverse community of bacteria and microeukaryotes exposed to light, O₂ and pH gradients, but how such physicochemical gradients affect the coral skeleton microbiome remains unclear. In this study, we employed chemical imaging of O₂ and pH, hyperspectral reflectance imaging and spatially resolved taxonomic and inferred functional microbiome characterization to explore links between the skeleton microenvironment and microbiome in the reef-building corals Porites lutea and Paragoniastrea benhami. The physicochemical environment was more stable in the deep skeleton, and the diversity and evenness of the bacterial community increased with skeletal depth, suggesting that the microbiome was stratified along the physicochemical gradients. The bulk of the coral skeleton was in a low O₂ habitat, whereas pH varied from pH 6–9 with depth. Physicochemical gradients of O₂ and pH of the coral skeleton explained the β-diversity of the bacterial communities, and skeletal layers that showed O₂ peaks had a higher relative abundance of endolithic algae, reflecting a link between the abiotic environment and the microbiome composition. Our study links the physicochemical, microbial and functional landscapes of the coral skeleton and provides new insights into the involvement of skeletal microbes in the coral holobiont metabolism.     

PHOTOSYNTHESIS AND RESPIRATION OF THE CONCHOCELIS STAGE OF ALASKAN PORPHYRA (BANGIALES,RHODOPHYTA) SPECIES IN RESPONSE TO ENVIRONMENTAL VARIABLES1

Photosynthesis and respiration of three Alaskan Porphyra species, P. abbottiae V. Krishnam., P. pseudolinearis Ueda species complex (identified as P. “pseudolinearis” below), and P. torta V. Krishnam., were investigated under a range of environmental parameters. Photosynthesis versus irradiance (P–I) curves revealed that maximal photosynthesis (P_max), irradiance at maximal photosynthesis (I_max), and compensation irradiance (I_c) varied with salinity, temperature, and species. The P_max of Porphyra abbottiae conchocelis varied between 83 and 240 μmol O₂ · g dwt^?1 · h^?1 (where dwt indicates dry weight) at 30–140 μmol photons · m^?2 · s^?1 (I_max) depending on temperature. Higher irradiances resulted in photoinhibition. Maximal photosynthesis of the conchocelis of P. abbottiae occurred at 11°C, 60 μmol photons · m^?2·s^?1, and 30 psu (practical salinity units). The conchocelis of P. “pseudolinearis” and P. torta had similar P_max values but higher I_max values than those of P. abbottiae. The P_max of P. “pseudolinearis” conchocelis was 200–240 μmol O₂ · g dwt^?1 · h^?1 and for P. torta was 90–240 μmol O₂ · g dwt^?1 · h^?1. Maximal photosynthesis for P. “pseudolinearis” occurred at 7°C and 250 μmol photons · m^?2 · s^?1 at 30 psu, but P_max did not change much with temperature. Maximal photosynthesis for P. torta occurred at 15°C, 200 μmol photons · m^?2 · s^?1, and 30 psu. Photosynthesis rates for all species declined at salinities <25 or >35 psu. Estimated compensation irradiances (I_c) were relatively low (3–5 μmol · photons · m^?2 · s^?1) for intertidal macrophytes. Porphyra conchocelis had lower respiration rates at 7°C than at 11°C or 15°C. All three species exhibited minimal respiration rates at salinities between 25 and 35 psu.     

PRELIMINARY EVALUATION OF THE BIOREMEDIATION POTENTIAL OF PORPHYRA: PHOTOSYNTHETIC PRODUCTION BY BLADES AND CONCHOCELIS

Given their rapid growth and nutrient assimilation rates, Porphyra spp. are good candidates for bioremediation. The production potential of two northeast U.S. Porphyra species currently in culture (P. purpurea and P. umbilicalis) was evaluated by measuring rates of photosynthesis (as O₂ evolution) of samples grown at 20° C. Gametophytes of P. umbilicalis photosynthesized at rates that were 80% higher than those of P. purpurea over 5–20° C at both sub‐saturating and saturating irradiances (37 and 289 μmol photons m^?2 s^?1). Porphyra umbilicalis was both more efficient at low irradiances (higher alpha) and had a higher P_max than did P. purpurea (23.0 vs. 15.6 μmol O₂ g^?1 DW min^?1), suggesting that P. umbilicalis is a better choice for mass culture where self‐shading may be severe. The photosynthesis‐irradiance relationship for the Conchocelis stage of P. purpurea was also examined. Tufts of filaments, grown at 10, 15, and 20° C, were assayed at growth temperatures at irradiances ranging from 0–315 μmol photons m^?2 s^?1. Tufts were slightly more productive at 15° than at 10° C, but only ca. 4–6% as productive as gametophytes. Maximum rates of net photosynthesis were reduced by 66–74% in tufts grown at 20° C (only about 2% of gametophytes). The Conchocelis stage, however, need not limit mariculture operations; once Conchocelis cultures are established, they can be maintained over the long‐term as ready sources of spores for net seeding.     

HETEROGENEITY OF OXYGEN PRODUCTION AND CONSUMPTION IN A PHOTOSYNTHETIC MICROBIAL MAT AS STUDIED BY PLANAR OPTODES

By applying planar optodes and imaging techniques to a benthic photosynthetic mat, we demonstrated an extensive vertical and horizontal variation in O₂ concentrations, O₂ consumption, and O₂ production. In light, the oxic zone could be divided into three horizons: 1) an upper zone dominated by diatoms that had a moderate net O₂ production, 2) another zone dominated by Microcoleus-like cyanobacteria with a high net O₂ production, and 3) a lower zone with disintegrating microalgae and cyanobacteria with a high O₂ consumption rate. From the O₂ images, the net O₂ production/consumption was calculated at a spatial resolution of 130 μM. This allowed us to identify microsites with high rates of O₂ turnover within the photic zone. Sites with high net O₂ consumption (>1.5 nmol·cm^?3·s^?1) were typically situated next to sites with a relatively high net production (>2 nmol·cm^?3·s^?1), revealing a mosaic in which the highest O₂ consumption sites were surrounded by the highest O₂ production sites. This suggested a tight spatial coupling between production and consumption of O₂ within the photic zone. Light stimulated the O₂ consumption within the photic zone. At irradiances above 400 μmol photons·m^?2·s^?1, the stimulated O₂ production was almost completely balanced by enhanced O₂ consumption at microsites exhibiting net consumption of O₂ even at maximum irradiance (578 μmol photons·m^?2·s^?1). Our observations strongly supported the idea that light-stimulated respiration was caused by stimulated heterotrophic activity fueled by organic carbon leakage from the phototrophs. Despite microsites with high net O₂ consumption, anoxic microniches were not encountered in the investigated mat. Images of gross photosynthetic rates also revealed an extensive horizontal variation in gross rates, with microsites of low or no photosynthesis within the otherwise photic zone. Calculations based on the obtained images revealed that at maximum light (578 μmol photons·m^?2·s^?1), 90% of the O₂ produced was consumed within the photic zone. The presented data demonstrate the great potential offered by planar optode for studies of benthic photosynthetic communities.     

Ocean acidification and warming scenarios increase microbioerosion of coral skeletons

Biological mediation of carbonate dissolution represents a fundamental component of the destructive forces acting on coral reef ecosystems. Whereas ocean acidification can increase dissolution of carbonate substrates, the combined impact of ocean acidification and warming on the microbioerosion of coral skeletons remains unknown. Here, we exposed skeletons of the reef‐building corals, Porites cylindrica and Isopora cuneata, to present‐day (Control: 400 μatm – 24 °C) and future pCO₂–temperature scenarios projected for the end of the century (Medium: +230 μatm – +2 °C; High: +610 μatm – +4 °C). Skeletons were also subjected to permanent darkness with initial sodium hypochlorite incubation, and natural light without sodium hypochlorite incubation to isolate the environmental effect of acidic seawater (i.e., Ω_aragonite <1) from the biological effect of photosynthetic microborers. Our results indicated that skeletal dissolution is predominantly driven by photosynthetic microborers, as samples held in the dark did not decalcify. In contrast, dissolution of skeletons exposed to light increased under elevated pCO₂–temperature scenarios, with P. cylindrica experiencing higher dissolution rates per month (89%) than I. cuneata (46%) in the high treatment relative to control. The effects of future pCO₂–temperature scenarios on the structure of endolithic communities were only identified in P. cylindrica and were mostly associated with a higher abundance of the green algae Ostreobium spp. Enhanced skeletal dissolution was also associated with increased endolithic biomass and respiration under elevated pCO₂–temperature scenarios. Our results suggest that future projections of ocean acidification and warming will lead to increased rates of microbioerosion. However, the magnitude of bioerosion responses may depend on the structural properties of coral skeletons, with a range of implications for reef carbonate losses under warmer and more acidic oceans.     

THE EFFECT OF IRON NUTRITION ON PHOTOSYNTHESIS AND NITROGEN FIXATION IN CULTURES OF TRICHODESMIUM (CYANOPHYCEAE)1

Cultures of Trichodesmium NIBB 1067 were grown in the synthetic medium AQUIL with a range of iron added from none to 5 × 10^?7 M Fe for 15 days. Chlorophyll-a, cell counts, and total cell volume were two or three times higher in medium with 10^?7 M Fe than with no added Fe. Oxygen production rate per chlorophyll-a was over 60% higher with higher iron. Increased iron stimulated photosynthesis at all irradiances from about 12–250 μE · m^?2· s^?1. Nitrogen fixation rate, estimated from acetylene reduction, for 10^?7 and 10^?8 M Fe cultures was approximately twice that of the cultures with no added Fe. The range of rates of O₂ production and N₂ fixation in cultures at the iron concentrations we used were similar to the rates from natural samples of Trichodesmium from both the Atlantic, and the Pacific oceans. This similarity may allow this clone to be used, with some caution, for future physiological ecology studies. This study demonstrates the importance of iron to photosynthesis and nitrogen fixation and suggests that Trichodesmium plays a central role in the biogeochemical cycles of iron, carbon and nitrogen.     

Coral-associated nitrogen fixation rates and diazotrophic diversity on a nutrient-replete equatorial reef

The role of diazotrophs in coral physiology and reef biogeochemistry remains poorly understood, in part because N₂ fixation rates and diazotrophic community composition have only been jointly analyzed in the tissue of one tropical coral species. We performed field-based ¹⁵N₂ tracer incubations during nutrient-replete conditions to measure diazotroph-derived nitrogen (DDN) assimilation into three species of scleractinian coral (Pocillopora acuta, Goniopora columna, Platygyra sinensis). Using multi-marker metabarcoding (16S rRNA, nifH, 18S rRNA), we analyzed DNA- and RNA-based communities in coral tissue and skeleton. Despite low N₂ fixation rates, DDN assimilation supplied up to 6% of the holobiont’s N demand. Active coral-associated diazotrophs were chiefly Cluster I (aerobes or facultative anaerobes), suggesting that oxygen may control coral-associated diazotrophy. Highest N₂ fixation rates were observed in the endolithic community (0.20 µg N cm⁻² per day). While the diazotrophic community was similar between the tissue and skeleton, RNA:DNA ratios indicate potential differences in relative diazotrophic activity between these compartments. In Pocillopora, DDN was found in endolithic, host, and symbiont compartments, while diazotrophic nifH sequences were only observed in the endolithic layer, suggesting a possible DDN exchange between the endolithic community and the overlying coral tissue. Our findings demonstrate that coral-associated diazotrophy is significant, even in nutrient-rich waters, and suggest that endolithic microbes are major contributors to coral nitrogen cycling on reefs.Subject terms: Microbial ecology, Biogeochemistry, Stable isotope analysis     

THE SHORT‐TERM EFFECT OF IRRADIANCE ON THE PHOTOSYNTHETIC PROPERTIES OF ANTARCTIC FAST‐ICE MICROALGAL COMMUNITIES1

Although sea‐ice represents a harsh physicochemical environment with steep gradients in temperature, light, and salinity, diverse microbial communities are present within the ice matrix. We describe here the photosynthetic responses of sea‐ice microalgae to varying irradiances. Rapid light curves (RLCs) were generated using pulse amplitude fluorometry and used to derive photosynthetic yield (Φ_PSII), photosynthetic efficiency (α), and the irradiance (E_k) at which relative electron transport rate (rETR) saturates. Surface brine algae from near the surface and bottom‐ice algae were exposed to a range of irradiances from 7 to 262 μmol photons · m^?2 · s^?1. In surface brine algae, Φ_PSII and α remained constant at all irradiances, and rETR_max peaked at 151 μmol photons · m^?2 · s^?1, indicating these algae are well acclimated to the irradiances to which they are normally exposed. In contrast, Φ_PSII, α, and rETR_max in bottom‐ice algae reduced when exposed to irradiances >26 μmol photons · m^?2 · s^?1, indicating a high degree of shade acclimation. In addition, the previous light history had no significant effect on the photosynthetic capacity of bottom‐ice algae whether cells were gradually exposed to target irradiances over a 12 h period or were exposed immediately (light shocked). These findings indicate that bottom‐ice algae are photoinhibited in a dose‐dependent manner, while surface brine algae tolerate higher irradiances. Our study shows that sea‐ice algae are able to adjust to changes in irradiance rapidly, and this ability to acclimate may facilitate survival and subsequent long‐term acclimation to the postmelt light regime of the Southern Ocean.     

ENDOLITHIC MICROFLORA ARE MAJOR PRIMARY PRODUCERS IN DEAD CARBONATE SUBSTRATES OF HAWAIIAN CORAL REEFS1

To quantify the contribution of endolithic phototrophs to primary production of dead carbonate substrates, experimental blocks of cleaned Porites lobata Dana skeleton were placed at three different sites in Kaneohe Bay, Hawaii: inshore, lagoonal, and oceanic. After 6 months of exposure, experimental blocks were colonized by communities characteristic of their estuarine (inshore, lagoonal) and oceanic (ocean) environments. Blocks were sub-sampled; net photosynthesis (NP) and chl a concentrations of the whole blocks (epi- and endoliths) and scrapped blocks (only endoliths) were quantified. Green turf algae colonized predominantly inshore and lagoonal blocks, while encrusting corallines were the dominant epiliths colonizing oceanic blocks. Four main species of endolithic phototrophs were identified in all blocks: Mastigocoleus testarum Lagerheim, Plectonema terebrans Bornet and Flahault (cyanobacteria), Phaeophila dendroides Crouan and Crouan, and Ostreobium quekettii Bornet and Flahault (Chlorophytes). While epiliths were very different between sites, NP rates and chl a concentration of endoliths did not vary significantly and were positively correlated (191±25 mmol C·m⁻²·day⁻¹ and 590±150 mg chl a·m⁻² of reef, respectively). Assimilation numbers for whole communities, including both epilithic and endolithic communities, were similar to those measured for endolithic communities alone (average of 0.3 g C·g chl a·h⁻¹). Under experimental conditions, the contribution of endolithic phototrophs to community NP rates of blocks ranged from 56% to 81%, and under natural conditions, we estimated that this contribution ranged between 32% and 46%. Thus, we showed that the endolithic phototrophs are one of the major primary producers in dead coral substrates in a wide range of coral reef environments.     

Effects of light and hydrogen peroxide on gene expression of newly identified antioxidant enzymes in the harmful algal bloom species Chattonella marina

ABSTRACT

Antioxidant enzymes are essential proteins that maintain cell proliferation potential by protecting against oxidative stress. They are present in many organisms including harmful algal bloom (HAB) species. We previously identified the antioxidant enzyme 2-Cys peroxiredoxin (PRX) in the raphidophyte Chattonella marina. This enzyme specifically decomposes a hydrogen peroxide (H₂O₂). PRX is the only antioxidant enzyme so far identified in C. marina. This study used mRNA-seq, using Trinity assemble and blastx for annotation, to identify a further five antioxidant enzymes from C. marina: Cu Zn superoxide dismutase (Cu/Zn-SOD), glutathione peroxidase (GPX), catalase (CAT), ascorbate peroxidase (APX) and thioredoxin (TRX). In the gene expression analysis of six enzymes (Cu/Zn-SOD, GPX, CAT, APX, TRX and PRX) using light-acclimated (100 μmol photons m^?2 s^?1) C. marina cells, only PRX gene expression levels were significantly increased by strong light irradiation (1000 μmol photons m^?2 s^?1). H₂O₂ concentration and scavenging activity were also increased and significantly positively correlated with PRX gene expression levels. In dark-acclimated cells, expression levels of all antioxidant enzymes except APX were significantly increased by light irradiation (100 μmol photons m^?2 s^?1). Expression decreased the following day, with the exception of PRX expression. With the exception of CAT, gene expression of antioxidant enzymes was not significantly induced by artificial H₂O₂ treatment, although average gene expression levels were slightly increased in some enzymes. Thus, we suggest that light is the main trigger of gene expression, but the resultant oxidative stress is also a possible factor affecting the gene expression of antioxidant enzymes in C. marina.     

Effect of irradiance and temperature on the photosynthesis of a cultivated red alga,Pyropia tenera (= Porphyra tenera), at the southern limit of distribution in Japan

The effect of irradiance and temperature on the photosynthesis of the red alga, Pyropia tenera, was determined for maricultured gametophytes and sporophytes collected from a region that is known as one of the southern limits of its distribution in Japan. Macroscopic gametophytes were examined using both pulse‐amplitude modulated fluorometry and/or dissolved oxygen sensors. A model of the net photosynthesis–irradiance (P‐E) relationship of the gametophytes at 12°C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 46 μmol photons m^?2 s^?1, and the compensation irradiance was 9 μmol photons m^?2 s^?1. Gross photosynthesis and dark respiration for the gametophytes were also determined over a range of temperatures (8–34°C), revealing that the gross photosynthetic rates of 46.3 μmol O₂ mg_chl‐a^?1 min^?1 was highest at 9.3 (95% Bayesian credible interval (BCI): 2.3–14.5)°C, and the dark respiration rate increased at a rate of 0.93 μmol O₂ mg_chl‐a^?1 min^?1°C^?1. The measured dark respiration rates ranged from ?0.06 μmol O₂ mg_chl‐a^?1 min^?1 at 6°C to ?25.2 μmol O₂ mg_chl‐a^?1 min^?1 at 34°C. The highest value of the maximum quantum yield (Fv/Fm) for the gametophytes occurred at 22.4 (BCI: 21.5–23.3) °C and was 0.48 (BCI: 0.475–0.486), although those of the sporophyte occurred at 12.9 (BCI: 7.4–15.1) °C and was 0.52 (BCI: 0.506–0.544). This species may be considered well‐adapted to the current range of seawater temperatures in this region. However, since the gametophytes have such a low temperature requirement, they are most likely close to their tolerable temperatures in the natural environment.     

RESPONSE OF CATALASE ACTIVITY TO ENVIRONMENTAL STRESS IN THE FRESHWATER DINOFLAGELLATE PERIDINIUM GATUNENSE1

Catalase activity increased in Peridinium gatunense (formerly P. cinctum fa. westii) cells during the decline of the seasonal spring bloom period in Lake Kinneret. This was correlated with the low ambient total CO₂ concentration. The relationship was confirmed in laboratory experiments where maximum catalase activity occurred under an atmosphere composed of 30% O₂ and 0.003% CO₂. Conversely, high CO₂ concentrations inhibited catalase activity. The rise in catalase activity was not directly due to increasing environmental pH, as in vitro and in vivo measurements showed a characteristic broad pH curve with a constant activity from pH 6–10 for catalase. Photoinhibition of catalase occurred above 250 μmol photons · m^?2· s^?1. However, at high photoinactivating irradiances, photoinhibition was ameliorated under high pO₂/pCO₂. Such conditions prevail in the Kinneret at the end of the spring. We propose that the enhancement of photorespiration (under high pO₂/pCO₂) induces a temporary burst in catalase activity despite the progressively photoinhibitory conditions of early summer.     

Stable isotopic records of bleaching and endolithic algae blooms in the skeleton of the boulder forming coral Montastraea faveolata

Within boulder forming corals, fixation of dissolved inorganic carbon is performed by symbiotic dinoflagellates within the coral tissue and, to a lesser extent, endolithic algae within the coral skeleton. Endolithic algae produce distinctive green bands in the coral skeleton, and their origin may be related to periods of coral bleaching due to complete loss of dinoflagellate symbionts or “paling” in which symbiont populations are patchily reduced in coral tissue. Stable carbon isotopes were analyzed in coral skeletons across a known bleaching event and 12 blooms of endolithic algae to determine whether either of these types of changes in photosynthesis had a clear isotopic signature. Stable carbon isotopes tended to be enriched in the coral skeleton during the initiation of endolith blooms, consistent with enhanced photosynthesis by endoliths. In contrast, there were no consistent δ¹³C patterns directly associated with bleaching, suggesting that there is no unique isotopic signature of bleaching. On the other hand, isotopic values after bleaching were lighter 92% of the time when compared to the bleaching interval. This marked drop in skeletal δ¹³C may reflect increased kinetic fractionation and slow symbiont recolonization for several years after bleaching.     

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.     

Photosynthetic activity of a temperate coral Acropora pruinosa (Scleractinia, Anthozoa) with symbiotic algae in Japan

Physiological properties of the temperate hermatypic coral Acropora pruinosa Brook with symbiotic algae (zooxanthellae) on the southern coast of the Izu Peninsula, Shizuoka Prefecture, central Japan, were compared between summer and winter. Photosynthesis and respiration rates of the coral with symbiotic zooxanthellae were measured in summer and winter under controlled temperatures and irradiances with a differential gasvolumeter (Productmeter). Net photosynthetic rate under all irradiances was higher in winter than in summer at the lower range of temperature (12–20°C), while lower than in summer at the higher range of temperature (20–30°C). The optimum temperature for net photosynthesis was apt to fall with the decrease of irradiance both in summer and winter, whereas it was higher in summer than in winter under each irradiance. At 25/ 50/100 μmol photons nr² s^?1, it was nearly the sea‐water temperature in each season. Dark respiration rate was higher in winter than in summer, especially in the range from 20–30°C. In both seasons the optimum temperature for gross photosynthesis was 28°C under 400 μmol photons nr² s^?1 and lowered with decreasing irradiance up to 22°C under 25 μmol photons nr² s^?1 in summer, while 20°C under the same irradiance in winter. The optimum temperature for production/respiration (P/R) ratio was higher in summer than in winter under each irradiance. Results indicated that metabolism of coral and zooxanthellae is adapted to ambient temperature condition under nearly natural irradiance in each season.     

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.     

LIGHT‐INDUCED RESPONSES OF OXYGEN PHOTOREDUCTION,REACTIVE OXYGEN SPECIES PRODUCTION AND SCAVENGING IN TWO DIATOM SPECIES1

Diatoms are frequently exposed to high light (HL) levels, which can result in photoinhibition and damage to PSII. Many microalgae can photoreduce oxygen using the Mehler reaction driven by PSI, which could protect PSII. The ability of Nitzschia epithemioides Grunow and Thalassiosira pseudonana Hasle et Heimdal grown at 50 and 300 μmol photons · m^?2 · s^?1 to photoreduce oxygen was examined by mass spectrometric measurements of ¹⁸O₂. Both species exhibited significant rates of oxygen photoreduction at saturating light levels, with cells grown in HL exhibiting higher rates. HL‐grown T. pseudonana had maximum rates of oxygen photoreduction five times greater than N. epithemoides, with 49% of electrons transported through PSII being used to reduce oxygen. Exposure to excess light (1,000 μmol photons · m^?2 · s^?1) produced similar decreases in the operating quantum efficiency of PSII (F_q′/F_m′) of low light (LL)‐ and HL‐grown N. epithemoides, whereas HL‐grown T. pseudonana exhibited much smaller decreases in F_q′/F_m′ than LL‐grown cells. HL‐grown T. pseudonana and N. epithemioides exhibited greater superoxide and hydrogen peroxide production, higher activities (in T. pseudonana) of superoxide dismutase (SOD) and ascorbate peroxidase (APX), and increased expression of three SOD‐ and one APX‐encoding genes after 60 min of excess light compared to LL‐grown cells. These responses provide a mechanism that contributes to the photoprotection of PSII against photodamage.     

Effects of salinity,light intensity and sediment on growth,pigments, agar production and reproduction in Gracilaria tenuistipitata from Songkhla Lagoon in Thailand

The effects of salinity, light intensity and sediment on Gracilaria tenuistipitata C.F. Chang & B.M. Xia on growth, pigments, agar production, and net photosynthesis rate were examined in the laboratory under varying conditions of salinity (0, 25 and 33 psu), light intensity (150, 400, 700 and 1000 µmol photons m^?2 s^?1) and sediment (0, 0.67 and 2.28 mg L^?1). These conditions simulated field conditions, to gain some understanding of the best conditions for cultivation of G. tenuistipitata. The highest growth rate was at 25 psu, 700 µmol photons m^?2 s^?1 with no sediments, that provided a 6.7% increase in weight gain. The highest agar production (24.8 ± 3.0 %DW) was at 25 psu, 150–400 µmol photons m^?2 s^?1 and no sediment. The highest pigment contents were phycoerythrin (0.8 ± 0.5 mg g^?1FW) and phycocyanin (0.34 ± 0.05 mg g^?1 FW) produced in low light conditions, at 150 µmol photons m^?2 s^?1. The highest photosynthesis rate was 161.3 ± 32.7 mg O₂ g^?1 DW h^?1 in 25 psu, 400 µmol photons m^?2 s^?1 without sediment in the short period of cultivation, (3 days) and 60.3 ± 6.7 mg O₂ g^?1 DW h^?1 in 25 psu, 700 µmol photons m^?2 s^?1 without sediment in the long period of cultivation (20 days). The results indicated that salinity was the most crucial factor affecting G. tenuistipitata growth and production. This would help to promote the cultivation of Gracilaria cultivation back into the lagoon using these now determined baseline conditions. Extrapolation of the results from the laboratory study to field conditions indicated that it was possible to obtain two crops of Gracilaria a year in the lagoon, with good yields of agar, from mid‐January to the end of April (dry season), and from mid‐July to the end of September (first rainy season) when provided sediment was restricted.