The photosynthetic performance of the tropical seagrass Halophila ovalis in the upper intertidal

Some ecophysiological adaptation strategies of the tropical seagrass Halophila ovalis were investigated with respect to this plant's ability to grow in the upper intertidal in either monospecific pools (but not together with other intertidal species) or emergent and exposed to high temperatures and irradiances during several hours every day. It was found that Halophila ovalis could raise the pH in simulated pools to 8.6, while the two other major (biomass wise) intertidal seagrasses raised the pH to 8.8 (Cymodocea rotundata) and 9.2 (Thalassia hemprichii). In situ, midday pH values of 8.5, 8.7 and 9.0 were recorded in pools inhabiting the three species, respectively. It was further found that photosynthetic electron transport rates (ETR) measured continuously in situ for one leaf during a diurnal cycle followed the irradiance up to a daily maximal value of 2200 μmol photons m⁻² s⁻¹, but dropped by ca. 50% when the leaf became air exposed just before noon. This drop in ETR upon emergence was verified by “point measurements”, while ETRs of leaves that grew in small pools formed as the tide receded followed the irradiance more closely. Similarly, F_v/F_m measured after 15 min of dark adaptation decreased by ca. 50% in emergent leaves during midday, but maintained higher daily values in the submerged leaves.     

Effect of sediment type on microphytobenthos vertical distribution: Modelling the productive biomass and improving ground truth measurements

Intertidal flats are frequently colonised by microphytobenthos (MPB) assemblages that form transient biofilms at the sediment surface which are responsible for large fractions of estuarine primary production. The large spatio-temporal variability in MPB biomass distribution in concert with the fact that tidal flats can cover many km² makes the use of remote sensing particularly useful in assessing MPB distribution. Water content, sediment type and MPB vertical migration are variables that affect the relationship between ground truth measurements and remote sensing of benthic chlorophyll. The effect of chlorophyll depth distribution (top 2 mm) on the relationship between benthic chlorophyll and several remote sensing indices (NDVI, PI, R562/R647, derivative indices and PAM fluorescence) was investigated over a 2 year sampling period at 6 sites (Tagus estuary, Portugal). Additionally, the effect of the dark adaptation time required to measure the minimum fluorescence parameter (F₀) was also tested. Sediment type strongly affected MPB depth distribution with muddy sites showing a strong negative exponential decay in chlorophyll with distance from the surface while sandy sites had a homogenous distribution over the same scale (2 mm). Chlorophyll content (mass per unit mass, μg g^− 1) in the top 2 mm was better correlated with remote sensing indices than concentration (mass per unit volume, mg m^− 3), both for NDVI (0.72 vs. 0.45) and for PAM fluorescence (0.70 vs. 0.55). Separating the data by transect increased the correlation values in all situations. A fitted model of chlorophyll depth distribution showed that the effect of asymmetrical chlorophyll depth distribution was stronger on the correlations between chlorophyll concentration and NDVI than on chlorophyll content and NDVI (0.46-2 mm vs. 0.74-125 μm, muddy site) the same was valid for fluorescence (0.66-2 mm vs. 0.92-125 μm, muddy site). Dark adapting the samples for more than 5 min did not result in any significant difference in the relationship between F₀ and chlorophyll a. The residuals from the regression of chlorophyll content on NDVI were positively correlated (0.7) with the mass per unit of mass of sediment < 63 μm and negatively (− 0.6) with chlorophyll concentration, this indicates that if no correction is performed to account for chlorophyll depth distribution both units will be strongly affected by the mass of < 63 μm particles. The results demonstrate that although expressing chlorophyll a as concentration is generally a better option for ground truth measurements care should be taken to account for chlorophyll depth distribution since strong asymmetries within the sampling depth can introduce large errors.     

Small-scale burial of macroalgal detritus in marine sediments: Effects of Ulva spp. on the spatial distribution of macrofauna assemblages

Patches of dead seaweeds can deposit, bury, and age into the sediment. Decomposition and release of algal-derived nutrients can influence patterns of distribution of benthic organisms. Here, I investigated how small-scale burial of Ulva spp. affected spatial variation of macrofauna in intertidal sediment. I deliberately buried Ulva detritus under the surface of 50 × 50 cm² patches of sediment in three intertidal flats of the Oosterschelde estuary (The Netherlands). Results showed that there was no accumulation of particulate organic carbon and nitrogen in the sediment at the scales examined. The biomass of microphytobenthos did not show any change and there was evidence that grazing was important all over the study area. Burial did not alter composition and diversity of macrofauna, but some animals (Corophium volutator, Eteone spp. and Scoloplos armiger) had less numbers in the plots where detritus was buried than in the controls. These findings showed that burial of macroalgal detritus does not represent a major source of variation at the scales examined. It is suggested that in these sediments, recycling of detritus is fast and it buffers the effects of excess organic matter in the system.     

Increase in seagrass distribution at Bourgneuf Bay (France) detected by spatial remote sensing

The changes in spatial distribution of intertidal Zostera noltii seagrass beds were studied with multispectral visible-infrared remote sensing in Bourgneuf Bay (France) over a 14-year period, between 1991 and 2005. Six SPOT satellite images acquired at low tide were calibrated using in situ spectroradiometric data and processed with the Normalized Difference Vegetation Index (NDVI). A steady and linear increase in meadow areas was observed between 1991 and 2005 with total surfaces colonized by Z. noltii increasing from 208 to 586 ha, respectively. A greater increase in the densest part of the meadow (NDVI > 0.4) was also observed: it represented only 15% of total meadow surfaces in 1991 vs. 35% in 2005. The seagrass expansion took place mainly towards the lower part of the intertidal zone, while in the upper intertidal zone the meadow appeared strictly limited by the +4 m (Lowest Astronomical Tide) bathymetric level. The influence of Z. noltii above-ground biomass variations on spectral reflectance was analyzed experimentally by spectrometry. Z. noltii displays a characteristic steep slope from 700 to 900 nm, increasing with increasing biomass. A quantitative relationship obtained experimentally between NDVI and the dry weight of leaves was used to produce a biomass distribution map. The distribution of Bourgneuf Bay intertidal seagrass beds is certainly constrained by the water turbidity and we suggest that tidal flat accretion could be a significant variable explaining the observed expansion downwards. With very limited spatial interactions, oyster aquaculture cannot be considered as a threat, while a recent increase in recreational hand fishing of Manila clams within the beds could become problematic.     

Non-invasive monitoring of photosynthetic activity and water content in forest lichens by spectral reflectance data and RGB colors from photographs

There is a need for non-invasive monitoring of temporal and spatial variation in hydration and photosynthetic activity of red-listed poikilohydric autotrophs. Here, we simultaneously recorded kinetics in RGB-colors (photos), reflectance spectra, water content, maximal (F_V/F_M), and effective quantum yield of PSII (Φ_PSII) during desiccation in foliose lichens differing in cortical characteristics and photobionts. The spectral absorbance peaks of chlorophyll a, phycocyanin, and phycoerythrin were clearly displayed at high hydration levels. Brightness and total RGB colors of the lichens strongly increased during desiccation. The normalized difference vegetation index (NDVI) efficiently estimated hydration level and Φ_PSII – a proxy for lichen photosynthesis – in all species, including threatened old forest lichens. Color and reflectance indices based on green wavelengths gave good estimates of water content in cephalo- and chlorolichens, but not in cyanolichens with a wider range of photosynthetic pigments. Due to species-specific characteristics, species-wise calibration is essential for non-invasive assessments of lichen functioning.     

Comparative analysis of field and laboratory spectral reflectances of benthic diatoms with a modified Gaussian model approach

Benthic diatom biomass on an intertidal mudflat was estimated by field spectrometry, a non-intrusive optical method operating in the visible-infrared wavelength range. Spectral reflectance (400-900 nm) of natural assemblages was related to the amount of principal photosynthetic and accessory pigments measured by high-performance liquid chromatography (HPLC) in the top 2 mm of sediment. Relationships established in situ were compared with those obtained in the laboratory with monospecific cultures. However, to compare both types of reflectance spectra differing in their overall shape (continuum) and to isolate the pigment absorption features from other effects, we applied hyperspectral data processing, the Modified Gaussian Model (MGM), to remove the continuum and model the main absorption bands by a succession of Gaussian curves. The Gaussian band depths associated with the absorption by chlorophyll a and the pigments specific to diatoms (chlorophyll c, fucoxanthine, diadinoxanthin) displayed linear relationships with the logarithm of chlorophyll a.The most relevant spectral feature to quantify the microalgal biomass in situ was the 632 nm absorption band associated with chlorophyll c. A significant but equivocal statistical relationship was obtained at 675 nm, due to the overlapping absorption by chlorophyll breakdown products (pheophytine a and pheophorbide a) present in all the samples analyzed in the field. Fucoxanthin absorption at 550 nm can become an indicator of chlorophyll a biomass for benthic diatoms, but the effect of seasonal photophysiological adaptation should be considered. The comparative analysis of field and laboratory reflectance showed that the chlorophyll a present in the top 2 mm was not a good estimator of the photosynthetic active biomass (PAB) and that a shallower depth should be sampled in muddy sediments colonized by microphytobenthos. The approach of MGM band depth retrieval offers the possibility of quantifying PAB and has the potential to characterize (by accessory pigments) microalgae present in the photic zone of different sediment types, removing albedo variability due to grain-size scattering effects or sediment moisture content. Its accuracy relies, however, on an appropriate ground-truth sampling, excluding the contribution of redundant pigments not detected by the sensor.     

Desiccation‐induced changes in photochemical processes of photosynthesis and spectral reflectance in Nostoc commune (Cyanobacteria,Nostocales) colonies from polar regions

Cyanobacterium Nostoc commune is a species highly resistant against desiccation. In this study, we investigated changes in photochemical processes of photosynthesis and spectral reflectance indices during controlled desiccation of the colonies from Antarctica. In a dehydration process, water potential (WP) reached ?3 MPa and values of potential (F _v/F _m) and effective quantum yields (ΦPSII) of photosystem II were kept to high value until 90% of water was lost from the colony, and these values decreased rapidly by further loss of water. This indicates that the colony loses water mostly from the exopolysaccharidic envelope, not from cells during the initial part of dehydration (relative water content, RWC = 100–10%). Other suggestions of inhibition of photosynthetic processes after 90% loss of water were the increase of the chlorophyll fluorescence parameter F _p/F _s. The F _m′ was higher than F _m in hydrated colonies because of state transition which change energy distribution between PS I and PS II, but decreased to same level as F _m in dehydrated colonies. The Normalized Difference Vegetation Index (NDVI) and Photochemical Reflectance Index (PRI) showed concave‐ and convex‐curvilinear relationship with RWC, respectively. The changes of NDVI values were, however, statistically insignificant. PRI values were predominantly below 0 because of phycobiliprotein involvement. These results were compared with the same species in the Arctic region. This is, according to our best knowledge, the first measurement of changes in spectral reflectance indices during desiccation of cyanobacteria.     

Diurnal courses of net photosynthesis and photosystem II quantum efficiency of submerged Lagarosiphon major under natural light conditions

An optode device for net-photosynthesis measurements, based on oxygen-depending quenching of fluorescence from O₂-specific sensors, and PAM fluorometry have been used to study diurnal courses of net-photosynthesis and the F_v/F_m ratio of the submerged plant Lagarosiphon major. Plants were pre-cultivated and studied in large mesocosm flow-through outdoor tanks under 50% and 80% shade cloth, respectively. Growth under the different shade cloths resulted in similar light compensation points (∼20 μmol photons m⁻² s⁻¹), but strongly different light saturation levels, with about 150 μmol m⁻² s⁻¹ for plants grown under 80% shade cloth and about 350 μmol m⁻² s⁻¹ for plants grown under 50% shade cloth. Plants under both growth conditions showed a transient reduction of the maximum F_v/F_m value in the afternoon (down to 70% of the morning control values under 80% shade cloth and down to 85% under 50% shade cloth), which was not accompanied by a reduction of the net photosynthetic rate. This indicated that the fluorescence parameter F_v/F_m must not be a reliable indicator of the rate of photosynthesis under all conditions. The new photo-optical device became evidenced as a valuable tool not only for laboratory experiments, but also for field studies of gas exchange of submerged plants.     

Diurnal variation in relative photosynthetic performance in giant kelp Macrocystis pyrifera (Phaeophyceae, Laminariales) at different depths as estimated using PAM fluorometry

Pulse-amplitude modulated (PAM) fluorometry allows instantaneous estimates of photosynthetic rates, but may well produce variable measurements of photosynthetic activity depending on time of day, recent light history, internal fluctuations, and environmental variability. To investigate this, we compare estimates of diurnal variability in relative photosynthetic performance for the giant kelp, Macrocystis pyrifera (L.) C. Agardh, obtained from PAM fluorometry at three depths during 3 days characterized by different light conditions, and for two different blade ages. Sampling in the mid morning, late morning, early afternoon and late afternoon, we examined diurnal changes in relative photosynthetic performance in meristematic tissue and older blades occurring near the bottom, in the mid water, and at the water surface. Measures of maximum relative electron transport rates (rETR_max), minimum saturating irradiance (E_k), photosynthetic efficiency (α) and maximum quantum yield (F_v/F_m) show that giant kelp blades in the mid water and near the bottom exhibit little to no photosynthetic changes during the day. Near the surface, however, blades exhibit photosynthetic characteristics similar to light-adapted species in that they begin the day acclimated to low light, acclimate to increasing irradiance during the day, and end the day acclimated to low light. Consequently, while estimates of rETR_max were highest during the midday for all sample depths and days, they were also always highest near the surface for both old blades (112.16 ± 8.7, 98.6 ± 14.7, 70.16 ± 5.7) and meristematic tissue (109.0 ± 9.0, 86.9 ± 1.9, 59.2 ± 11.6, surface, mid water and bottom, respectively). Similar patterns were observed for E_k for both old blades (169.2 ± 5.4, 88.0 ± 11.2, 83.8 ± 5.2) and meristematic tissue (138.4 ± 11.5, 96.6 ± 4.69, 68.4 ± 10.6). In contrast, estimates of F_v/F_m were lowest near the surface during the midday for both old blades (0.6 ± 0.02, 0.73 ± 0.69, 0.75 ± 0.01) and meristematic tissue (0.58 ± 0.02, 0.69 ± 0.05, 0.74 ± 0.01, surface, mid water and bottom, respectively). These patterns coincided with similar patterns in ambient light, which was most variable and reached its greatest values near the surface during the midday.     

Spatial and temporal variation of photosynthesis in intertidal Mazzaella laminarioides (Bory)Fredericq (Rhodophyta, Gigartinales)

The red alga Mazzaella laminarioides is an economically important species with an extended latitudinal distribution along the Chilean coast. Its populations form mid-intertidal stands, several meters wide, and therefore are differentially exposed to environmental variables that result in temporal and spatial variability in productivity. We evaluated the effect of latitude and intertidal height on productivity by in situ measurement of photosynthetic performance. Daily and seasonal variations of O₂-evolution rate and maximal quantum yield (F _v/F _m) were determined in plants from the upper and lower intertidal zone at two localities 1500,km apart. Results suggest that plant responses were mainly affected by irradiation, temperature and desiccation. At local level, upper intertidal plants showed a reduced photosynthetic rate and quantum efficiency as compared to those displayed by plants from the lower intertidal, indicating their higher level of excitation energy acclimation. Stronger acclimation differences between upper and lower intertidal plants were observed in spring and summer. Differences in photosynthetic parameters between reproductive phases were recorded in autumn and winter, regardless of the position of the individuals in the intertidal zone. The effects of tidal elevation on seasonal patterns of photosynthesis were also influenced by latitude. Seasonal variation in photosynthetic efficiency was observed in plants from the northern population at both intertidal elevations, but only at the upper intertidal level in the southern population. This study shows that production variability in M. laminarioides results from differences in the intensity of environmental factors observed seasonally at local (intertidal) and latitudinal scales.     

Cytokinin-induced changes in the chlorophyll content and fluorescence of in vitro apple leaves

Cytokinins (CKs) are one of the main regulators of in vitro growth and development and might affect the developmental state and function of the photosynthetic apparatus of in vitro shoots. Effects of different cytokinin regimes including different types of aromatic cytokinins, such as benzyl-adenine, benzyl-adenine riboside and 3-hydroxy-benzyladenine alone or in combination were studied on the capacity of the photosynthetic apparatus and the pigment content of in vitro apple leaves after 3 weeks of culture. We found that the type of cytokinins affected both chlorophyll a and b contents and its ratio. Chlorophyll content of in vitro apple leaves was the highest when benzyl-adenine was applied as a single source of cytokinin in the medium (1846–2176 μg/1 g fresh weight (FW) of the leaf). Increasing the concentration of benzyl-adenine riboside significantly decreased the chlorophyll content of the leaves (from 1923 to 1183 μg/1 g FW). The highest chl a/chl b ratio was detected after application of meta-topolin (TOP) at concentrations of 2.0 and 6.0 μM (2.706 and 2.804). Chlorophyll fluorescence was measured both in dark-adapted (F_v/F_m test) and in light-adapted leaf samples (Yield test; Y(II)). The maximum quantum yield and efficiency of leaves depended on the cytokinin source of the medium varied between 0.683 and 0.861 (F_v/F_m) indicating a well-developed and functional photosynthetic apparatus. Our results indicate that the type and concentration of aromatic cytokinins applied in the medium affect the chlorophyll content of the leaves in in vitro apple shoots. Performance of the photosynthetic apparatus measured by chlorophyll fluorescence in the leaves was also modified by the cytokinin supply. This is the first ever study on the relationship between the cytokinin supply and the functionability of photosystem II in plant tissue culture and our findings might help to increase plantlet survival after transfer to ex vitro conditions.     

Photosynthesis and crop growth of spring oilseed rape and broccoli under elevated tropospheric ozone

The impact of a season-long exposure to moderately elevated tropospheric O₃ concentrations on the canopy growth and photosynthetic capacity of two important Brassica crops, spring oilseed rape and broccoli, was studied during three consecutive growing seasons (2007–2009). Brassica napus L. cv. Ability and Brassica oleracea L. cv. Monaco were exposed to non-filtered ambient air (NF) and non-filtered air with addition of 20 (NF+) and/or 40 ppb O₃ (NF++) in open-top chambers. Light saturated CO₂ assimilation (A_sat), stomatal conductance (g_st), maximum and actual quantum yield of photosystem II (F_v/F_m, F_v′/F_m′), performance index (PI) and leaf area index (LAI) were monitored on a weekly basis from emergence or planting until harvest. Before flowering, elevated O₃ did not have an influence on LAI nor on the photosynthetic capacity of the upper canopy leaves of either crops. This corresponded with the absence of a reduction of aboveground biomass of oilseed rape at maximum leaf area (MLA) and of broccoli plants harvested before flowering. After flowering, which coincided with MLA, the oilseed rape canopy showed a faster decline of LAI and of the chlorophyll content in NF+ compared to NF. In the NF++ treatment, this effect was intensified with an additional decrease of A_sat, g_st, F_v/F_m, F_v′/F_m′ and PI. In broccoli these detrimental O₃ effects were only detected in the lower canopy leaves. The changes in canopy development and photosynthetic performance of the upper canopy leaves can unravel the underlying mechanisms leading to the contrasting yield effects of O₃ on broccoli and spring oilseed rape that were previously reported (De Bock et al., 2011).     

Influence of carbon source and inoculum type on anaerobic biomass adhesion on polyurethane foam in reactors fed with acid mine drainage

This paper analyzes the influence of carbon source and inoculum origin on the dynamics of biomass adhesion to an inert support in anaerobic reactors fed with acid mine drainage. Formic acid, lactic acid and ethanol were used as carbon sources. Two different inocula were evaluated: one taken from an UASB reactor and other from the sediment of a uranium mine. The values of average colonization rates and the maximum biomass concentration (C_max) were inversely proportional to the number of carbon atoms in each substrate. The highest C_max value (0.35 g TVS g⁻¹ foam) was observed with formic acid and anaerobic sludge as inoculum. Maximum colonization rates (v_max) were strongly influenced by the type of inoculum when ethanol and lactic acid were used. For both carbon sources, the use of mine sediment as inoculum resulted in a v_max of 0.013 g TVS g⁻¹ foam day⁻¹, whereas 0.024 g TVS g⁻¹ foam day⁻¹ was achieved with anaerobic sludge.     

Responses of Gmelina arborea, a tropical deciduous tree species, to elevated atmospheric CO2: Growth, biomass productivity and carbon sequestration efficacy

The photosynthetic response of trees to rising CO₂ concentrations largely depends on source-sink relations, in addition to differences in responsiveness by species, genotype, and functional group. Previous studies on elevated CO₂ responses in trees have either doubled the gas concentration (>700 μmol mol⁻¹) or used single large addition of CO₂ (500-600 μmol mol⁻¹). In this study, Gmelina arborea, a fast growing tropical deciduous tree species, was selected to determine the photosynthetic efficiency, growth response and overall source-sink relations under near elevated atmospheric CO₂ concentration (460 μmol mol⁻¹). Net photosynthetic rate of Gmelina was ∼30% higher in plants grown in elevated CO₂ compared with ambient CO₂-grown plants. The elevated CO₂ concentration also had significant effect on photochemical and biochemical capacities evidenced by changes in F_V/F_M, ABS/CSm, ET₀/CSm and RuBPcase activity. The study also revealed that elevated CO₂ conditions significantly increased absolute growth rate, above ground biomass and carbon sequestration potential in Gmelina which sequestered ∼2100 g tree⁻¹ carbon after 120 days of treatment when compared to ambient CO₂-grown plants. Our data indicate that young Gmelina could accumulate significant biomass and escape acclimatory down-regulation of photosynthesis due to high source-sink capacity even with an increase of 100 μmol mol⁻¹ CO₂.     

Impacts of chlorination and heat shocks on growth, pigments and photosynthesis of Phaeodactylum tricornutum (Bacillariophyceae)

The main impacts of cooling water from thermal (nuclear) power plants on aquatic organisms were caused by chlorination and temperature increase. In this study, we investigated the impacts of residual chlorine and short-term heat shocks on growth, pigment contents and photosynthesis of Phaeodactylum tricornutum. Growth of P. tricornutum was completely inhibited; Chlorophyll a and carotenoids contents deceased about 63.3% and 61.4% in 24 h treated with 0.2 mg L^− 1 chlorine. The negative effects of chlorination increased with enhanced concentration and prolonged exposure time. Relative electrode transfer rate (rETR) of P. tricornutum was significantly suppressed when treated with 0.2 mg L^− 1 residual chlorine for 24 h. Furthermore, the effective quantum yield (F_v'/F_m') decreased first but then recovered with prolonged exposure when residual chlorine ranged between 0.1 and 0.2 mg L^− 1. The cells were less sensitive to heat shocks compared with chlorination: the rETR and F_v'/F_m' was suppressed only when the temperature exceeded 35 °C for 1 h. When P. tricornutum was exposed to chlorination combined with heat shocks, the rETR was further inhibited at 35 °C. It indicated that both chlorination and heat shocks had negative impacts on the primary producers living in discharging coastal waters; furthermore, there were synergistic effects of heat shocks on chlorination toxicity.     

Effects of experimental harvesting on recruitment of an alien mussel Mytilus galloprovincialis

Successful fisheries management is underpinned by an understanding of the processes that underlie the population dynamics of exploited stocks. This study considered the effects of experimental harvesting on recruitment of Mytilus galloprovincialis along the west coast of South Africa, where harvesting of this alien species is being contemplated. In particular, the role of settlement habitat availability in the form of adult mussels was analysed. To track the effects of a spectrum of harvesting intensities, five treatments were implemented: F = 0 (i.e. a control), F = 0.3, F = 0.6, F = 0.9 and F = 1. At these harvesting intensities 0%, 30%, 60%, 90% or 100% of mussel biomass was removed respectively at the outset of the experiment. A significant negative exponential relationship (p < 0.01) was found between M. galloprovincialis recruit density and harvesting intensity, with intensities greater than F = 0.3 dramatically reducing recruitment. This pattern was recorded throughout the intertidal zone and remained temporally constant over 2 years. Significant positive linear relationships (p < 0.01) between recruit density and adult mussel biomass or density indicate a strong correlation between availability of settlement habitat and recruitment. It is likely that the high recruit density recorded at low harvesting intensities (2000-20 000 per 0.01 m²) exceeds the level required for population maintenance. However, if settlement habitat is eliminated or significantly reduced, as was achieved by F = 0.3 or above, recruitment may become limiting. Thus, to protect stock replenishment, harvesting of M. galloprovincialis in this region should take place at intensities less than F = 0.3.     

Photophysiological stress in scleractinian corals in response to short-term sedimentation

Effects of short-term sedimentation on common coastal coral species were investigated in laboratory and field experiments on the Great Barrier Reef (GBR) using pulse-amplitude modulated (PAM) chlorophyll fluorometry. In the laboratory, changes in maximal quantum yields of photosystem II (F_v/F_m) in Montipora peltiformis were examined in response to the amount of sedimentation (79-234 mg cm⁻²) and duration of exposure (0-36 h). In control colonies, F_v/F_m ranged from 0.67 to 0.71, and did not show any temporal trend, while maximum yields of sediment-covered fragments declined steadily and reached levels below 0.1 in most colonies after 36 h coverage. Maximal quantum yield in M. peltiformis declined linearly in relation to both the amount of sediment deposited per unit surface area and the duration of exposure. Zooxanthellae densities and chlorophyll concentrations per unit area of sediment-treated corals decreased in the same manner, however, their responses were not quite as strong as the changes in F_v/F_m. Within the ranges measured, sedimentation stress of colonies exposed to large amounts of sediment for short periods of time was similar to that exposed to low amounts of sediments for prolonged periods of time. Colonies were recovered from short-term, or low-level, sedimentation within <36 h, whereas long-term exposure, or high levels of sedimentation, killed exposed colony parts. Field experiments comparing susceptibilities of common coastal coral species towards sedimentation showed significant reductions in effective quantum yields (ΔF/F_m′) in 9 out of 12 common coastal species after 22 h of exposure. Three out of twelve investigated species were not affected by the experimental application of sediments (Galaxea fascicularis, Fungia crassa, and Pectinia lactuca). Our results suggest that anthropogenic sediment deposition can negatively affect the photosynthetic activity of zooxanthellae and thus the viability of corals. However, the results also showed the ability of corals to compartmentalise sedimentation stress, as the photosynthetic activity only from tissues directly underneath the sediment declined, whereas that of adjacent clean tissues did not change measurably.     

Reevaluation of the nutrient mineralization process by infaunal bivalves (Ruditapes philippinarum) in a shallow lagoon in Hokkaido, Japan

Previous estimations of nutrient mineralization in the water column by infaunal bivalves might have been overestimated because of underestimation of the uptake process by microphytobenthos in the field. We conducted field surveys of environmental conditions and quantitative sampling of Ruditapes philippinarum in a shallow lagoon system (Hichirippu Lagoon, eastern Hokkaido, Japan) in August 2006. We recorded the spatial distribution pattern and the molar ratio of dissolved inorganic nutrients to determine the limiting nutrients for microphytobenthos, to evaluate the input and output of nutrients at the entrance of the lagoon station, and to estimate potential nutrient mineralization by R. philippinarum. Our aim was to reevaluate the nutrient mineralization process by infaunal bivalve species. In this study, the mean standing stock of microphytobenthos inhabiting surface sediment (5 mm thick) on the tidal flats was 100 times higher than that of phytoplankton (1 m depth). Low N/P and high Si/N ratios (mean = 2.6 and 17.6, respectively) near the entrance of the lagoon compared to those of microphytobenthos (N:P:Si = 10.1:1:18) clearly suggest N deficiency. The flux of NH₄-N coming into the lagoon was 3.4 kmolN d^− 1, and the flux out was − 3.7 kmolN d^− 1. Thus, assuming that there would have been no phytoplankton and microphytobenthos uptake during the day, 0.3 kmolN d^− 1 of NH₄-N was produced within the lagoon. However, the NH₄-N mineralization rate of the clams has been estimated to be approximately 7.7 ± 6.8 kmolN d^− 1. Thus, 96% (7.4 kmolN d^− 1, i.e., 7.7 kmolN d^− 1 minus 0.3 kmolN d^− 1) of the NH₄-N mineralized by the clam was consumed by microphytobenthos. In contrast, if all the NH₄-N inflow (3.1 kmolN d^− 1) was consumed by the microalgae before outflow, 52% (4.0 kmolN d^− 1, i.e., 7.7 kmolN d^− 1 minus 3.7 kmolN d^− 1) of the NH₄-N mineralized by the clams should have been consumed by microphytobenthos. Microphytobenthos on the tidal flats (11.3 ± 11.8 kmolN) used all of the surplus nutrients (between 4.0 and 7.4 kmolN d^− 1), and the temporal division rate [=(NH₄-N uptake)/(standing stock of microphytobenthos)] of microphytobenthos would have to be between 0.35 and 0.65 d^− 1. Residual NH₄-N (0.3 - 3.7 kmolN d^− 1) was the water-column source and accounted for 12-148% of NH₄-N in the water column near the entrance of the lagoon (2.5 ± 1.4 kmolN) per day. This is the first field-based observation with a quantitative evaluation of nutrient mineralization by infaunal bivalves and nutrient uptake by microphytobenthos.     

The role of ammonium in photoprotection against high irradiance in the red alga Grateloupia lanceola

Combined and/or interactive effects of inorganic nitrogen (as ammonium) and irradiance on the accumulation of nitrogenous compounds, like UV-absorbing mycosporine-like amino acids (MAAs), chlorophyll a and phycobiliproteins, were examined in the red alga Grateloupia lanceola (J. Agardh) J. Agardh in a high irradiance laboratory exposure and a subsequent recovery period under low light. Also, photosynthetic activity as in vivo chlorophyll fluorescence of photosystem II, i.e. optimum quantum yield (F_v/F_m), electron transport rate (ETR) and quantum efficiency, were examined. Photosynthetic activity, phycobiliproteins and internal nitrogen content declined during the 3-day PAR (photosynthetically active radiation; 600 μmol s⁻¹ m⁻²) and PAR + UVR (ultraviolet radiation; UVB 280–315 nm 0.8 W m⁻², UVA 315–400 nm 16 W m⁻²) exposure. Ammonium supplied in the culture medium (0, 100 and 300 μM NH₄Cl) modified the responses of the alga to high irradiance exposures in a concentration dependent manner, mainly with respect to recovery, as the highest recovery during a 10-day low light period was produced under elevated concentration of ammonium (300 μM). The recovery of photosynthetic activity and phycobiliproteins was enhanced in the algae previously incubated under PAR + UVR as compared to exposure to only PAR, suggesting a beneficial effect of UVR on recovery or photoprotective processes under enriched nitrogen conditions. However, the content of MAAs did not follow the same pattern and thus it could not be concluded as the cause of observed enhanced recovery.