Urea uptake by the scleractinian coral Stylophora pistillata

Urea can be one of the major sources of nitrogen for phytoplankton, but little is known about its importance for corals. Experiments were therefore designed to assess the uptake rates of urea by the scleractinian coral Stylophora pistillata; ¹⁵N-urea was used to follow the incorporation of nitrogen into the zooxanthellae and animal tissue. The uptake kinetics of urea in the tissue of S. pistillata showed that there is a concentration-dependent uptake of urea. The transport of urea was composed of a linear component (diffusion) at concentrations higher than 6 μmol N-urea l^− 1 and an active carrier-mediated component, at lower concentrations. The value of the carrier affinity (K_m = 1.05 μmol urea l^− 1) indicates a good adaptation of the corals to low levels of urea in seawater. At the in situ concentration of ca. 0.2 μmol N-urea l^− 1, the uptake rate was equal to 0.1 nmol N h^− 1 cm^− 2. Urea uptake was at least four times higher in the animal than in the algal fraction, and five times higher when corals were incubated in the light than in the dark. These results could be explained by the involvement of urea in the calcification process, which is also enhanced by light. Comparison of urea uptake rates with nitrate or ammonium uptake rates for the same S. pistillata species, at in situ concentrations, showed that urea is preferred to nitrate and may therefore be an important source of nitrogen for scleractinian corals.     

Oxygen and Heterotrophy Affect Calcification of the Scleractinian Coral Galaxea fascicularis

Heterotrophy is known to stimulate calcification of scleractinian corals, possibly through enhanced organic matrix synthesis and photosynthesis, and increased supply of metabolic DIC. In contrast to the positive long-term effects of heterotrophy, inhibition of calcification has been observed during feeding, which may be explained by a temporal oxygen limitation in coral tissue. To test this hypothesis, we measured the short-term effects of zooplankton feeding on light and dark calcification rates of the scleractinian coral Galaxea fascicularis (n = 4) at oxygen saturation levels ranging from 13 to 280%. Significant main and interactive effects of oxygen, heterotrophy and light on calcification rates were found (three-way factorial repeated measures ANOVA, p<0.05). Light and dark calcification rates of unfed corals were severely affected by hypoxia and hyperoxia, with optimal rates at 110% saturation. Light calcification rates of fed corals exhibited a similar trend, with highest rates at 150% saturation. In contrast, dark calcification rates of fed corals were close to zero under all oxygen saturations. We conclude that oxygen exerts a strong control over light and dark calcification rates of corals, and propose that in situ calcification rates are highly dynamic. Nevertheless, the inhibitory effect of heterotrophy on dark calcification appears to be oxygen-independent. We hypothesize that dark calcification is impaired during zooplankton feeding by a temporal decrease of the pH and aragonite saturation state of the calcifying medium, caused by increased respiration rates. This may invoke a transient reallocation of metabolic energy to soft tissue growth and organic matrix synthesis. These insights enhance our understanding of how oxygen and heterotrophy affect coral calcification, both in situ as well as in aquaculture.     

Efficiencies and costs of larval growth in different food environments (Asteroidea: Asterina miniata)

The ocean is a nutritionally heterogeneous environment. For feeding larval forms, food variability has significant consequences for growth and later recruitment success. In this study, the physiological and biochemical responses to a range of different food concentrations (unfed, 4, 20, and 40 algal cells μl^− 1) were examined in larvae of the asteroid, Asterina miniata. Measurements of growth, protein synthesis rates, and the energetic cost of protein synthesis were made. Under conditions of rapid growth, protein comprised a larger percent (66%) of a larva's organic biomass compared to similar-aged, slower-growing larvae (26%). Larvae fed at the highest food concentration tested (40 algal cells μl^− 1) had a protein depositional efficiency of 80% (± 16%), a value 3-fold higher than larvae fed 20 algal cells μl^− 1 (28% ± 11%). Also, faster-growing larvae required 3-fold less energy per unit mass of protein growth. Larvae fed 40 algal cells μl^− 1 deposited protein at a respiratory cost of 65 ± 11 pmol O₂ h^− 1 (μg protein)^− 1; larvae fed 20 algal cells μl^− 1 had a cost of 192 ± 47 pmol O₂ h^− 1 (μg protein)^− 1. While there were differences in the cost to deposit protein (i.e., protein growth, the balance of synthesis and degradation), there were no differences in the energetic cost of protein synthesis for all food concentrations tested. The energetic cost of protein synthesis was fixed at 13.8 (± 0.92) Joules (mg protein synthesized)^− 1 and was independent of developmental stage, growth rates, and large changes (58-fold) in protein synthesis rates. A major conclusion from this study is that larvae grown in high-food environments not only grew faster, but did so for considerably less energy. Defining the complex relationships of food availability and metabolic efficiency will provide more accurate predictions of larval growth under variable food conditions in the ocean.     

Effect of natural zooplankton feeding on the tissue and skeletal growth of the scleractinian coral<Emphasis Type="Italic"> Stylophora pistillata</Emphasis>

Laboratory experiments were designed to estimate the ingestion rates of the scleractinian coral Stylophora pistillata under varying prey concentrations and feeding regimes and to assess the effect of feeding on the tissue and skeletal growth. Six sets of corals were incubated under two light (80 and 300 µmol photons m^–2 s^–1) and three feeding levels (none, fed twice, and fed six times per week) using freshly collected zooplankton. Results showed that the number of prey ingested was proportional to prey density, and no saturation of feeding capability was reached. Capture rates varied between 0.5 and 8 prey items 200 polyp^–1 h^–1. Corals starved for several days ingested more plankton than did fed corals. Fed colonies exhibited significantly higher levels of protein, chlorophyll a, and chlorophyll c₂ per unit surface area than starved colonies. Feeding had a strong effect on tissue growth, increasing it by two to eight times. Calcification rates were also 30% higher in fed than in starved corals. Even moderate levels of feeding enhanced both tissue and skeletal growth, although the processes involved in this enhancement remain to be determined.     

Density dependent effects of an infaunal suspension-feeding bivalve (Austrovenus stutchburyi) on sandflat nutrient fluxes and microphytobenthic productivity

We examined in situ the density dependent effects of an infaunal suspension-feeding bivalve, Austrovenus stutchburyi (hereafter Austrovenus) on sandflat nutrient fluxes and microphytobenthic (MPB) production. Nine experimental plots (0.64 m^− 2) were established at two locations separated by 300 m. Ambient fauna was left intact and Austrovenus added to plots creating a density range from 20 to 2000 ind. m^− 2. Three weeks later, light and dark benthic chambers (area = 0.114 m^− 2) were deployed to measure MPB production and nutrient fluxes. Austrovenus density was positively correlated with organic content and porosity but did not affect other sediment properties (grain size, pigment content) or resident macrofauna. In dark chambers there was a net influx of oxygen (O₂) into the sediments which increased with Austrovenus density (from − 0.45 to − 1.21 mmol m^− 2 h^− 1) whereas in light chambers there was a net efflux from the sediments which decreased with density (from 0.90 to 0.31 mmol m^− 2 h^− 1). Significant (p < 0.01) multiple linear regression models explained respectively 42% and 72% of the variability in the dark and light chamber O₂ fluxes with Austrovenus density as the most important predictor variable. When the effects of significant co-variables (light intensity, grain size) were accounted for, the negative relationship between O₂ flux and Austrovenus density was less steep in light chambers (ANCOVA p < 0.001) suggesting a stimulation of MPB production at higher densities. Estimates of gross MPB primary production indicated a 30% increase in rates of carbon fixation with Austrovenus density (from 36 to 48 mg C m^− 2 h^− 1). Ammonium (NH₄⁺) was released from the sediments in both light and dark chambers and increased with Austrovenus density by a factor of 5.9-6.9×. Multiple linear regression models were significant for light and dark chambers (p < 0.001; r² 86-87%) with Austrovenus again as the most important variable influencing fluxes. ANCOVA results (p < 0.001) indicated that in dark chambers NH₄⁺ efflux increased with Austrovenus density at a rate 1.76× greater than in light chambers. These results indicate that the greater efflux of NH₄⁺ at high densities was being trapped by photosynthesising MPB at the sediment-water interface supporting higher rates of primary production. Our results suggest that a reduction in Austrovenus density will lower nutrient fluxes potentially influencing system productivity by reducing MPB production.     

High rates of nitrogen fixation (acetylene reduction) on coral skeletons following bleaching mortality

Nubbins of the coral Acropora aspera were artificially bleached and nitrogen fixation (acetylene reduction) rates were measured on the developing epilithic communities. Seasonal comparisons were made between corals that died in summer of heat stress and corals that died in winter from natural cold stress. Rates of acetylene reduction from artificially bleached corals peaked at 26.66 nmol cm⁻² h⁻¹ 2 weeks after summer mortality, while rates from natural winter mortality peaked at 18.07 nmol cm⁻² h⁻¹ 12 days after coral death. Comparative rates of acetylene reduction taken from live corals and coral rubble ranged between 0.56 and 1.16 nmol cm⁻² h⁻¹, and 0.15 and 12.77 nmol cm⁻² h⁻¹, respectively. N₂-fixation rates from dead corals were up to 30 times greater than those measured on live corals. The observed increase in N₂-fixation from dead corals may increase the availability of nitrogen for use in trophic processes within the reef for an extended period following the initial mortality event. If the spatial scale over which coral mortality has occurred in past thermal bleaching events is considered the ramifications of such an increase may be substantial.     

Calcification by juvenile corals under heterotrophy and elevated CO2

Ocean acidification (OA) threatens the existence of coral reefs by slowing the rate of calcium carbonate (CaCO₃) production of framework-building corals thus reducing the amount of CaCO₃ the reef can produce to counteract natural dissolution. Some evidence exists to suggest that elevated levels of dissolved inorganic nutrients can reduce the impact of OA on coral calcification. Here, we investigated the potential for enhanced energetic status of juvenile corals, achieved via heterotrophic feeding, to modulate the negative impact of OA on calcification. Larvae of the common Atlantic golf ball coral, Favia fragum, were collected and reared for 3 weeks under ambient (421 μatm) or significantly elevated (1,311 μatm) CO₂ conditions. The metamorphosed, zooxanthellate spat were either fed brine shrimp (i.e., received nutrition from photosynthesis plus heterotrophy) or not fed (i.e., primarily autotrophic). Regardless of CO₂ condition, the skeletons of fed corals exhibited accelerated development of septal cycles and were larger than those of unfed corals. At each CO₂ level, fed corals accreted more CaCO₃ than unfed corals, and fed corals reared under 1,311 μatm CO₂ accreted as much CaCO₃ as unfed corals reared under ambient CO₂. However, feeding did not alter the sensitivity of calcification to increased CO₂; ? calcification/?Ω was comparable for fed and unfed corals. Our results suggest that calcification rates of nutritionally replete juvenile corals will decline as OA intensifies over the course of this century. Critically, however, such corals could maintain higher rates of skeletal growth and CaCO₃ production under OA than those in nutritionally limited environments.     

Primary Production and Calcification by the Soritid Foraminifer Archais angulatus (Fichtel & Moll)*

SYNOPSIS. Symbiosis between Chlamydomonas hedleyi (Lee, Crockett, Hagen & Stone) and Archais angulatus (Fichtel & Moll) was examined during laboratory studies of primary production and light-enhanced calcification. Photosynthesis and calcification are directly proportional to light intensity in the range of 0–200 μEinsteins m^-2 sec^-1. Calcification in the light is directly proportional to photosynthesis and proceeds at rates that are 2–3 times that observed in the dark. The herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), in concentrations of 1–100 μM, completely inhibits photosynthesis and light-enhanced calcification. Calcification of the foraminiferan test is therefore due to the photosynthetic activity of the symbiote. Calcification rates for foraminifers incubated in the dark or with DCMU are not significantly different from the calcification rates obtained for dead foraminifers. Rates of calcification obtained with ⁴⁵Ca are twice that obtained with ¹⁴C.     

Light-dependent phosphate uptake of a submersed macrophyte Myriophyllum spicatum L.

The uptake kinetics of phosphate (Pi) by Myriophyllum spicatum was determined from adsorption and absorption under light and dark conditions. Pi uptake was light dependent and showed saturation following the Michaelis-Menten relation (in light: V = 16.91 × [Pi](1.335 + [Pi]), R² = 0.90, p < 0.001; in the dark: V = 5.13 × [Pi](0.351 + [Pi]), R² = 0.77, p < 0.001). Around 77% of the loss of Pi in the water column was absorbed into the tissue of M. spicatum, and only 23% was adsorbed on the surface of the plant shoots. Our study shows that M. spicatum shoots have a much higher affinity (in light: 3.9 μmol g⁻¹ dw h⁻¹ μM⁻¹; in the dark: 3.7 μmol g⁻¹ dw h⁻¹ μM⁻¹) and V_max (maximum uptake rate, shoot light) for Pi uptake than many other aquatic macrophytes (in light: 0.002-0.23 μmol g⁻¹ dw h⁻¹ μM⁻¹; in the dark: 0.002-0.19 μmol g⁻¹ dw h⁻¹ μM⁻¹), which may provide a competitive advantage over other macrophytes across a wide range of Pi concentrations.     

Bicarbonate use in three aquatic plants

Our study aimed to test the ability of aquatic plants to use bicarbonate when acclimated to three different bicarbonate concentrations. To this end, we performed experiments with the three species Ceratophyllum demersum, Egeria densa, Lagarosiphon major to determine photosynthetic rates under varying bicarbonate concentrations. We measured bicarbonate use efficiency, photosynthetic performance and respiration. For all species, our results revealed that photosynthetic rates were highest in replicates grown at low alkalinity. Thus, E. densa had approx. five times higher rates at low (264 ± 15 μmol O₂ g⁻¹ DW h⁻¹) than at high alkalinity (50 ± 27 μmol O₂ g⁻¹ DW h⁻¹), C. demersum had three times higher rates (336 ± 95 and 120 ± 31 μmol O₂ g⁻¹ DW h⁻¹), and L. major doubled its rates at low alkalinity (634 ± 114 and 322 ± 119 μmol O₂ g⁻¹ DW h⁻¹). Similar results were obtained for bicarbonate use efficiency by E. densa (136 ± 44 and 43 ± 10 μmol O₂ mequiv. L⁻¹ g⁻¹ DW h⁻¹) and L. major (244 ± 29 and 82 ± 24 μmol O₂ mequiv. L⁻¹ g⁻¹ DW h⁻¹). As to C. demersum, efficiency was high but unaffected by alkalinity, indicating high adaptation ability to varied alkalinities. A pH drift experiment supported these results. Overall, our results suggest that the three globally widespread worldwide species of our study adapt to low inorganic carbon availability by increasing their efficiency of bicarbonate use.     

Sea surface temperature and the growth of the West Atlantic reef-building coral Montastraea annularis

Relationships were analyzed between sea surface temperature (SST) and annual growth characteristics (density, extension rate and calcification rate) of the Caribbean reef-building coral Montastraea annularis. Colonies were collected from 12 localities in the Gulf of Mexico and the Caribbean Sea. Two well-separated relationships were found, one for the Gulf of Mexico and the other for the Caribbean Sea. Calcification rate and skeletal density increased with increasing SST in both regions, while extension rate tended to decrease. Calcification rate increased ∼0.57 g cm⁻² year⁻¹ for each 1 °C increase in SST. Zero calcification was projected to occur at 23.7 °C in corals from the Gulf of Mexico and at 25.5 °C in corals from the Caribbean Sea. The 24 °C annual average SST isotherm marks the northern limit of distribution of M. annularis. Montastraea annularis populations of the Gulf of Mexico are isolated from those of the Caribbean Sea, and results indicate that corals from the Gulf of Mexico are adapted to growth at lower minimum and average annual SST. Corals from both the Gulf of Mexico and the Caribbean Sea, growing at lower SSTs and having lower calcification rates, extend their skeletons the same or more than those growing at higher SSTs. They achieve this by putting more of their calcification resources into extension and less into thickening, i.e., by sacrificing density.     

Temperature effects on calcification rate and skeletal deposition in the temperate coral, Plesiastrea versipora (Lamarck)

The geographic range of the coral, Plesiastrea versipora (Lamarck, 1816), extends into temperate waters outside the southern limit for hermatypic corals. In the present study, calcification in Plesiastrea collected from Port Phillip Bay, Victoria was examined over the coral's normal annual temperature range (10-21 °C), which is well below the normal optimum for coral calcification in tropical corals (25-28 °C). Calcification rate in Plesiastrea was considerably lower than in reef corals, but showed a similar pattern in temperature responses, with a trend towards higher rates at ∼18 °C. The light/dark calcification ratio was markedly lower than that in tropical corals. Autoradiography showed that calcification occurred primarily by deposition of calcium carbonate at the upper surfaces of the septo-costae. Scanning electron microscopy (SEM) showed that skeletal deposition in Plesiastrea had a temperature-dependent diel pattern. In the light, calcium carbonate was deposited as small spheroidal crystals and, at higher temperatures, small needle-shaped crystals. In the dark, calcium carbonate deposition appeared to be in the form of an amorphous sheet-like cementation. Compared with other scleractinian corals, calcification rate in Plesiastrea was relatively slow and showed different patterns of skeletal deposition.     

Factors affecting photosynthesis and its seasonal variation in the seagrasses Cymodocea nodosa (Ucria) Aschers,and Posidonia oceanica (L.) Delile in the mediterranean

Measurements of photosynthesis, dark respiration, and leaf chlorophyll content were made in the laboratory on both shallow (1 to 5 m) and deep (25 to 33 m) leaves of Cymooceu nodosa (Ucria) Aschers, and Posidonia oceanica (L.) Delile in Malta in April and August. Light saturated photosynthetic rates in Cymodocea were similar in spring (18 μg C cm^?2h^?1) and summer (25μg Ccm^?2h^?1) if the 9 C increase in water temperature in summer is taken into account: however, photosynthetic rates in Posidonia were higher in spring than in summer, especially in shallow leaves which fixed ≈ 10 μg C cm^?2h^?1 in spring but less than half that in summer when rates of carbon accretion were close to compensation point. Levels of irradiance at which photosynthesis was light saturated (Iλ were ≈ 3 mW cm^?2 PAR for Cymodocea and 2 mW cm^?2 PAR for Posidonia: underwater irradiance at the lower depth limit for these plants (≈33 m) was ≈3 mW cm^?2 PAR. corresponding closely to the saturation irradiances. Compensation irradiance for both species was between 0.3 and 0.5 mW cm^?2 PAR.Photosynthesis in both species had a temperature optimum at about 30 C (slightly higher in Cymodocea in summer). Dark respiration rates were generally similar in spring and summer, in the region of 3 μg C cm^?2 h^?1 in Cymodocea and 1.5 to 2 μg C cm^?2 h^?1 in Posidonia. Increase in dark respiration rates with increased temperature was considerably greater in spring than in summer in both species. Photosynthesis was directly proportional to chlorophyll content in Posidonia in the range encountered (up to 58 μg Chl cm^?2) and the summer reduction in photosynthesis was closely correlated with reduction in chlorophyll content. It seems unlikely that environmental factors such as seasonal changes in light intensity, nutrient availability or water temperature were directly responsible for this loss of chlorophyll and it is suggested that this is a manifestation of general leaf senescence, probably induced by daylength changes but possibly enhanced by increased water temperature. Cymodocea showed a similar reduction in chlorophyll content in summer but this was not reflected in reduced photosynthesis. Thus, although Cymodocea may grow rapidly throughout the spring and summer with an overall productivity of 3.6 g C m^?2 day^?1 in shallow water, the luxuriant growths of Posidonia must develop in the first half of the year when a dense meadow may produce up to 2.1 g C m^?2 day^?1 in shallow water, declining to ?0.6 g C m^?2 day^?1 in summer.     

Thermal preference and tolerance of Megastrea (Lithopoma) undosa (Wood, 1828; Gastropoda: Turbinidae)

The thermoregulatory behavior of the wavy turban snail Megastrea (Lithopoma) undosa was determined in a horizontal thermal gradient and was 16.31 in day cycle and 14.4 °C in night cycle. Displacement velocity of adults was 29.3±4.2 cm h⁻¹ during the light phase and 26.1±3.2 cm h⁻¹ during the dark phase. The critical thermal maxima of the wavy turban snail were determined. As a measure of thermal tolerance, snails were subjected to increasing water temperatures at a rate of 1 °C every 30 min until they were detached from the substrate. The critical thermal maximum at 50% was 29.7 °C.     

Effects of host nutritional status and seasonality on the nitrogen status of zooxanthellae in the temperate coral Plesiastrea versipora (Lamarck)

The nitrogen status of endosymbiotic dinoflagellates (zooxanthellae) in the temperate coral Plesiastrea versipora (Lamarck) was determined by measuring the extent to which ammonium (40 μM NH₄⁺) enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the enhancement ratio was expressed as [dark NH₄⁺ rate/dark FSW rate]. V_D′/V_L, a further index of nitrogen status, was also calculated where V_D′ = [dark NH₄⁺ rate − dark FSW rate] and V_L = rate of carbon fixation in the light. When corals were starved for 2-8 weeks, zooxanthellar nitrogen deficiency became apparent at ≥ 4 weeks, with NH₄⁺/FSW and V_D′/V_L averaging up to 2.08 and 0.0061, respectively. A decrease in light-saturated photosynthesis per zooxanthella also occurred, with the photosynthetic rate after 4-6 weeks being just 81% of that seen prior to starvation. In comparison, when corals were fed 5 times per week for 8 weeks the addition of ammonium had little effect, indicating nitrogen sufficiency; NH₄⁺/FSW and V_D′/V_L were 1.03 and 0.0003, respectively. Photosynthetic rates of zooxanthellae from well-fed corals were up to 1.7 times greater than those of zooxanthellae from starved corals. The nitrogen status of zooxanthellae from corals in the field exhibited seasonal differences. Autumn samples were nitrogen sufficient, with NH₄⁺/FSW = 1.003 and V_D′/V_L = 0.0001. In contrast, a small degree of nitrogen deficiency was seen in winter and spring, when NH₄⁺/FSW averaged 1.075 and 1.249, and V_D′/V_L averaged 0.0013 and 0.0014, respectively. The greatest degree of nitrogen deficiency was observed in summer, when NH₄⁺/FSW averaged 1.318 and V_D′/V_L averaged 0.0036. Given the clear links between food supply and nitrogen status seen under experimental conditions, and the likelihood that the zooxanthellae are also able to take up nutrients directly from the seawater, the fluctuations in nitrogen status may reflect temporal fluctuations in seawater nutrient concentrations and plankton abundance. The nutrient status of these temperate zooxanthellae in the field is in contrast to the marked nitrogen deficiency seen in zooxanthellae from nutrient-poor coral reef waters, and raises the possibility that temperate zooxanthellae can store nitrogen for use when exogenous nutrients and food are less readily available. This, in turn, may contribute to the considerable stability of temperate zooxanthellar populations under highly variable environmental conditions.     

The effect of natural populations of the burrowing and grazing soldier crab (Mictyris longicarpus) on sediment irrigation, benthic metabolism and nitrogen fluxes

The aim of this study was to determine the effect of sediment grazing and burrowing activities of natural populations of Mictyris longicarpus on benthic metabolism, nitrogen flux and irrigation rates by comparing sediments taken from minimum disturbance exclusion cages and adjacent sediments subject to M. longicarpus activities. M. longicarpus reduced sediment surface chlorophyll a (approximately 77%), organic carbon (approximately 95%) and total nitrogen concentrations (approximately 99%) in comparison to ungrazed sediments. Consequently, they significantly reduced gross benthic O₂ production (about 71%) and sediment O₂ consumption (approximately 46%). Mean N₂ fluxes showed net effluxes (276-430 μmol m⁻² day⁻¹) in the presences of M. longicarpus and net uptakes (194.09-449.21 μmol m⁻² day⁻¹) where they were excluded. The net uptake of N₂ was most likely due to cyanobacteria fixing of N₂, as dense microbial mats became established over the sediment surface in the absence of M. longicarpus grazing activity. Sediment irrigation/transport rates calculated from CsCl tracer dilution indicated greater irrigation rates in the exclusions (12.12-16.22 l m⁻² h⁻¹) compared to inhabited sediments (6.33-11.73 l m⁻² h⁻¹) and this was again was most likely due to the lack of grazing pressure which allowed large populations of small burrowing polychaetes to inhabit the organic matter rich exclusion sediments. As such, the main influence of M. longicarpus was the interception and consumption of transported organic material, benthic microalgae and other small infaunal organisms resulting in the removal of approximately 0.06 g m⁻² day⁻¹ of nitrogen and 12.12 g m⁻² day⁻¹ of organic carbon. This “cleansing” of the sediments reduced sediment metabolism and the flux of solutes across the sediment water interface and ultimately the heavy predation of M. longicarpus by transient species such as stingrays, results in a net loss of carbon and nitrogen from the system.     

Nitrogen uptake responses of Gracilaria vermiculophylla (Ohmi) Papenfuss under combined and single addition of nitrate and ammonium

The ammonium (NH₄⁺) and nitrate (NO₃⁻) uptake responses of tetrasporophyte cultures from a Portuguese population of Gracilaria vermiculophylla were studied. Thalli were incubated at 5 nitrogen (N) levels, including single (50 μM of NH₄⁺ or NO₃⁻) and combined addition of each of the N sources. For the combined additions, the experimental conditions attempted to simulate 2 environments with high N availability (450 μM NO₃⁻ + 150 μM NH₄⁺; 250 μM NO₃⁻ + 50 μM NH₄⁺) and the mean N concentrations occurring at the estuarine environment of this population (30 μM NO₃⁻ + 5 μM NH₄⁺). The uptake kinetics of NH₄⁺ and NO₃⁻ were determined during a 4 h time-course experiment with N deprived algae. The experiment was continued up to 48 h, with media exchanges every 4 h. The uptake rates and efficiency of the two N sources were calculated for each time interval. For the first 4 h, G. vermiculophylla exhibited non-saturated uptake for both N sources even for the highest concentrations used. The uptake rates and efficiency calculated for that period (V_0-4 h), respectively, increased and decreased with increasing substrate concentration. NO₃⁻ uptake rates were superior, ranging from 1.06 ± 0.1 to 9.65 ± 1.2 μM g(dw)⁻¹ h⁻¹, with efficiencies of 19% to 53%. NH₄⁺ uptake rates were lower (0.32 ± 0.0 to 5.75 ± 0.08 μM g(dw)⁻¹ h⁻¹) but G. vermiculophylla removed 63% of the initial 150 μM and 100% at all other conditions. Uptake performance of both N sources decreased throughout the duration of the experiment and with N tissue accumulation. Both N sources were taken up during dark periods though with better results for NH₄⁺. Gracilaria vermiculophylla was unable to take up NO₃⁻ at the highest concentration but compensated with a constant 27% NH₄⁺ uptake through light and dark periods. N tissue accumulation was maximal at the highest N concentration (3.9 ± 0.25% dw) and superior under NH₄⁺ (3.57 ± 0.2% dw) vs NO3 (3.06 ± 0.1% dw) enrichment. The successful proliferation of G. vermiculophylla in estuarine environments and its potential utilization as the biofilter component of Integrated Multi-Trophic Aquaculture (IMTA) are discussed.     

Nitrogen nutrition of Canna indica: Effects of ammonium versus nitrate on growth, biomass allocation, photosynthesis, nitrate reductase activity and N uptake rates

The effects of inorganic nitrogen (N) source (NH₄⁺, NO₃⁻ or both) on growth, biomass allocation, photosynthesis, N uptake rate, nitrate reductase activity and mineral composition of Canna indica were studied in hydroponic culture. The relative growth rates (0.05-0.06 g g⁻¹ d⁻¹), biomass allocation and plant morphology of C. indica were indifferent to N nutrition. However, NH₄⁺ fed plants had higher concentrations of N in the tissues, lower concentrations of mineral cations and higher contents of chlorophylls in the leaves compared to NO₃⁻ fed plants suggesting a slight advantage of NH₄⁺ nutrition. The NO₃⁻ fed plants had lower light-saturated rates of photosynthesis (22.5 μmol m⁻² s⁻¹) than NH₄⁺ and NH₄⁺/NO₃⁻ fed plants (24.4-25.6 μmol m⁻² s⁻¹) when expressed per unit leaf area, but similar rates when expressed on a chlorophyll basis. Maximum uptake rates (V_max) of NO₃⁻ did not differ between treatments (24-35 μmol N g⁻¹ root DW h⁻¹), but V_max for NH₄⁺ was highest in NH₄⁺ fed plants (81 μmol N g⁻¹ root DW h⁻¹), intermediate in the NH₄NO₃ fed plants (52 μmol N g⁻¹ root DW h⁻¹), and lowest in the NO₃⁻ fed plants (28 μmol N g⁻¹ root DW h⁻¹). Nitrate reductase activity (NRA) was highest in leaves and was induced by NO₃⁻ in the culture solutions corresponding to the pattern seen in fast growing terrestrial species. Plants fed with only NO₃⁻ had high NRA (22 and 8 μmol NO₂⁻ g⁻¹ DW h⁻¹ in leaves and roots, respectively) whereas NRA in NH₄⁺ fed plants was close to zero. Plants supplied with both forms of N had intermediate NRA suggesting that C. indica takes up and assimilate NO₃⁻ in the presence of NH₄⁺. Our results show that C. indica is relatively indifferent to inorganic N source, which together with its high growth rate contributes to explain the occurrence of this species in flooded wetland soils as well as on terrestrial soils. Furthermore, it is concluded that C. indica is suitable for use in different types of constructed wetlands.     

Comparison of stress susceptibility of in hospite and isolated zooxanthellae among five coral species

Coral species in a similar habitat often show different bleaching susceptibilities. It is not understood which partner of coral-zooxanthellae complexes is responsible for differential stress susceptibility. Stress susceptibilities of in hospite and isolated zooxanthellae from five species of corals collected from shallow water in Okinawa were compared. To estimate stress susceptibility, we measured the maximum quantum yields (F_v/F_m) of in hospite and isolated zooxanthellae after 3-h exposure to either 28 or 34 °C at various light intensities and their recovery after 12 h under dim light at 26 °C. Significant reduction in photochemical efficiency (F_v/F_m) of photosystem II (PSII) was observed in in hospite zooxanthellae exposed to high light intensity (1000 μmol quanta m⁻² s⁻¹), while PSII activity of isolated zooxanthellae decreased significantly even at a lower light intensity (70 μmol quanta m⁻² s⁻¹). The recovery of the PSII activity after 12 h was incomplete in both in hospite and isolated zooxanthellae, indicating the presence of chronic photoinhibition. The stress susceptibility of isolated zooxanthellae was more variable among species than in hospite zooxanthellae. The order of stress susceptibility among the five coral species was different between in hospite and isolated zooxanthellae. The present results suggest that the host plays a significant role in determining bleaching susceptibility of corals, though zooxanthellae from different host have different stress susceptibilities.     

Kinetics and bioreactor studies of immobilized invertase on polyurethane rigid adhesive foam

A new support, polyurethane rigid adhesive foam (PRAF), which can be used to cover internal surface of metallic tubes, was used to immobilize invertase for application in an enzymatic bioreactor. The kinetic parameters were: K_m - 46.5 ± 1.9 mM (PRAF-invertase) and 61.2 ± 0.1 mM (free enzyme) and V_max 42.0 ± 4.3 U/mg protein/min (PRAF-invertase) and 445.3 ± 24.0 U/mg protein/min (free invertase). The PRAF-invertase derivative maintained 50.1% of initial activity (69.17 U/g support) for 8 months (4 °C) and was not observed microbial contamination. The bioreactor showed the best production of inverted sugar syrup using up-flow rate (0.48 L/h) with average conversion of 10.64 ± 1.5% h⁻¹ at feeding rate (D) of 104 h⁻¹. The operational inactivation rate constant (k_opi) and half-life were 1.92 × 10⁻⁴ min⁻¹ and 60 h (continue use). The PRAF spray support looks promising as a new alternative to produce immobilized derivatives on reactor surfaces.