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.     

Critical swimming speeds of wild‐caught sand‐smelt Atherina presbyter larvae

Swimming abilities of wild‐caught sand‐smelt Atherina presbyter larvae were assessed as critical swimming speed (U_crit) throughout ontogeny. The mean U_crit increased with size, ranging from 3·6 to 18·7 cm s^?1, over the size range of 6·6–21·0 mm L_T. This indicates that at hatching A. presbyter larvae, far from being passive floaters, are already capable of active behaviours, which may influence their dispersal patterns.     

Consequences of electroshock‐induced narcosis in fish muscle: from mitochondria to swim performance

Adult zebrafish Danio rerio were exposed to an electric shock of 3 V and 1A for 5 s delivered by field backpack electrofishing gear, to induce a taxis followed by a narcosis. The effect of such electric shock was investigated on both the individual performances (swimming capacities and costs of transport) and at cellular and mitochondrial levels (oxygen consumption and oxidative balance). The observed survival rate was very high (96·8%) independent of swimming speed (up to 10 body length s^?1). The results showed no effect of the treatment on the metabolism and cost of transport of the fish. Nor did the electroshock trigger any changes on muscular oxidative balance and bioenergetics even if red muscle fibres were more oxidative than white muscle. Phosphorylating respiration rates rose between (mean 1 s.e. ) 11·16 ± 1·36 pmol O₂ s^?1 mg^?1 and 15·63 ± 1·60 pmol O₂ s^?1 mg^?1 for red muscle fibres whereas phosphorylating respiration rates only reached 8·73 ± 1·27 pmol O₂ s^?1 mg^?1 in white muscle. Such an absence of detectable physiological consequences after electro‐induced narcosis both at organismal and cellular scales indicate that this capture method has no apparent negative post‐shock performance under the conditions of this study.     

SHORT‐TERM EFFECT OF TEMPERATURE ON THE PHOTOKINETICS OF MICROALGAE FROM THE SURFACE LAYERS OF ANTARCTIC PACK ICE1

Microalgae growing within brine channels (85 psu salinity) of the surface ice layers of Antarctic pack ice showed considerable photosynthetic tolerance to the extreme environmental condition. Brine microalgae exposed to temperatures above ?5°C and at irradiances up to 350 μmol photons·m^?2·s^?1 showed no photosynthetic damage or limitations. Photosynthesis was limited (but not photoinhibited) when brine microalgae were exposed to ?10°C, provided the irradiance remained under 50 μmol photons·m^?2·s^?1. The highest level of photosynthetic activity (maximum relative electron transport rate [rETR_max]) in brine microalgae growing within the surface layer of sea ice was at approximately 18 μmol electrons·m^?2·s^?1, which occurred at ?1.8°C. Effective quantum yield of PSII and rETR_max of the halotolerant brine microalgae exhibited a temperature‐dependent pattern, where both parameters were higher at ?1.8°C and lower at ?10°C. Relative ETR_max at temperatures above ?5°C were stable across a wide range of irradiance.     

Meiospores produced in sori of nonsporophyllous laminae of Macrocystis pyrifera (Laminariales,Phaeophyceae) may enhance reproductive output

Different lamina of Macrocystis pyrifera sporophytes (i.e., sporophylls, pneumatocyst‐bearing blades, and apical scimitars) in a wave‐sheltered site were found to be fertile. We quantified their sorus surface area, reproductive output (number of spores released) and the viability of released spores (germination rate). Sorus area was greatest on the sporophylls, with sporangia developing on >57% of the total area and smallest on the pneumatocyst‐bearing blades with 21% of the total area bearing sporangia. The apical scimitar released the greatest number of meiospores (cells · mL^?1 · cm^?2) and the sporophylls the least. Meiospores produced from all types of fertile laminae were equally viable. This reproductive plasticity may enhance reproductive output, and contribute to short and long‐distance spore dispersal and the cryptic gametophyte propagule bank for the next generation of sporophytes.     

Ontogeny of critical and prolonged swimming performance for the larvae of six Australian freshwater fish species

Critical (<30 min) and prolonged (>60 min) swimming speeds in laboratory chambers were determined for larvae of six species of Australian freshwater fishes: trout cod Maccullochella macquariensis, Murray cod Maccullochella peelii, golden perch Macquaria ambigua, silver perch Bidyanus bidyanus, carp gudgeon Hypseleotris spp. and Murray River rainbowfish Melanotaenia fluviatilis. Developmental stage (preflexion, flexion, postflexion and metalarva) better explained swimming ability than did length, size or age (days after hatch). Critical speed increased with larval development, and metalarvae were the fastest swimmers for all species. Maccullochella macquariensis larvae had the highest critical [maximum absolute 46·4 cm s^?1 and 44·6 relative body lengths (L_B) s^?1] and prolonged (maximum 15·4 cm s^?1, 15·6 L_B s^?1) swimming speeds and B. bidyanus larvae the lowest critical (minimum 0·1 cm s^?1, 0·3 L_B s^?1) and prolonged swimming speeds (minimum 1·1 cm s^?1, 1·0 L_B s^?1). Prolonged swimming trials determined that the larvae of some species could not swim for 60 min at any speed, whereas the larvae of the best swimming species, M. macquariensis, could swim for 60 min at 44% of the critical speed. The swimming performance of species with precocial life‐history strategies, with well‐developed larvae at hatch, was comparatively better and potentially had greater ability to influence their dispersal by actively swimming than species with altricial life‐history strategies, with poorly developed larvae at hatch.     

The influence of light on copper‐limited growth of an oceanic diatom,Thalassiosira oceanica (Coscinodiscophyceae)

Thalassiosira oceanica (CCMP 1005) was grown over a range of copper concentrations at saturating and subsaturating irradiance to test the hypothesis that Cu and light were interacting essential resources. Growth was a hyperbolic function of irradiance in Cu‐replete medium (263 fmol Cu′ · L^?1) with maximum rates achieved at 200 μmol photons · m^?2 · s^?1. Lowering the Cu concentration at this irradiance to 30.8 fmol Cu′ · L^?1 decreased cellular Cu quota by 7‐fold and reduced growth rate by 50%. Copper‐deficient cells had significantly slower (P < 0.0001) rates of maximum, relative photosynthetic electron transport (rETR_max) than Cu‐sufficient cells, consistent with the role of Cu in photosynthesis in this diatom. In low‐Cu medium (30.8 fmol Cu′ · L^?1), growth rate was best described as a positive, linear function of irradiance and reached the maximum value measured in Cu‐replete cells when irradiance increased to 400 μmol photons · m^?2 · s^?1. Thus, at high light, low‐Cu concentration was no longer limiting to growth: Cu concentration and light interacted strongly to affect growth rate of T. oceanica (P < 0.0001). Relative ETR_max and Cu quota of cells grown at low Cu also increased at 400 μmol photons · m^?2 · s^?1 to levels measured in Cu‐replete cells. Steady‐state uptake rates of Cu‐deficient and sufficient cells were light‐dependent, suggesting that faster growth of T. oceanica under high light and low Cu was a result of light‐stimulated Cu uptake.     

Relationships between metabolic rate, muscle electromyograms and swim performance of adult chinook salmon

Oxygen consumption rates of adult spring chinook salmon Oncorhynchus tshawytscha increased with swim speed and, depending on temperature and fish mass, ranged from 609 mg O₂ h^?1 at 30 cm s^?1 (c. 0·5 BL s^?1) to 3347 mg O₂ h^?1 at 170 cm s^?1 (c. 2·3 BL s^?1). Corrected for fish mass, these values ranged from 122 to 670 mg O₂ kg^?1 h^?1, and were similar to other Oncorhynchus species. At all temperatures (8, 12·5 and 17° C), maximum oxygen consumption values levelled off and slightly declined with increasing swim speed >170 cm s^?1, and a third‐order polynomial regression model fitted the data best. The upper critical swim speed (U_crit) of fish tested at two laboratories averaged 155 cm s^?1 (2·1 BL s^?1), but U_crit of fish tested at the Pacific Northwest National Laboratory were significantly higher (mean 165 cm s^?1) than those from fish tested at the Columbia River Research Laboratory (mean 140 cm s^?1). Swim trials using fish that had electromyogram (EMG) transmitters implanted in them suggested that at a swim speed of c. 135 cm s^?1, red muscle EMG pulse rates slowed and white muscle EMG pulse rates increased. Although there was significant variation between individual fish, this swim speed was c. 80% of the U_crit for the fish used in the EMG trials (mean U_crit 168·2 cm s^?1). Bioenergetic modelling of the upstream migration of adult chinook salmon should consider incorporating an anaerobic fraction of the energy budget when swim speeds are ≥80% of the U_crit.     

Di‐2‐ethylhexyl phthalate (DEHP) exposure disturbs lipid metabolism in juvenile yellow catfish Tachysurus fulvidraco

This study was conducted to determine the mechanism by which di‐2‐ethylhexyl phthalate (DEHP) exposure influences lipid metabolism of juvenile yellow catfish Tachysurus fulvidraco. Fish were exposed to three DEHP concentrations (0, 0·1 and 0·5 mg l^?1 DEHP) for 8 weeks. Fatty acid synthase (FAS) activity significantly decreased with increasing DEHP concentrations, the highest value was in the Tween control group, whereas the lowest activities of carnitine palmitoyltransferase (CPT) and lipoprotein lipase (LPL) were in this group. The messenger (m)RNA levels of 6‐phospho‐gluconate dehydrogenase (6PGD), FAS and acetyl‐CoA carboxylase a (ACCa) significantly increased with increasing DEHP concentration, the highest values were in the 0·5 mg l^?1 DEHP group. The mRNA level of peroxisome proliferator‐activated receptor γ (PPARγ) was lower in Tween control than in fish exposed to 0·1 and 0·5 mg l^?1 DEHP. The highest mRNA level of ACCb was in the 0·1 mg l^?1 DEHP group. These results indicate that DEHP exposure can disturb lipid metabolism at the enzymatic and mRNA levels in Pelteobagrus fulvidraco.     

VARIATIONS IN GROWTH,EROSION, PRODUCTIVITY,AND MORPHOLOGY OF ECKLONIA RADIATA (ALARIACEAE; LAMINARIALES) ALONG A FJORD IN SOUTHERN NEW ZEALAND1

Spatial and temporal patterns of growth, erosion, productivity, and morphology of the dominant habitat‐forming kelp Ecklonia radiata (C. Agardh) J. Agardh were studied bimonthly over 1.5 years in a southern New Zealand fjord characterized by strong gradients in light and wave exposure. Spatial differences in growth were observed with rates at two outer coast, high‐light, wave‐exposed sites reaching 0.42 and 0.45 cm · d^?1, respectively, compared to 0.27 cm · d^?1 at an inner, more homogeneous site. Sporophyte productivity was similar among sites, although population productivity was greater at the outer sites due to population density being 5‐fold greater than at the inner site. It was expected that the inner site would have no pronounced seasonal pattern in growth and productivity due to its homogeneity; however, all three sites displayed maximum rates in late winter/spring and minimal in autumn. Growth rates were 2‐fold greater during the first growth period than the following year. This discrepancy was not correlated to inorganic nitrogen (N) levels, which remained low year‐round (<4 μM), and is likely a result of an interaction between light and temperature, and the photosynthetic capability of E. radiata. Variable pigment content indicated photoacclimation at the inner site. Morphological differences were observed between sites, with E. radiata from the inner site having longer, wider, thinner blades and longer stipes. While E. radiata displayed spatial differences in growth, erosion, productivity, and morphology, populations displayed no temporal differences. These results highlight the need for greater understanding of the mechanisms influencing kelp growth and productivity in a unique marine environment.     

Effects of surgically implanted acoustic transmitters >2% of body mass on the swimming performance, survival and growth of juvenile sockeye and Chinook salmon

The influence of surgical implantation of an acoustic transmitter on the swimming performance, growth and survival of juvenile sockeye salmon Oncorhynchus nerka and Chinook salmon Oncorhynchus tshawytscha was examined. The transmitter had a mass of 0·7 g in air while sockeye salmon had a mass of 7·0–16·0 g and Chinook salmon had a mass of 6·7–23·1 g (a transmitter burden of 4·5–10·3% for sockeye salmon and 3·1–10·7% for Chinook salmon). Mean critical swimming speeds (U_crit) for Chinook salmon ranged from 47·5 to 51·2 cm s^?1 [4·34–4·69 body lengths (fork length, L_F) s^?1] and did not differ among tagged, untagged and sham‐tagged groups. Tagged sockeye salmon, however, did have lower U_crit than control or sham fish. The mean U_crit for tagged sockeye salmon was 46·1 cm s^?1 (4·1 L_F s^?1), which was c. 5% less than the mean U_crit for control and sham fish (both groups were 48·6 cm s^?1 or 4·3 L_F s^?1). A laboratory evaluation determined that there was no difference in L_F or mass among treatments (control, sham or tag) either at the start or at the end of the test period, suggesting that implantation did not negatively influence the growth of either species. None of the sockeye salmon held under laboratory conditions died from the influence of surgical implantation of transmitters. In contrast, this study found that the 21 day survival differed between tagged and control groups of Chinook salmon, although this result may have been confounded by the poor health of Chinook salmon treatment groups.     

THE INFLUENCE OF FLUCTUATING LIGHT ON DIVERSITY AND SPECIES NUMBER OF NUTRIENT‐LIMITED PHYTOPLANKTON1

The influence of fluctuating light on diversity and species number of a natural phytoplankton assemblage competing for nutrients was investigated for 48 days under semicontinuous culture conditions. Light conditions were either changed periodically from high (65 μmol photons·m^?2·s^?1) to low intensity (15 μmol photons·m^?2·s^?1) at intervals of 1, 3, 6, and 12 days or fixed at constant light conditions of intermediate intensity (40 μmol photons·m^?2·s^?1). Fluctuating light at intervals of 1–12 days significantly affected phytoplankton diversity. The development of phytoplankton communities differed in treatments with different light regimes. In treatments with long light intervals, species abundance oscillated with the light phases. Differences in the temporal development of phytoplankton communities resulted in hump‐shaped relations between the interval length of the light phases and both species number and diversity index and can be explained by the intermediate disturbance hypothesis. Fluctuating light tends to sustain phytoplankton diversity under nutrient limitation if the light regime changes in the order of several days. This indicates that temporal changes in weather regime are important in preventing competitive exclusion of phytoplankton species in nature.     

Validated annual band‐pair periodicity and growth parameters of blue‐spotted maskray Neotrygon kuhlii from south‐east Queensland,Australia

Age and growth parameters were derived for blue‐spotted maskray Neotrygon kuhlii from Moreton Bay in subtropical eastern Australia. Maximum age estimates of 13 and 10 years were obtained from female (n = 76) and male (n = 44) N. kuhlii, respectively. Estimated ages at maturity for 50% of females and males were 6·32 and 3·95 years, respectively. A three‐parameter power function provided the best statistical fit to size at age data in both sexes, providing parameter estimates of y⁰ = 163·13, a = 58·52 and b = 0·58 for females and y⁰ = 165·13, a = 59·02 and b = 0·54 in males. The two‐parameter von Bertalanffy growth function was used to estimate biological parameters based on disc width (W_D) for both female (W_D∞ = 465·81 mm, K = 0·13 year^?1, b = 0·63) and male N. kuhlii (W_D∞ = 385·19 mm, K = 0·20 year^?1, b = 0·54). Annual band‐pair deposition was observed in three calcein‐injected N. kuhlii after periods of liberty ranging from 631 to 1081 days. Centrum edge analysis indicated that annual band‐pair formation was generally consistent within this population, with translucent bands formed over spring and summer and opaque bands formed in autumn and winter. Individual growth rates obtained from tagged specimens were similar to power function growth predictions. These results support previous characterizations of this common trawl by‐catch species as comparatively resilient to non‐targeted catches, although higher catch rates outside Australia infer a need for cautious management.     

Pelagic to demersal transition in a coral‐reef fish,the orbicular batfish Platax orbicularis

Behavioural and ecological observations were made on young, reared Platax orbicularis in Opunohu Bay, Moorea, French Polynesia, during their transition from the pelagic, dispersive stage to the reef‐orientated demersal stage. Seventy‐two young P. orbicularis (17–75 mm standard length, L_S) were released in the pelagic zone and 20 (40–70 mm L_S) adjacent to the reefs. Swimming speed was slow (mean 5·2 cm s^?1) and independent of size. An ontogenetic descent was observed: the smallest P. orbicularis swam at the surface, medium‐sized P. orbicularis swam in midwater (mean 5–13 m) and the largest P. orbicularis swam to the bottom, where many lay on their sides. Platax orbicularis swam southerly on average, away from the ocean and into the bay. Smaller P. orbicularis were more likely to swim directionally than larger individuals. Young P. orbicularis released near reef edges swam at similar, but more variable speeds (mean 6·6 cm s^?1). About half of those released near reefs swam away, but fewer swam away from an inshore fringing reef than from a patch reef near the bay mouth. Many P. orbicularis swam up the slope onto the reef top, but the little settlement observed was near the reef base. Average, near‐reef swimming direction was also southerly. Some reef residents, in particular the triggerfish Balistapus undulatus, harassed young P. orbicularis.     

How does coarse gravel augmentation affect early‐stage Chinook salmon Oncorhynchus tshawytscha embryonic survivorship?

Early‐stage Chinook salmon Oncorhynchus tshawytscha embryos were incubated in artificial redds that mimicked hyporheic conditions in gravel‐augmented habitat to assess survivorship. Two complementary experiments were conducted where units varied along gradients of (1) increasing interstitial flow velocity (0·05–2·50 cm s^?1) in a uniformly coarse (particles ≥22 mm) sediment mixture and (2) increasing sediment porosity with interstitial flow velocity held constant. Embryonic survivorship increased moderately along a gradient of interstitial flow velocity, while survivorship among units with varying sediment porosities was consistent. No evidence for flow‐induced agitation and mortality was observed. Results suggest that high interstitial flow velocities may confer a moderate advantage for incubating salmonid embryos when conditions that typically reduce embryonic mortality (i.e. low concentrations of fine particles) are ideal.     

Temperature response of photosynthetic light‐ and carbon‐use characteristics in the red seaweed Gracilariopsis lemaneiformis (Gracilariales,Rhodophyta)

The red seaweed Gracilariopsis is an important crop extensively cultivated in China for high‐quality raw agar. In the cultivation site at Nanao Island, Shantou, China, G. lemaneiformis experiences high variability in environmental conditions like seawater temperature. In this study, G. lemaneiformis was cultured at 12, 19, or 26°C for 3 weeks, to examine its photosynthetic acclimation to changing temperature. Growth rates were highest in G. lemaneiformis thalli grown at 19°C, and were reduced with either decreased or increased temperature. The irradiance‐saturated rate of photosynthesis (P_max) decreased with decreasing temperature, but increased significantly with prolonged cultivation at lower temperatures, indicating the potential for photosynthesis acclimation to lower temperature. Moreover, P_max increased with increasing temperature (~30 μmol O₂ · g^?1FW · h^?1 at 12°C to 70 μmol O₂ · g^?1FW · h^?1 at 26°C). The irradiance compensation point for photosynthesis (I_c) decreased significantly with increasing temperature (28 μmol photons · m^?2 · s^?1 at high temperature vs. 38 μmol photons · m^?2 · s^?1 at low temperature). Both the photosynthetic light‐ and carbon‐use efficiencies increased with increasing growth or temperatures (from 12°C to 26°C). The results suggested that the thermal acclimation of photosynthetic performance of G. lemaneiformis would have important ecophysiological implications in sea cultivation for improving photosynthesis at low temperature and maintaining high standing biomass during summer. Ongoing climate change (increasing atmospheric CO₂ and global warming) may enhance biomass production in G. lemaneiformis mariculture through the improved photosynthetic performances in response to increasing temperature.     

Temperature‐dependent phagotrophy and phototrophy in a mixotrophic chrysophyte

The roles of temperature and light on grazing and photosynthesis were examined for Dinobryon sociale, a common freshwater mixotrophic alga. Photosynthetic rate was determined for D. sociale adapted to temperatures of 8, 12, 16, and 20°C under photosynthetically active radiation light irradiances of 25, 66, and 130 μmol photons · m^?2 · s^?1, with concurrent measurement of bacterial ingestion at all temperatures under medium and high light (66 and 130 μmol photons · m^?2 · s^?1). Rates of ingestion and photosynthesis increased with temperature to a maximum at 16°C under the two higher light regimes, and declined at 20°C. Although both light and temperature had a marked effect on photosynthesis, there was no significant difference in bacterivory at medium and high irradiances at any given temperature. At the lowest light condition (25 μmol photons · m^?2 · s^?1), photosynthesis remained low and relatively stable at all temperatures. D. sociale acquired the majority of carbon from photosynthesis, although the low photosynthetic rate without a concurrent decline in feeding rate at 8°C suggested 20%–30% of the carbon budget could be attributed to bacterivory at low temperatures. Grazing experiments in nutrient‐modified media revealed that this mixotroph had increased ingestion rates when either dissolved nitrogen or phosphorus was decreased. This work increases our understanding of environmental effects on mixotrophic nutrition. Although the influence of abiotic factors on phagotrophy and phototrophy in pure heterotrophs and phototrophs has been well studied, much less is known for mixotrophic organisms.     

GROWTH AND CARBON CONTENT OF THREE DIFFERENT‐SIZED DIAZOTROPHIC CYANOBACTERIA OBSERVED IN THE SUBTROPICAL NORTH PACIFIC1

To develop tools for modeling diazotrophic growth in the open ocean, we determined the maximum growth rate and carbon content for three diazotrophic cyanobacteria commonly observed at Station ALOHA (A Long‐term Oligotrophic Habitat Assessment) in the subtropical North Pacific: filamentous nonheterocyst‐forming Trichodesmium and unicellular Groups A and B. Growth‐irradiance responses of Trichodesmium erythraeum Ehrenb. strain IMS101 and Crocosphaera watsonii J. Waterbury strain WH8501 were measured in the laboratory. No significant differences were detected between their fitted parameters (±CI) for maximum growth rate (0.51 ± 0.09 vs. 0.49 ± 0.17 d^?1), half‐light saturation (73 ± 29 vs. 66 ± 37 μmol quanta · m^?2 · s^?1), and photoinhibition (0 and 0.00043 ± 0.00087 [μmol quanta · m^?2 · s^?1]^?1). Maximum growth rates and carbon contents of Trichodesmium and Crocosphaera cultures conformed to published allometric relationships, demonstrating that these relationships apply to oceanic diazotrophic microorganisms. This agreement promoted the use of allometric models to approximate unknown parameters of maximum growth rate (0.77 d^?1) and carbon content (480 fg C · μm^?3) for the uncultivated, unicellular Group A cyanobacteria. The size of Group A was characterized from samples from the North Pacific Ocean using fluorescence‐activated cell sorting and real‐time quantitative PCR techniques. Knowledge of growth and carbon content properties of these organisms facilitates the incorporation of different types of cyanobacteria in modeling efforts aimed at assessing the relative importance of filamentous and unicellular diazotrophs to carbon and nitrogen cycling in the open ocean.     

Nitrogen Utilization and Toxin Production by Two Diatoms of the Pseudo‐nitzschia pseudodelicatissima Complex: P. cuspidata and P. fryxelliana

The toxigenic diatom Pseudo‐nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m^?2 · s^?1) and subsaturating (40 μmol photons · m^?2 · s^?1) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO₃^?), ammonium (NH₄⁺), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO₃^?‐supported cells being significantly more toxic than both NH₄⁺‐ and urea‐supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO₃^? they exhibited maximum specific uptake rates (V_max) of 22.7, 29.9, 8.98 × 10^?3 · h^?1, half‐saturation constants (K_s) of 1.34, 2.14, 0.28 μg‐at N · L^?1, and affinity values (α) of 17.0, 14.7, 32.5 × 10^?3 · h^?1/(μg‐at N · L^?1) for NO₃^?, NH₄⁺ and urea, respectively. These labo‐ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo‐cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.     

Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Schreb.)

The water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s^?1 m^?2), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2·4 mmol s^?1 m^?2. Measured axial hydraulic conductivity, K_h (mmol s^?1 mm MPa^?1), of excised leaf segments was three times lower than theoretical hydraulic conductivity (K_t) calculated using the Poiseuille equation and measurements of vessel number and diameter. K_t was corrected (K_t*) to account for the discrepancy between K_h and K_t and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of K_t* was bell‐shaped with a maximum near the sheath–blade joint (≈ 19 mmol s^?1 mm MPa^?1). In elongating leaves, immature xylem in the basal growing region led to a much lower K_t*. As the first metaxylem matured, K_t* increased by 10‐fold. The hydraulic conductances of the whole root system, (mmol s^?1 MPa^?1) and leaf blades, (mmol s^?1 MPa^?1) were measured by a vacuum induced water flow technique. and were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric‐analogue computer model was used to calculate the leaf blade area‐specific radial hydraulic conductivity, (mmol s^?1 m^?2 MPa^?1), using , K_t* and water flux values. values decreased with leaf age, from 21·2 mmol s^?1 m^?2 MPa^?1 in rapidly elongating leaf to 7·2 mmol s^?1 m^?2 MPa^?1 in mature leaf. Comparison of and values showed that ≈ 90% of the resistance to water flow within the blades resided in the liquid extra‐vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (≈ 0·3 MPa m^?1) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low K_t* in the basal growth zone dramatically increased the local xylem water potential gradient (≈ 2·0 MPa m^?1) there. In the leaf elongation zone the growth‐induced water potential difference was ≈ 0·2 MPa.