Long-Term Temperature Acclimation of Photosynthesis in Steady-State Cultures of the Polar Diatom <Emphasis Type="Italic">Fragilariopsis cylindrus</Emphasis>

Cultures of the obligate psychrophilic diatom Fragilariopsis cylindrus (Grunow) were grown for 4 months under steady-state conditions at −1 °C and +7 °C (50 μmol photons m⁻² s⁻¹) prior to measurements in order to investigate long-term acclimation of photosynthesis to both temperatures. No differences in maximum intrinsic quantum yield of PS II (F_V/F_M) and relative electron transport rates could be detected at either temperature after 4 months of acclimation. Measurements of photosynthesis (relative electron transport rates) vs. irradiance (P vs. E curves) revealed similar values for relative light utilization efficiency (α = 0.57 at −1 °C, α = 0.60 at +7 °C) but higher values for irradiance levels at which photosynthesis saturates (E_K) at −1 °C and, therefore, higher maximum photosynthesis (P_MAX = 54 (relative units) at −1 °C, P_MAX = 49 at +7 °C). Nonphotochemical quenching (NPQ) measurements at 385 μmol photons m⁻² s⁻¹ indicated higher (37%) NPQ for diatoms grown at −1 °C compared to +7 °C, which was possibly related to a 2-fold increase in the concentration of the pigment diatoxanthin and a 9-fold up-regulation of a gene encoding a fucoxanthin chlorophyll a,c-binding protein. Expression of the D1 protein encoding gene psbA was ca. 1.5-fold up-regulated at −1 °C, whereas expression levels of other genes from Photosystem II (psbC, psbU, psbO), as well as rbcL, the gene encoding the Rubisco large subunit were similar at both temperatures. However, a 2-fold up-regulation of a plastid glyceraldehyde-P dehydrogenase at −1 °C indicated enhanced Calvin cycle activity. This study revealed for the first time that a polar diatom could efficiently acclimate photosynthesis over a wide range of polar temperatures given enough time. Acclimation of photosynthesis at −1 °C was probably regulated similarly to high light acclimation.     

Nitrogen utilization by the raphidophyte Heterosigma akashiwo: Growth and uptake kinetics in laboratory cultures

The nitrogen uptake and growth capabilities of the potentially harmful, raphidophycean flagellate Heterosigma akashiwo (Hada) Sournia were examined in unialgal batch cultures (strain CCMP 1912). Growth rates as a function of three nitrogen substrates (ammonium, nitrate and urea) were determined at saturating and sub-saturating photosynthetic photon flux densities (PPFDs). At saturating PPFD (110 μE m⁻² s⁻¹), the growth rate of H. akashiwo was slightly greater for cells grown on NH₄⁺ (0.89 d⁻¹) compared to cells grown on NO₃⁻ or urea, which had identical growth rates (0.82 d⁻¹). At sub-saturating PPFD (40 μE m⁻² s⁻¹), both urea- and NH₄⁺-grown cells grew faster than NO₃⁻-grown cells (0.61, 0.57 and 0.46 d⁻¹, respectively). The N uptake kinetic parameters were investigated using exponentially growing batch cultures of H. akashiwo and the ¹⁵N-tracer technique. Maximum specific uptake rates (V_max) for unialgal cultures grown at 15 °C and saturating PPFD (110 μE m⁻² s⁻¹) were 28.0, 18.0 and 2.89 × 10⁻³ h⁻¹ for NH₄⁺, NO₃⁻ and urea, respectively. The traditional measure of nutrient affinity—the half saturation constants (K_s) were similar for NH₄⁺ and NO₃⁻ (1.44 and 1.47 μg-at N L⁻¹), but substantially lower for urea (0.42 μg-at N L⁻¹). Whereas the α parameter (α = V_max/K_s), which is considered a more robust indicator for substrate affinity when substrate concentrations are low (<K_s), were 19.4, 12.2 and 6.88 × 10⁻³ h⁻¹/(μg-at N L⁻¹) for NH₄⁺, NO₃⁻ and urea, respectively. These laboratory results demonstrate that at both saturating and sub-saturating N concentrations, N uptake preference follows the order: NH₄⁺ > NO₃⁻ > urea, and suggests that natural blooms of H. akashiwo may be initiated or maintained by any of the three nitrogen substrates examined.     

A mesocosm study of Phaeocystis globosa population dynamics: I. Regulatory role of viruses in bloom control

The regulatory role of viruses on population dynamics of the prymnesiophyte Phaeocystis globosa was studied during a mesocosm experiment in relation to growth and loss by microzooplankton grazing and cell lysis. The mesocosms were conducted under varying light conditions (20 and 150 μmol photons m⁻² s⁻¹) and nutrient regime (inorganic nitrogen to phosphorus ratios of 4, 16 and 44). Overall, viruses infecting P. globosa (PgV) were found to be an important cause of cell lysis (30–100% of total lysis) and a significant loss factor (7–67% of total loss). We demonstrate that the morphology of P. globosa cells (solitary versus colonial) differently regulated viral control of P. globosa bloom formation. Reduced irradiance (20 μmol photons m⁻² s⁻¹) was provided for 11 days to select for the solitary cell morphotype. Viruses were able to restrict P. globosa bloom formation even after irradiance became saturating again (150 μmol photons m⁻² s⁻¹). Saturating light conditions from the start of the experiment allowed colony formation and because the colony-morphotype acted as a mechanism reducing viral infection bloom formation succeeded. Nutrient depletion, however, affected specifically the colonies that disintegrated while releasing single cells. Virus infection of these solitary cells resulted in the termination of the bloom. The nature of phytoplankton growth-limiting nutrient (nitrate and/or orthophosphate) did not seem to noticeably affect the level of viral control.     

Effects of temperature and ammonium on growth,pigment production and nitrogen uptake by four species of <Emphasis Type="BoldItalic">Porphyra</Emphasis> (Bangiales,Rhodophyta) native to the New England coast

Porphyra is one of the world’s most valued maricultured seaweeds and has been cultivated for several hundred years in Asia. The objective of this study was to produce critical information as a guide for the selection of an appropriate Porphyra species from coastal New England for the development of a land-based aquaculture system. Four Northwest Atlantic Porphyra species: P. leucosticta, P. amplissima, P. linearis and P. umbilicalis, were cultivated for 1 and 2 weeks at saturated light intensities (100–150 μmol photons m⁻²s⁻¹) and six combinations of ammonium (25 and 250 μmoles L⁻¹) and temperature (10, 15 and 20°C). Specific growth rate (SGR) increased with decreasing temperature in P. leucosticta, P. linearis and P. umbilicalis and increased with increasing temperature in P. amplissima. The SGR of all species was greater at the higher ammonium concentration. Porphyra linearis had the highest SGR, increasing in biomass by approximately 16% day⁻¹. Phycoerythrin (PE) content was higher at 10°C and 250 μmoles L⁻¹ in all species except P. amplissima. The PE content, measured as fresh weight (FW), of P. linearis (29 mg g⁻¹ FW⁻¹) and P. umbilicalis (26 mg g⁻¹ FW⁻¹) was significantly higher than the other two species. Tissue nitrogen content of all species measured in dry weight was on average 1.45% higher at 250 μmoles L⁻¹ than at 25 μmoles L⁻¹ ammonium concentration. Porphyra umbilicalis had the highest tissue nitrogen contents (6.76%) at 10°C and 250 μmoles L⁻¹ ammonium. Based on these results, P. linearis and P. umbilicalis should be considered as potential candidates for bioremediation with finfish and shellfish mariculture.     

Growth, uptake, and assimilation of ammonium, nitrate, and urea, by three strains of Karenia brevis grown under low light

Observations of near-bottom populations of Karenia brevis suggest that these cells may derive nutrients from the sediment–water interface. Cells undergoing a metabolic-mediated migration may be in close proximity to enhanced concentrations of nutrients associated with the sediment during at least a fraction of their diel cycle. In this study, the growth, uptake and assimilation rates of ammonium, nitrate, and urea by K. brevis were examined on a diel basis to better understand the potential role of these nutrients in the near-bottom ecology of this species. Three strains of K. brevis, C6, C3, and CCMP 2229, were grown under 12:12 light dark cycle under 30 μmol photons m⁻² s⁻¹ delivered to the surface plain of batch cultures. Nitrogen uptake was evaluated using ¹⁵N tracer techniques and trichloroacetic acid extraction was used to evaluate the quantity of nitrogen (N) assimilated into cell protein. Growth rates ranged from a low of 0.12 divisions day⁻¹ for C6 and C3 grown on nitrate to a high of 0.18 divisions day⁻¹ for C3 grown on urea. Diurnal maximum uptake rates, ρ_max, varied from 0.41 pmol-N cell⁻¹ h⁻¹ for CCMP 2229 grown on nitrate, to 1.29 pmol-N cell⁻¹ h⁻¹ for CCMP 2229 grown on urea. Average nocturnal uptake rates were 29% of diurnal rates for nitrate, 103% of diurnal uptake rates for ammonium and 56% of diurnal uptake rates for urea. Uptake kinetic parameters varied between substrates, between strains and between day and night measurements. Highest maximum uptake rates were found for urea for strains CCMP2229 and C3 and for ammonium for strain C6. Rates of asmilation into protein also varied day and night, but overall were highest for urea. The comparison of maximal uptake rates as well as assimilation efficiencies indicate that ammonium and urea are utilized (taken up and assimilated) more than twice was fast as nitrate on a diel basis.     

The effect of ampicillin plus streptomycin on growth and photosynthesis of two halotolerant chlorophyte algae

Streptomycin and ampicillin are antibiotics commonly used to eliminate prokaryotes from the cultures of eukaryotic algae. We studied the effects of 25 mg l⁻¹ streptomycin plus 50 mg l⁻¹ ampicillin on the growth and photosynthesis of two broadly halotolerant algae, Picochlorum oklahomensis and Dunaliella sp. (Chlorophyceae). We measured growth rate, oxygen evolution, chlorophyll fluorescence kinetics, and pigment content in low (150 μmol photons m⁻² s⁻¹) and high (600 μmol photons m⁻² s⁻¹) light grown batch cultures. Our results show only a minor effect of the antibiotics on P. oklahomensis, and none on Dunaliella sp., so this combination of antibiotics is suitable for maintenance of stock cultures for physiological experiments. We also show that these antibiotics can be used in turbidostat cultures of P. oklahomensis, which otherwise tend to succumb to bacteria.     

Factors Affecting Swimming Speed in the Rotifer <Emphasis Type="Italic">Brachionus plicatilis</Emphasis>

This study examines the swimming speed in amictic females of Brachionus plicatilis in laboratory cultures. Five different stages were examined: recently hatched females, juveniles, adult non-ovigerous females, ovigerous females with 1 attached egg and ovigerous females with 2 attached eggs. We tested the speed at two temperatures, 15 °C and 25 °C, and two feeding conditions, presence and absence of microalgal cells. An automated motion analysis system was used to measure speed which was then video recorded. Swimming speed (μm s⁻¹) increased with increasing body size. There was a slight decrease in the speed of adult females as the number of attached eggs increased. Swimming activity was higher at 25 °C than at 15 °C and in the absence of food than if microalgae were present. Average values under the different experimental conditions ranged between 500 μm s⁻¹ for the recently hatched and fed females and 1500 μm s⁻¹ for the adult non-ovigerous females in the absence of microalgae. Mass-specific swimming speed decreased with body mass increase.     

Evaluation of the potential role of the macroalga Laminaria japonica for alleviating coastal eutrophication

The rapid development of human activities has caused serious eutrophication of coastal waters in China in the recent decades. The study of the biofiltration capacity of Laminaria japonica under laboratory conditions showed a significant nutrient uptake. After 36 h of incubation, around 42%, 46%, 44% of N and 45%, 42%, 35% of P were removed from three gradients of medium concentrations, respectively. In the conditions of different ratios of N/P and NO₃–N/NH₄–N, the optimum N/P ratio for nutrient uptake was 7.4 and L. japonica prefered NO₃–N rather than NH₄–N as nitrogen source. Temperature and irradiance affected uptake rates significantly. The maximal N uptake rate appeared at 10 °C and 18 μmol photons m⁻² s⁻¹ and the maximal P uptake rate was found at 15 °C and 144 μmol photons m⁻² s⁻¹. Moreover, further studies were needed to investigate the bioremediation potential of L. japonica in the open sea.     

Life history and growth in culture of the endemic New Zealand kelp <Emphasis Type="Italic">Lessonia variegata</Emphasis> J. Agardh in response to differing regimes of temperature,photoperiod and light

Lessonia variegata J. Agardh (Laminariales, Phaeophyta) is endemic to New Zealand, where it occurs in subtidal kelp forests on wave exposed coasts in the North, South and Stewart Islands. This is the first account of the growth in culture and life history of L. variegata. Microscopic gametophytes alternate with macroscopic sporophytes, characteristic of members of the order Laminariales. The life history was completed in culture within 14 days under growth conditions of 12 °C, 12:12 (L:D) and 15 °C, 15:9 (L:D). Maximum growth of sporophytes occurred at 15 °C, 15:9, and slowest growth at 10 °C, 9:15. Under low light conditions (8–9 μmol photon m⁻² s⁻¹) filamentous growth of gametophytes predominated, and both the proportion of gametogenesis and the growth of sporophytes after 30 days was much reduced from equivalent cultures grown under conditions of higher light (16–17.5 μmol photon m⁻² s⁻¹). Interest in this species relates both to its potential for commercial utilisation as well as in the role it plays in coastal rocky reef ecosystems.     

Growth of dinoflagellates, Ceratium furca and Ceratium fusus in Sagami Bay, Japan: The role of temperature, light intensity and photoperiod

Seasonal changes of field populations and growth rates of two dinoflagellates, Ceratium furca and Ceratium fusus, were examined in the temperate coastal water of Sagami Bay, Japan. Weekly field sampling was conducted from August 2002 to August 2003, and laboratory experiments were also carried out to investigate effects of temperature, irradiance and photoperiod on the growth rates of these two Ceratium species. In the field, the abundances of both species increased significantly from April to August 2003, were gradually decreased from November 2002 and were not observed in January 2003. C. fusus was able to increase at lower temperatures in February 2003 compared to C. furca. In the laboratory, the two species did not grow at <10 °C or >32 °C. The highest specific growth rate of C. furca was 0.72 d⁻¹ at 24 °C and 600 μmol m⁻² s⁻¹. Optimum growth rates (>0.4 d⁻¹) of C. furca were observed at temperatures from 18 to 28 °C and at irradiances from 216 to 796 μmol m⁻² s⁻¹. The highest growth rate of C. fusus was 0.56 d⁻¹ at 26 °C and 216 μmol m⁻² s⁻¹. Optimum growth rates of C. fusus were observed at the same irradiance rage of C. furca, whereas optimum temperature range was narrower (26–28 °C). The growth curves of both species indicated saturation of the growth rates when light intensity was above 216 μmol m⁻² s⁻¹, and did not show photoinhibition at irradiances up to 796 μmol m⁻² s⁻¹. The specific growth rates of both Ceratium species were clearly decreased at L:D = 10:14 relative to those at L:D = 14:10 and L:D = 12:12. The present study indicates the two Ceratium species can adapt to a wide range of temperature and irradiance.     

Nitrogen source effects on the growth and toxicity of two strains of the ciguatera-causing dinoflagellate Gambierdiscus toxicus

Two Caribbean strains (1651 and 1655) of the ciguatera-causing dinoflagellate Gambierdiscus toxicus were grown in xenic, batch culture under defined, measured nutrient conditions with nitrate, ammonium, urea, a mix of free amino acids (FAA), or putrescine as the nitrogen source. Cultures were maintained at 27 °C, salinity 35, 110 μmol m⁻² s⁻¹ (12 h:12 h light:dark cycle) on L2 medium at an initial nitrogen concentration of 50 μM N. Toxicity was determined using a ouabain/veratridine-dependent cytotoxicity assay (N2A assay) standardized to a ciguatoxin standard. Nitrate, ammonium, FAA, and putrescine supported growth, but urea did not. The appearance of ammonium in the organic nitrogen cultures indicated that G. toxicus and/or associated bacteria remineralized the available organic nitrogen. Both strains were exposed to nitrogen-limiting conditions as evidenced by chlorophyll a content per cell, nitrogen content, and nitrogen (N) to phosphorus (P) (N:P) ratio significantly declining once nitrogen was no longer available in the medium and cells entered stationary phase. Strain 1651 grew significantly faster than strain 1655 when nitrate, FAA, and putrescine was the nitrogen source, but not ammonium. Nitrogen source had no effect on growth rate (0.14 d⁻¹) in strain 1651. The growth rate of strain 1655 (0.10–0.13 d⁻¹) was significantly faster on ammonium than the other nitrogen sources. Strain 1655 was significantly more toxic (10-fold) than strain 1651 except when growing on ammonium at exponential phase. Toxicity ranged from 1.3 to 8.7 fg C-CTX1-Eq cell⁻¹ in strain 1651 and from 30.7 to 54.3 fg C-CTX1-Eq cell⁻¹ in strain 1655. Nitrogen source had no significant affect on toxicity. Toxicity was greater in stationary versus exponential phase cells for strain 1651 when grown on nitrate and strain 1655 regardless of nitrogen source. The difference in toxicity between growth phases may result from an increase in ciguatoxin and/or maitotoxin. Our results suggest that some strains of G. toxicus when associated with bacteria are able to take advantage of organic as well as inorganic nitrogen sources on short time scales to support future growth. The uncoupling of total nitrogen and phosphorus pools from conditions in the water column suggest that instantaneous growth rates can be supported by nutrients acquired hours to days earlier.     

The effect of environmental factors on the growth rate of Karenia brevis (Davis) G. Hansen and Moestrup

The red tide dinoflagellate Karenia brevis (Davis) G. Hansen and Moestrup is noted for causing mass mortalities of marine organisms in the Gulf of Mexico. Most research has focused on culture isolates from the eastern Gulf of Mexico. In this investigation, we examine the effects of light, temperature and salinity on the growth rate of K. brevis from the western Gulf of Mexico. Growth rates of K. brevis were determined under various combinations of irradiance (19, 31, 52, 67, and 123 μmol m⁻² s⁻¹), salinity (25, 30, 35, 40 and 45), and temperature (15, 20, 25, and 30 °C). Maximum growth rates varied from 0.17 to 0.36 div day⁻¹ with exponential growth rates increasing with increasing irradiance. Little or no growth was supported at 19 μmol photons m⁻² s⁻¹ for any experiment. Maximum growth rates at 15 °C were much lower than at other temperatures. Maximum growth rates of the Texas clone (SP3) fell within the range of Florida clones reported in the literature (0.17–0.36 div day⁻¹ versus 0.2–1.0 div day⁻¹). The Texas clone SP3 had a very similar light saturation point compared to that of a Florida isolate (Wilson's clone) (67 μmol m⁻² s⁻¹ versus 65 μmol m⁻² s⁻¹), and light compensation (20–30 μmol m⁻² s⁻¹1). The upper and lower salinity tolerance of the Texas clone was similar than that of some Florida clones (45 versus 46 and 25 versus 22.5, respectively). In our study, the Texas clone had the same temperature tolerance reported for Florida clones (15–30 °C). While individual clones can vary considerably in maximum growth rates, our results indicate only minor differences exist between the Texas and Florida strains of K. brevis in their temperature and salinity tolerance for growth. While the literature notes lower salinity occurrences of K. brevis in nearby Louisiana, our isolate from the southern Texas coast has the higher salinity requirements typical of K. brevis in the eastern Gulf of Mexico.     

Nitrogenous preference of toxigenic Pseudo-nitzschia australis (Bacillariophyceae) from field and laboratory experiments

Field and laboratory experiments were designed to determine the differential growth and toxin response to inorganic and organic nitrogen additions in Pseudo-nitzschia spp. Nitrogen enrichments of 50 μM nitrate (KNO₃), 10 μM ammonium (NH₄Cl), 20 μM urea and a control (no addition) were carried out in separate carboys with seawater collected from the mouth of the San Francisco Bay (Bolinas Bay), an area characterized by high concentrations of macronutrients and iron. All treatments showed significant increases in biomass, with chlorophyll a peaking on days 4–5 for all treatments except urea, which maintained exponential growth through the termination of the experiment. Pseudo-nitzschia australis Frenguelli abundance was 10³ cells l⁻¹ at the start of the experiment and increased by an order of magnitude by day 2. Particulate domoic acid (pDA) was initially low but detectable (0.15 μg l⁻¹), and increased throughout exponential and stationary phases across all treatments. At the termination of the experiment, the urea treatment produced more than double the amount of pDA (9.39 μg l⁻¹) than that produced by the nitrate treatment (4.26 μg l⁻¹) and triple that of the control and ammonium treatments (1.36 μg l⁻¹ and 2.64 μg l⁻¹, respectively). The mean specific growth rates, calculated from increases in chlorophyll a and from cellular abundance of P. australis, were statistically similar across all treatments.These field results confirmed laboratory experiments conducted with a P. australis strain isolated from Monterey Bay, CA (isolate AU221-a) grown in artificial seawater enriched with 50 μM nitrate, 50 μM ammonium or 25 μM of urea as the sole nitrogen source. The exponential growth rate of P. australis was significantly slower for cells grown on urea (ca. 0.5 day⁻¹) compared to the cells grown on either nitrate or ammonium (ca. 0.9 day⁻¹). However the urea-grown cells produced more particulate and dissolved domoic acid (DA) than the ammonium- or nitrate-grown cells. The field and laboratory experiments demonstrate that P. australis is able to grow effectively on urea as the primary source of nitrogen and produced more pDA when grown on urea in both natural assemblages and unialgal cultures. These results suggest that the influence of urea from coastal runoff may prove to be more important in the development or maintenance of toxic blooms than previously thought, and that the source of nitrogen may be a determining factor in the relative toxicity of west coast blooms of P. australis.     

Relative Uptake of Urea and Ammonium by Dinoflagellates or Cyanobacteria in Shrimp Mesocosms

The relative role of the organic nitrogen source, urea, versus ammonium as a nitrogen source for two species of dinoflagellates was compared with one species of cyanobacteria. Experiments were conducted opportunistically in nutrient-rich marine water during blooms of 34either cyanobacteria or dinoflagellates in outdoor mesocosms. These replicate mesocosms, which were stocked with shrimp fed high-protein formulated feeds, contained high biomasses of phytoplankton (mean chlorophyll a concentrations, 439.2–811.2 μg l⁻¹). ¹⁵N-urea and ammonium uptake rates for dinoflagellate-dominated blooms (Gymnodinium pulchellum-complex (Larsen), Karlodinium micrum (Larsen) (Dinophyceae)) were compared with blooms of the cyanobacterium, Romeria sp. (Cyanophyceae) in mesocosms with mean urea and ammonium concentrations ranging from 2.32 to 3.24 μM, and 7.39 to 64.85 μM, respectively. Urea uptake rates were significantly (p < 0.005) lower than ammonium uptake rates irrespective of which algal species dominated the bloom. Additionally urea uptake rates were not significantly higher in G. pulchellum-complex or K. micrum-dominated blooms than in Romeria sp. blooms. These results suggest that G pulchellum complex and K. micrum may not be gaining a competitive advantage in waters high in dissolved organic matter simply by preferentially utilizing urea. The periodic dominance of these species in highly organic environments, such as shrimp ponds, is likely to have a more complex explanation.     

In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

Bottom-up controls on a mixed-species HAB assemblage: A comparison of sympatric Chattonella subsalsa and Heterosigma akashiwo (Raphidophyceae) isolates from the Delaware Inland Bays, USA

Recent novel mixed blooms of several species of toxic raphidophytes have caused fish kills and raised health concerns in the highly eutrophic Inland Bays of Delaware, USA. The factors that control their growth and dominance are not clear, including how these multi-species HAB events can persist without competitive exclusion occurring. We compared and contrasted the relative environmental niches of sympatric Chattonella subsalsa and Heterosigma akashiwo isolates from the bays using classic Monod-type experiments. C. subsalsa grew over a temperature range from 10 to 30 °C and a salinity range of 5–30 psu, with optimal growth occurring from 20 to 30 °C and 15 to 25 psu. H. akashiwo had similar upper temperature and salinity tolerances but also lower limits, with growth occurring from 4 to 30 °C and 5 to 30 psu and optimal growth between 16 and 30 °C and 10 and 30 psu. These culture results were confirmed by field observations of bloom occurrences in the Inland Bays. Maximum nutrient-saturated growth rates (μ_max) for C. subsalsa were 0.6 d⁻¹ and half-saturation concentrations for growth (K_s) were 9 μM for nitrate, 1.5 μM for ammonium, and 0.8 μM for phosphate. μ_max of H. akashiwo (0.7 d⁻¹) was slightly higher than C. subsalsa, but K_s values were nearly an order of magnitude lower at 0.3 μM for nitrate, 0.3 μM for ammonium, and 0.2 μM for phosphate. H. akashiwo is able to grow on urea but C. subsalsa cannot, while both can use glutamic acid. Cell yield experiments at environmentally relevant levels suggested an apparent preference by C. subsalsa for ammonium as a nitrogen source, while H. akashiwo produced more biomass on nitrate. Light intensity affected both species similarly, with the same growth responses for each over a range from 100 to 600 μmol photons m⁻² s⁻¹. Factors not examined here may allow C. subsalsa to persist during multi-species blooms in the bays, despite being competitively inferior to H. akashiwo under most conditions of nutrient availability, temperature, and salinity.     

Growth of dinoflagellates, Ceratium furca and Ceratium fusus in Sagami Bay, Japan: The role of nutrients

In order to study the influence of nutrients on the growth characteristics of the dominant dinoflagellates, Ceratium furca and Ceratium fusus, in the temperate coastal area of Sagami Bay, Japan, we conducted field monitoring from January 2000 to December 2005 and performed laboratory culture experiments. In the field study, population densities of C. furca and C. fusus were high, even in low nutrient concentrations (N: 1.58 μM, P: 0.17 μM). Both species were more abundant in the surface and sub-surface layers than in the bottom layers during the stratification periods. In the laboratory study, the specific growth rates of C. furca and C. fusus increased gradually along with increasing nutrients up to the T₅ (N: 5 μM, P: 0.5 μM) and T₁₀ (N: 10 μM, P: 1 μM) concentration levels, after which the growth rate plateaued at the T₅₀ (N: 50 μM, P: 5 μM) concentration level. In contrast, the nutrient uptake rates of both species continuously increased, indicating “luxury consumption”, i.e., excessive cellular storage not related to growth rate. The half-saturation constants (K_s) of C. furca for nitrate (0.49 μM) and phosphate (0.05 μM) were slightly higher than C. fusus (0.32 and 0.03 μM, respectively). We offer two reasons why the two Ceratium population densities were maintained at high levels in low nutrient conditions. First, these two species have a competitive advantage over other algal species because of low K_s values and specific characteristics for nutrient uptake such as luxury consumption. Their ability to obtain nutrients through alternative methods, such as phagotrophy, might contribute to bloom formation and population persistence. Second, the cell densities of both Ceratium species increased along with nitrate concentrations in the media even when phosphorus was held constant. In particular, the growth of C. furca was directly supported by various nitrogen sources such as nitrate, ammonium, and urea, although the highest growth rates were observed only in the nitrate-enriched cultures. Our field and laboratory results revealed that the growth rates of the two Ceratium species increased readily in high N:P nutrient conditions (i.e., conditions of P limitation) indicating an advantage over other algal species in phosphorus-limited environments such as Sagami Bay.     

Water relations and carbon dioxide exchange of epiphytic lichens in the Namib fog desert

Although there is only negligible rainfall, frequent nocturnal fog, dew and high air humidity support a luxurious lichen vegetation in the coastal zone of the central Namib Desert (Namibia). In earlier publications, we have studied ecophysiological performance of a series of epilithic and terrestrial lichens. Here, we have extended this work to three epiphytic species (Heterodermia namaquana, Ramalina lacera, and Xanthoria turbinata) that inhabit the sparse perennial shrubs growing in this area. Our intention, monitoring lichen CO₂ exchange, their water relations and microclimate conditions, was to determine the functional mechanisms that allow these epiphytes to exist under the special conditions of a fog desert. Measurements were conducted mainly during the spring season.The epiphytic lichens showed response patterns very similar to the epilithic and epigaeic species at the same site. Their metabolism was activated through moistening by dew and/or fog during the night and, in the very early morning, they exhibited the typical brief peak of net photosynthesis (NP) between sunrise and desiccation. The thalli were almost completely dry for the remainder of the day. Average duration of the positive NP during the morning peak was about 3 h. Dew condensation, alone, resulted in activation that provided 58–63% of integrated carbon income (ΣNP) as compared to fog (plus dew). In the late afternoon, there was a tendency for hydration to increase again, due to water vapour uptake at higher air humidity, and this allowed on some days a brief additional period of very low rates of photosynthesis shortly before sunset.Light response of photosynthesis showed “sun-plant” characteristics with saturation around 1000 μmol m⁻² s⁻¹ photosynthetically active photon flux density (PPFD). Light compensation point (LCP) of CO₂ exchange after sunrise was highly dependent on actual water content (WC) for X. turbinata: at low hydration it was ca. 10 μmol m⁻² s⁻¹ PPFD whilst, at high WC, it was almost 80 μmol m⁻² s⁻¹ PPFD. In contrast, LCP of R. lacera was almost independent of WC. This phenomenon was probably due to differences in thallus structure.Maximal attained NP and daily ΣNP both showed a saturation-type response to previous maximal nocturnal WC. Neither parameter was increased substantially when higher maximal thallus WCs were produced by experimental moistening in the night. All three species, despite their different morphologies, performed optimally at the highest nocturnal moistening achieved by natural fog and were not able to make use of higher hydration.The three studied epiphytes were similar in their chlorophyll-related rates of NP. Due to lower chlorophyll content, dry weight and carbon-related NP of X. turbinata was only about one-third of that of the other two species. The average carbon income on days with fog and/or dew hydration during the spring season amounted to 2.4 and 2.1 mgC (gC)⁻¹ day⁻¹ (related to thallus carbon content) for H. namaquana and R. lacera, respectively. This primary production was of similar magnitude to those found for the terrestrial species at the same site.     

Nutrition and growth kinetics in nitrogen- and phosphorus-limited cultures of the novel red tide flagellate Chattonella ovata (Raphidophyceae)

In the summer of 2004, a harmful bloom caused by Chattonella ovata (Raphidophyceae) occurred over almost the entire area of the Seto Inland Sea and caused fishery damages. This incident was the first record of a bloom and damage to the fisheries caused by this species in Japanese waters. In order to elucidate the mechanism of the bloom outbreak, we examined the nutrition and the growth kinetics in nitrogen (N)- and phosphorus (P)-limited semi-continuous cultures of this species. Inorganic N compounds, such as nitrate, nitrite, and ammonium, were found to be good nitrogen sources for the growth of C. ovata, while organic nitrogen (urea and uric acid) was not utilized. This species was capable of using ATP, ADP and inorganic phosphorus compounds, but did not utilize phosphate monoesters as a sole P source. Under both N-limited and P-limited steady state conditions, the growth rate as a function of cell nitrogen and phosphorus quota, respectively, followed the Droop equation. Kinetic parameters μ_m (maximum growth rate) and k_q (minimum cell quota) obtained for N- and P-limited cultures were 0.79 day⁻¹ and 5.5 pmol N cell⁻¹ and 0.86 day⁻¹ and 0.48 pmol P cell⁻¹, respectively. The minimum cell quotas were 23–30% lower than those of C. antiqua. The nutrient availability and kinetic parameters of C. ovata are compared with other harmful algae and the ecological implications of these characteristics discussed.     

Ammonium excretion by a mutant of the nitrogen-fixing cyanobacterium Anabaena siamensis

Anabaena siamensis isolated from rice fields in Thailand is a fast growing cyanobacterium with a high nitrogen-fixing activity. Mutant strains resistant to the l-glutamate analogue, l-methionine sulfoximine (MSX) were isolated by ethyl methanesulfonate mutagenesis. A stable mutant named A. siamensis SS1, which released ammonium to the medium, was studied further. In batch cultures the rate of ammonium production peaked at the early log phase and gradually decreased until the 4th day of growth when the cultures reached a density of 90 μg chl ml⁻¹. To obtain constant release of ammonium by SS1, continuous culture experiments were performed at a cell density of 5 μg chl ml⁻¹ and the following results were obtained: (1) growth rate as the parent (μ:0·123 h⁻¹) in the presence and absence of 500 μm MSX; (2) 48% GS transferase activity when compared with the parent; (3) ammonium excretion at a rate of 8 μmol (mg chl)⁻¹ h⁻¹ as measured up to 20 generations (120 h); (4) depressed nitrogenase activity; and (5) 30% higher nitrogenase activity than that of the parent. SS1 immobilized in alginate beads (5 μg chl ml⁻¹) exhibited values of glutamine synthetase and nitrogenase activity similar to those of free cells. However, ammonium excretion at the rate of 11·61 μmol (mg chl)⁻¹ h⁻¹ was obtained only up to 20 h after loading in bioreactors, due to the fast growth of SS1 as also occurred in batch cultures.