Irradiance and temperature regulation of oxygenic photosynthesis and O2 consumption in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

 Short-term effects of temperature and irradiance on oxygenic photosynthesis and O₂ consumption in a hypersaline cyanobacterial mat were investigated with O₂ microsensors in a laboratory. The effect of temperature on O₂ fluxes across the mat–water interface was studied in the dark and at a saturating high surface irradiance (2162 μmol photons m⁻² s⁻¹) in the temperature range from 15 to 45 °C. Areal rates of dark O₂ consumption increased almost linearly with temperature. The apparent activation energy of 18 kJ mol⁻¹ and the corresponding Q ₁₀ value (25 to 35 °C) of 1.3 indicated a relative low temperature dependence of dark O₂ consumption due to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperature up to 40 °C and exhibited a Q ₁₀ value (20 to 30 °C) of 2.8. Both O₂ dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 °C, with the most pronounced increase of gross photosynthesis at the mat surface between 25 and 35 °C (Q ₁₀ of 3.1). In another mat sample, measurements at increasing surface irradiances (0 to 2319 μmol photons m⁻² s⁻¹) were performed at 25, 33 (the in situ temperature) and 40 °C. At all temperatures, areal rates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of saturation (E _k = 148 to 185 μmol photons m⁻² s⁻¹) estimated from P versus E _d curves showed no clear trend with temperature, while maximal photosynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 μmol photons m⁻² s⁻¹, where oxygenic gross photosynthesis and also the thickness of the photic zone was significantly reduced at 40 °C. The compensation irradiance increased with temperature, from 32 μmol photons m⁻² s⁻¹ at 25 °C to 77 μmol photons m⁻² s⁻¹ at 40 °C, due to increased rates of O₂ consumption relative to gross photosynthesis. Areal rates of O₂ consumption in the illuminated mat were higher than dark O₂ consumption at corresponding temperatures, due to an increasing O₂ consumption in the photic zone with increasing irradiance. Both light and temperature enhanced the internal O₂ cycling within hypersaline cyanobacterial mats. Received: 30 November 1999 / Accepted: 11 April 2000     

Effect of an organophosphorus insecticide, fenitrothion, on survival and osmoregulation of various developmental stages of the shrimp Penaeus japonicus (Crustacea: Decapoda)

The toxicity of fenitrothion was determined in larvae (nauplii, Zoeae 1 to 3, Mysis 1 to 3), postlarvae (PL stages) and juvenile shrimp (Penaeus japonicus Bate), in two media, seawater (SW) and diluted seawater (DSW) (1100 and 550 mosM kg⁻¹, ≃ 37 and 19‰ S). The effects of fenitrothion on the osmoregulatory capacities (OC) of juveniles were recorded. A gill and epipodite histopathological study was also conducted. For larvae in seawater, 24 and 48 h LC₅₀s ranged from 32.9 μg l⁻¹ (Zoeae 2) to 10.7 μg l⁻¹ (Mysis 3), and from 3.9 μg l⁻¹ (Zoeae 3) to 2.0 μg l⁻¹ (Mysis 3), respectively; 48 and 96 h  LC₅₀s in postlarvae (PL) at the same salinity ranged from 1.8 μg l⁻¹ (PL1) to 0.6 μg l⁻¹ (PL5), and from 0.3 μg l⁻¹ (PL7) to 0.4 μg l⁻¹ (PL15). In juveniles, 96 h LC₅₀s were 0.8 μg l⁻¹ in seawater and 1.5 μg l⁻¹ in diluted seawater. From hatching to juvenile stages, the overall trend was a rapid decrease (from nauplii to PL5–PL7) followed by a slight increase (from PL7 to PL15 and juveniles) in the shrimp's ability to tolerate the insecticide. In juveniles kept in seawater and in diluted seawater, fenitrothion decreased the osmoregulatory capacity (OC = difference between the hemolymph osmotic pressure and the osmotic pressure of the medium) at both lethal and sublethal concentrations. This effect was time- and dose-dependent. In SW, the decrease in hypo-OC was ˜ 25% at sublethal concentrations and ˜ 35% at the 96 h LC₅₀. In DSW, the decrease in hyper-OC was ˜ 10 to 15% at sublethal concentrations. In SW, shrimp were able to recover their OC in less than 48 h when transferred to water free of pesticide. In DSW, recovery at 48 h was only possible after exposure to the lowest tested sublethal concentration. Haemocytic congestions (thrombosis) of the gills, lamellae necrosis and other alterations of gills and epipodites (breakage of the cuticle, reduction of the hemolymph lacunae) were noted in juveniles exposed to lethal and sublethal concentrations of fenitrothion. Received: 7 October 1996 / Accepted: 13 November 1996     

Concentrations of inorganic elements in 20 municipal waters in Sweden before and after treatment – links to human health

The water chemistry of 20 municipal water treatment plants in southern Sweden, representing various bedrock situations, and water qualities, were investigated. Four water samples, raw and treated, were collected from each plant and analyzed by predominantly ICP-OES and ICP-MS at four occasions from June to December, 2001. The concentrations of Ca, Mg, K, Na, HCO₃ and a number of micronutrients, varied considerably in treated waters from the studied plants (ranges; Ca: 9.1–53.7 mg L⁻¹, Mg: 1.4–10.9 mg L⁻¹, K: 1.1–4.8 mg L⁻¹, Na; 5.4–75.6 mg L⁻¹, HCO₃: 27–217 mg L⁻¹). The elimination of Fe and Mn from raw water was efficient in all treatments investigated, giving concentrations in treated waters below the detection limits at some plants. Softening filters gave waters with Ca-concentrations comparable to the softest waters in this study. Adjustment of pH by use of chemicals like lye, soda or lime, modified the consumer water composition significantly, besides raising the pH. It was estimated that drinking water contributed to approximately 2.2–13% of the daily Ca uptake, if the gastrointestinal uptake efficiency from food and water was estimated to be around 50%. The corresponding figures for Mg was 1.0–7% and for F 0–59%. None of the studied elements showed any significant time trends in raw or treated waters during the follow-up period. The concentrations of potentially toxic metals such as Al, Pb and U were low and did not indicate risks for adverse health effects (ranges; Al: 0.5–2.3 μg L⁻¹, Pb: 0–0.3 μg L⁻¹, U: 0.2.5 μg L⁻¹).     

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

 The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O₂, CO₂, pH, Ca²⁺ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O₂ up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO₂ fixation decreased the CO₂ at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O₂ level to 91% air saturation and the CO₂ concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O₂, CO₂ and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O₂ cm⁻³ s⁻¹) than in stagnant water (1.6 nmol O₂ cm ⁻³ s⁻¹), whereas net photosynthesis rates were unaffected by flow conditions. The Ca²⁺ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca²⁺ gradients at the shell surface showed total Ca²⁺ uptake rates of 0.6 to 4.2 nmol cm⁻² h⁻¹ in A. lobifera and 1.7 to 3.6 nmol cm⁻² h⁻¹ in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis. Received: 6 July 1999 / Accepted: 28 April 2000     

Microsensor studies of photosynthesis and respiration in the symbiotic foraminifer Orbulina universa

Oxygen and pH microelectrodes were used to investigate the microenvironment of the planktonic foraminifer Orbulina universa and its dinoflagellate endosymbionts. A diffusive boundary layer surrounds the foraminiferal shell and limits the O₂ and proton transport from the shell to the ambient seawater and vice versa. Due to symbiont photosynthesis, high O₂ concentrations of up to 206% air saturation and a pH of up to 8.8, i.e. 0.5 pH units above ambient seawater, were measured at the shell surface of the foraminifer at saturating irradiances. The respiration of the host–symbiont system in darkness decreased the O₂ concentration at the shell surface to <70% of the oxygen content in the surrounding air-saturated water. The pH at the shell surface dropped to 7.9 in darkness. We measured a mean gross photosynthetic rate of 8.5 ± 4.0 nmol O₂ h⁻¹ foraminifer⁻¹. The net photosynthesis averaged 5.3 ± 2.7 nmol O₂ h⁻¹. In the light, the calculated respiration rates reached 3.9 ± 1.9 nmol O₂h⁻¹, whereas the dark respiration rates were significantly lower (1.7 ± 0.7 nmol O₂ h⁻¹). Experimental light–dark cycles demonstrated a very dynamic response of the symbionts to changing light conditions. Gross photosynthesis versus scalar irradiance curves (P vs E _o curves) showed light saturation irradiances (E _k) of 75 and 137 μmol photons m⁻² s⁻¹ in two O. universa specimens, respectively. No inhibition of photosynthesis was observed at irradiance levels up to 700 μmol photons m⁻² s⁻¹. The light compensation point of the symbiotic association was 50 μmol photons m⁻² s⁻¹. Radial profile measurements of scalar irradiance (E _o) inside the foraminifera showed a slight increase at the shell surface up to 105% of the incident irradiance (E _d). Received: 26 January 1998 / Accepted: 11 April 1998     

Cadmium accumulation in the female shore crab Carcinus maenas during the moult cycle and ovarian maturation

 The influence of moulting and ovarian maturation on cadmium accumulation in the tissues of female shore crabs Carcinus maenas exposed to 1 mg Cd l⁻¹ in the water was investigated. Cadmium accumulation in all tissues examined was markedly increased in crabs in the postmoult stages (A and B) compared to crabs in all other moult stages. During the moult cycle, average cadmium accumulation in the midgut gland ranged from 29 μg Cd g⁻¹ dw at premoult stage (D₂) to 589 μg Cd g⁻¹ dw at postmoult stage (A). Average cadmium concentrations in the haemolymph ranged from 0.56 μg Cd ml⁻¹ at intermoult stage (C₄) to 4.6 μg Cd ml⁻¹ at postmoult stage (A), while the gills accumulated from 103 μg Cd g⁻¹ dw in intermoult stage (C₃) to 352 μg Cd g⁻¹ dw in postmoult stage (A). Cadmium concentration in gills and haemolymph was also significantly higher in crabs in late premoult stage (D₃) compared to C₄-crabs, while midgut gland cadmium concentration remained elevated in C₁- and C₃- intermoult stages relative to C₄. During ovarian maturation the cadmium accumulation in midgut gland, gills, ovaries and haemolymph decreased. Average cadmium concentration in the midgut gland decreased from 63 μg g⁻¹ dw in ovarian Stage I to 19 μg g⁻¹ dw in ovarian Stage VI. The same pattern was observed for gills, haemolymph and ovaries. The present study demonstrates that cadmium accumulation in the female shore crab strongly depends on the physiological status of the animal. A possible association between physiological calcium requirements and cadmium accumulation during moulting is discussed. Received: 20 January 2000 / Accepted: 20 July 2000     

Effects of ambient ammonia levels on blood ammonia, ammonia excretion and heart and scaphognathite rates of Nephrops norvegicus

During commercial handling of Nephropsnorvegicus (L.) there are a number of situations when the prawns may be exposed to very high ambient ammonia levels. These experiments evaluated the effects of increased levels of ambient total ammonia (TA = NH₃ + NH₄ ⁺) on␣blood ammonia, ammonia efflux rates and on the cardio-ventilatory performance of N. norvegicus. When prawns were taken from <1 to 2000 μmol TA l⁻¹ medium, blood TA concentrations increased rapidly for the first 2 h but tended to drop thereafter. Original blood TA levels were restored 6 h after the prawns were transferred back from seawater containing 2000 to <1 μmol TA l⁻¹. Sudden exposure to 500, 1000, 2000 or 4000 μmol TA l⁻¹ medium induced blood TA concentrations to increase respectively to 50, 30, 33 and 36% of external concentrations (normally, internal TA values are much higher than external levels). Immediately after transfer back to seawater with low ammonia concentration ( <1 μmol TA l⁻¹), excretion rates were higher than those of control prawns, and the absolute amounts of TA excreted were considerably higher than those calculated to have accumulated in the haemolymph. Heart rate (HR) and scaphognathite rate (SR) were not altered when prawns were subjected to sudden alterations in ambient ammonia ( <1 to 2000 to <1 μmol TA l⁻¹). When water ammonia concentrations were altered more gradually, both rates increased, but only at 4000 μmol TA l⁻¹. These results show that N. norvegicus is able to remove ammonia from the haemolymph and/or transform ammonia into some other substance when subjected to increased levels of ambient ammonia. Possible mechanisms involved (e.g. active transport across the gills; storage in some other tissue; glutamate synthe sis) are discussed. Received: 20 May 1996 / Accepted: 1 July 1996     

Resource acquisition in two intertidal fucoid seaweeds, Fucus serratus and Himanthalia elongata : seasonal variation and effects of reproductive development

Seasonal variations and the effect of reproductive development on resource acquisition by two intertidal fucoid species, the iteroparous Fucus serratus L. and the semelparous Himanthalia elongata (L.) S. F. Gray were examined. The oxygen-exchange characteristics of vegetative apical tissue of both non-fertile and fertile plants and receptacle tissue were compared at monthly intervals throughout reproductive development. Respiratory rates in non-fertile F. serratus varied seasonally between 1.5 and 8.0 μmol g⁻¹ fresh wt h⁻¹; in fertile plants the receptacle had a significantly lower respiratory rate than the vegetative tissue. The respiratory rate of the vegetative button of fertile H. elongata displayed less seasonal variation and was lower than that of the receptacle, which varied from a maximum of 9.5 μmol g⁻¹ fresh wt h⁻¹ at receptacle initiation in October to a minimum of 2.0 μmol g⁻¹ fresh wt h⁻¹ in February. The maximum photosynthetic rate (P _max) of non-fertile plants of both species did not vary in a distinct seasonal manner (∼60 μmol g⁻¹ fresh wt h⁻¹ for F. serratus and ∼12 μmol g⁻¹ fresh wt h⁻¹ for H. elongata). In fertile plants, the P _max of the receptacle tissue was (∼50% lower in F. serratus, and at its peak three times higher in H. elongata, than that of vegetative tissue. The stable carbon-isotope ratio (δ¹³C) did not differ between different tissue types in F. serratus, but values did vary seasonally, being less negative in the summer than in the winter (−13.5‰ compared to −18‰). The receptacle tissue of H. elongata also displayed a distinct seasonal variation in δ¹³C values (−12‰ in summer, −16‰ in winter), whilst the δ¹³C of the vegetative button did not vary seasonally. The rate of uptake of inorganic nitrogen by the vegetative thallus was lower in H. elongata than in F. serratus. The receptacle tissue of F. serratus had lower uptake rates than the vegetative tissue, whilst the uptake rate by H. elongata receptacle tissue was higher than that of the vegetative button. Received: 14 March 1997 / Accepted: 22 April 1997     

The effect of ration size, temperature and body weight on specific dynamic action of the common cuttlefish Sepia officinalis

The effect of meal size (shrimp Crangon crangon) [0.83–18.82% dry body weight (Dw)] on specific dynamic action (SDA) was assessed in cuttlefish Sepia officinalis (1.03–6.25 g Dw) held at 15 and 20°C. Cuttlefish <2 g significantly expended less energy in feeding and digesting their meal than cuttlefish >2 g when given the same quantity of food. Handling, eating and digesting a shrimp meal was temperature dependent with cuttlefish processing and digesting a similar sized shrimp meal faster at 20°C than at 15°C. The proportional increase in oxygen consumption (2.07 ± 0.02) was not correlated with feeding rate (FR) and was independent of temperature and cuttlefish size. The SDA peak was not correlated with FRs, and increased as cuttlefish size and temperature increased. The mean SDA coefficient was 0.87 ± 0.07% of the ingested energy; one of the lowest SDA values recorded amongst vertebrates and invertebrates. Daily energy requirements (KJ day⁻¹) for S. officinalis were calculated from laboratory estimates of energy losses due to standard (MO_{2 Standard}), routine (MO_{2 Routine}) and feeding (MO_{2 SDA}) oxygen consumption. Laboratory estimates of daily metabolic expenditures were combined with results from previous investigations to construct an energy budget for 1 and 5 g cuttlefish consuming a meal of 5 and 15% Dw at 20°C and the amount of energy available for growth was estimated to be between 35 and 80.3% of the ingested energy.     

Photosynthetic utilisation of carbon and light by two tropical seagrass species as measured in situ

In situ measurements of seagrass photosynthesis in relation to inorganic carbon (Ci) availability, increased pH and an inhibitor of extracellular carbonic anhydrase were made using an underwater pulse amplitude modulated (PAM) fluorometer. By combining the instrument with a specially designed Perspex chamber, we were able to alter the water surrounding a leaf without removing it from the growing plant. Responses to Ci within the chamber showed that subtidal plants of the seagrasses Cymodocea serrulata and Halophila ovalis had photosynthetic rates that were limited by the ambient Ci concentration depending on the irradiance that was available during short-term photosynthesis–irradiance trials. Relative electron transport rates (RETRs) at light saturation (up to 500 μ mol photons m⁻² s⁻¹) increased by 66–100% when the Ci concentration was increased from ca. 2.2 to 6.2 mM. On the other hand, intertidal plants of the same species exhibited a much lesser limitation of photosynthesis by Ci at any irradiance (up to 1500 μ mol photons m⁻² s⁻¹). Both species were able to use HCO⁻ ₃ efficiently, and there was stronger evidence for direct uptake of HCO⁻ ₃ rather than extracellular dehydration of HCO⁻ ₃ to CO₂ prior to Ci uptake. Subtidally, H. ovalis and C. serrulata grew to 10 and 12 m, respectively, where ambient irradiances were approximately 16 and 11% of those at the surface. Maximum RETRs (at light saturation) were lower for these deep-growing plants than for the intertidally growing ones. For both species, the onset of light saturation of photosynthesis (E _k) occurred at approximately 100 μ mol photons m⁻² s⁻¹ for the deep water populations, which was four and two times lower than for the shallow populations of C. serrulata and H. ovalis, respectively. This, and the differences in maximal photosynthetic rates (RETR _max), reflects an acclimation of the deep-growing populations to the lower light environment. The results presented here show that photosynthesis, as measured in situ, was limited by the availability of Ci for the deeper growing plants in Zanzibar, while the intertidally growing plants photosynthesised at close to Ci saturation. The latter result is contrary to previous conclusions regarding Ci limitations for these intertidal plants, and, in general, our findings highlight the need for performing similar experiments in situ rather than under laboratory conditions. Received: 4 April 2000 / Accepted: 31 August 2000     

Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia)

Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The 2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics. High growth rates (0.9–1.04 day⁻¹), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l⁻¹) and carbon biomass (392–398 μg C l⁻¹) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day⁻¹) and biomass of algae (2.9–5.4 μg Chl a l⁻¹ and 209–260 μg C l⁻¹) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004 246–361 μg C l⁻¹), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%) in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day⁻¹) and a higher carbon demand (165–470 μg C l⁻¹ day⁻¹) than dinoflagellates (0.1–0.5 day⁻¹, 33–290 μg C l⁻¹ day⁻¹). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were more vigorous grazers of the large algae (diatoms) than were dinoflagellates.     

Effect of temperature on statolith growth of the European squid Loligo vulgaris during early life

Over the past decade, statolith interpretation has resulted in a major advance in our knowledge of squid population-dynamics, but the way in which environmental conditions affect the statolith increment-deposition ratio remains virtually unknown. The object of the present study was to determine the effect of temperature on this process, using tetracycline marks to validate statolith growth in Loligo vulgaris Lamarck, 1798 under rearing conditions equivalent to severe winter (11 °C) and summer (19 °C) temperature regimes. Tetracycline marking was performed every 10 d (at 10, 20, 30, 40, 50 and 60 d of age). The newly hatched squid paralarvae were slightly smaller in summer than those hatched in winter. Survival rates were similar in both cultures, but growth rates (wet mass) of summer squids were double those in winter. At hatching, statoliths were already longer in the summer squids, and growth rates were 2% d⁻¹ as opposed to 0.9% d⁻¹ for winter statoliths. For the dorsal dome area of the statolith, where more increment counts were made, statolith growth was of 3.25 μm d⁻¹ in summer, and daily increment deposition was confirmed in 87% of the statoliths. The slow growth of statoliths at winter temperatures yielded a mean growth of 1.1 μm d⁻¹– insufficient to discern the increments using light microscopy. Subsequent SEM observation enabled only 21% of the winter statoliths to be read; these also indicated a deposition rate of one increment d⁻¹. Since the life span of L. vulgaris is ≃1 yr, squids will experience at least one winter during their life cycle, and this might be visible on the statolith. Received: 28 June 1999 / Accepted: 20 December 1999     

Elemental composition of a tusk of a dugong, Dugong dugon, from Exmouth, Western Australia

Element analysis of the annual increments in a longitudinal section from the tusk of a female dugong Dugong dugon (Müller) from Exmouth, Western Australia, was carried out by X-ray fluorescence-imaging, inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS). Nine elements (Ba, Ca, Fe, Li, Mg, Na, P, Sr and Zn) were present in concentrations amenable to determination by these techniques. Most elements revealed both long-term trends and shorter, year-to-year, variations. For example, sodium concentrations increased from 0.55 to 0.72% with the age of the dugong. Strontium concentrations (mean 0.17%) were closely correlated (r = 0.86) with those of barium (mean 4.5 mg kg⁻¹). Zinc concentrations increased from ∼70 to 170 mg kg⁻¹ with dugong age, but also showed shorter-term fluctuations of ∼30 mg kg⁻¹ that were correlated (r = 0.41) with mean annual Fremantle sea level (a measure of the Southern Oscillation Index and strength of the Leeuwin Current). The concentrations of the elements and correlations with year and between pairs of elements are discussed. Received: 10 January 1997 / Accepted: 21 April 1997     

Kinetics of glucose and amino acid uptake by attached and free-living marine bacteria in oligotrophic waters

Kinetics of glucose and amino acid uptake by attached and free-living bacteria were compared in the upper 70 m of the oligotrophic north-western Mediterranean Sea. Potential uptake rates of amino acids were higher than those of glucose in all the samples analysed. Cell-specific potential uptake rates of attached bacteria were up to two orders of magnitude higher than those of total bacteria, both for amino acids and glucose (0.72–153 amol amino acids cell⁻¹ h⁻¹ and 0.05–58.42 amol glucose cell⁻¹ h⁻¹ for attached bacteria and 0.34–1.37 amol amino acids cell⁻¹ h⁻¹ and 0.07–0.22 amol glucose cell⁻¹ h⁻¹ for total bacteria). The apparent K _m values were also higher in attached bacteria than in total bacteria, both for amino acids and glucose. These results would reflect the presence of different uptake systems in attached and free-living bacteria, which is in accordance with the different nutrient characteristics of their microenvironments, ambient water and particles. Attached bacteria show transport systems with low affinity, which characterise a bacterial community adapted to high concentration of substrates. Received: 13 June 2000 / Accepted: 6 December 2000     

Interactions between NH+4 and NO−3 uptake and assimilation: comparison of diatoms and dinoflagellates at several growth temperatures

Ammonium concentrations of ∼1 M are commonly cited as being the threshold for inhibition of NO₃ ⁻ uptake, but the applicability of this threshold to phytoplankton from different taxonomic classes has rarely been examined. Additionally, little is known about the influence of environmental variables (e.g. growth temperature) on the interaction between ambient NH₄ ⁺ and NO₃ ⁻ uptake. Four species of estuarine phytoplankton, two diatom [Chaetoceros sp., and Thalassiosira weissflogii (Grunow) Fryxell et Hasle] and two dinoflagellate [Prorocentrum minimum (Pavillard) Schiller, and Gyrodinium uncatenum Hulburt], were grown on NO₃ ⁻ at several different temperatures (4, 10, 15, or 20 °C), and the impact of NH₄ ⁺ additions on NO₃ ⁻ uptake/assimilation (non-TCA-extracted) and assimilation (TCA-extracted) was assessed. For all species at all temperatures, NO₃ ⁻ uptake/assimilation and assimilation rates decreased in a roughly exponential manner with increasing NH₄ ⁺ concentrations but were not completely inhibited even at elevated NH₄ ⁺ concentrations of 200 μM. Estimated half-inhibition concentrations (K _i) were significantly greater in the diatom species (mean ± SE; 2.70 ± 0.67 μM) than in the dinoflagellate species (1.26 ± 0.55 μM). Half-inhibition constants were positively related to temperature-limited relative growth rate although not significantly. The observed inhibition of NO₃ ⁻ uptake and assimilation, as a percentage of NO₃ ⁻ uptake in the absence of NH₄ ⁺, averaged about 80% and ranged from 49 to 100%. For all species, a significant (P < 0.001) positive correlation was found between percent inhibition of NO₃ ⁻ assimilation and temperature-limited relative growth rate. Two experiments on Chesapeake Bay phytoplankton during an April 1998 diatom bloom showed that in short-term (∼1 h) temperature manipulation experiments, percent inhibition of NO₃ ⁻ uptake/assimilation was also positively related (P = 0.05) to experimental temperature. The observed relationships between temperature-limited relative growth rate and percent inhibition of NO₃ ⁻ assimilation rates for the species tested suggest that at the enzyme level, the inhibitory mechanism of NO₃ ⁻ assimilation is similar among species, but at the whole cell level may be regulated by species-specific differences in the accumulation of internal metabolites. These findings add not only to our understanding of species-specific variability and the role of growth temperature, but also provide additional data with which to evaluate current models of NH₄ ⁺ and NO₃ ⁻ interactions. Received: 31 August 1998 / Accepted: 7 December 1998     

Tissue distribution and neurotoxic effects of domoic acid in a prominent vector species, the northern anchovy Engraulis mordax

The planktivorous northern anchovy is a prominent vector of the phycotoxin domoic acid (DA) to organisms at higher trophic levels, including fish-eating seabirds and mammals. Although there are abundant data reporting DA-induced excitotoxic symptoms in higher vertebrates, to date there has been no reported evidence of neurotoxic effects in lower vertebrate vectors such as fish. To explain this apparent lack of toxicity, it has been suggested that DA may not reach the brain in anchovies and/or that fish are not as sensitive neurologically to DA. In the present study, intracoelomic (IC) injection of DA, at doses ranging from 1 to 14 μg DA g⁻¹ total fish weight, resulted in severe neurotoxic symptoms such as spinning, disorientation, inability to school, and mortality, indicating that anchovies are neurologically susceptible and that DA crosses the blood–brain barrier in fish. An ED₅₀ of 3.2 μg DA g⁻¹ total body weight was determined via IC injection of DA in 83 anchovies. Comparable intraperitoneal injection studies with mice, rats, and monkeys report similar DA-induced neurotoxic symptoms at doses near 3.2 μg DA g⁻¹, suggesting a similar neurologic sensitivity and mechanism of toxicity between anchovies and mammals. DA tissue distribution measurements from freshly collected field-exposed anchovies and orally gavaged anchovies indicate that DA uptake from the gastrointestinal tract does occur. Levels as high as 1,175 μg DA g⁻¹ were measured in anchovy viscera, while muscle and brain tissue DA levels were 3 orders of magnitude lower, indicating low but measurable DA uptake. Further evidence is needed to confirm that uptake is sufficient during field events to induce symptoms in anchovies. Our work provides the first reported evidence of neurotoxic symptoms in fish and suggests that anchovies may be affected by DA during toxic diatom blooms. If sufficient uptake occurs, DA-induced neurotoxic symptoms and mortality may make fish easier prey targets, thereby selecting for the highest toxin levels transferred, as well as providing a possible pathway for the transfer of DA to benthic communities. Received: 19 May 2000 / Accepted: 29 November 2000     

Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day⁻¹) and microzooplankton grazing (range: 0.00–2.10 day⁻¹) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l⁻¹). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l⁻¹. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs. Received: 12 November 1999 / Accepted: 4 October 2000     

Biomass partitioning of benthic microbes in a Baltic inlet: relationships between bacteria, algae, heterotrophic flagellates and ciliates

 The structure of a benthic microbial food web and its seasonal changes were studied in the shallow brackish waters of the island of Hiddensee, northeastern Germany, at two sites in close proximity by monthly or bimonthly sampling from July 1995 to June 1996. Abundance and biomass of phototrophic and non-phototrophic bacteria, heterotrophic flagellates (HF) and ciliates as well as the biomass of microphytobenthos were determined in the upper 0.3 cm sediment layer. Abundance of organisms showed strong positive correlation with water temperature, with the exception of the bacteria. Non-phototrophic bacterial numbers ranged from 7 × 10⁸ to 6.7 × 10⁹ cells cm⁻³ and phototrophic bacterial abundance from 4 × 10⁷ to 2.7 × 10⁸. Heterotrophic protist abundance ranged from 8 × 10³ to 104 × 10³ ind cm⁻³ for HF and from 39 to 747 ind cm⁻³ for ciliates. The biomass partitioning demonstrated the primary importance of non-phototrophic bacteria (min. 0.83, max. 84.87 μg C cm⁻³), followed by the microphytobenthos (min. 1.32, max. 50.93 μg C cm⁻³). The heterotrophic protists contributed roughly the same fraction to the total microbial biomass, with the biomass of the HF being slightly higher (HF 0.23 to 1.76 μg C cm⁻³, ciliates 0.04 to 1.17 μg C cm⁻³). Taxonomic classification of the benthic HF revealed the euglenids to be the most important group in terms of abundance and biomass, followed by thaumatomastigids and kinetoplastids. Other important groups were apusomonads, cercomonads, pedinellids and cryptomonads. The structure of the HF assemblage showed strong seasonal changes with euglenids being the most abundant taxa in summer, while apusomonads and thaumatomastigids were predominant in winter. Similar to the pelagic microbial food web, benthic picophototrophic bacteria were occasionally abundant, and the feeding modes of heterotrophic protists exhibited a great variety (predominantly omnivores, bacterivores, herbivores or predators). Filter-feeding HF were of little importance. Contrary to the pelagic environment, a top-down control on total benthic bacterial numbers by HF seemed unlikely at the studied stations which were characterised by muddy sand. Received: 6 January 1999 / Accepted: 21 October 1999     

Activity and inhibitor sensitivity of ATPases in the hydrothermal vent tubeworm Riftia pachyptila : a comparative approach

Phosphorylated ATPases may be involved in the effective pH regulation seen in the hydrothermal vent tubeworm Riftia pachyptila. R. pachyptila appears not only to have a large concentration of ATPases, but the main function of these ATPases seems to have shifted from other types of transport, such as Na⁺ and K⁺ movement, to the facilitation of H⁺ elimination. Plume and trophosome ATPase activity for R. pachyptila measured 646.2 ± 29.5 and 481.4 ± 32.0 μmol Pi (inorganic phosphate) g⁻¹ wet wth⁻¹, respectively. Plume tissue ATPase activity (both mass-specific and protein-specific) in R. pachyptila was higher (between 7% and 55%) than the activity measured in any tissue for 7 other shallow- and deep-living species, in this study. This supports the hypothesis that R. pachyptila regulates acid/base balance via high concentrations of H⁺-ATPases, including Na⁺/H⁺ and K⁺/H⁺ exchangers and possibly electrogenic H⁺-ATPases, as evidenced by a higher total ATPase concentration (646 μmol Pi g⁻¹ wet wt h⁻¹), lesser Na⁺/K⁺-ATPase activity (13% of the total, as compared to 20−40% found in other animals), and higher H⁺-ATPase activity (226–264 μmol Pi g⁻¹wet wt h⁻¹). Overall, R. pachyptila appears to demonstrate elevated ATPase activity, with a greater fraction of the enzymes devoted to proton elimination, in order to effectively control its extracellular pH in the face of processes acting to acidify the internal environment. Received: 9 May 2000 / Accepted: 4 October 2000     

Effect of nutrient enrichment in the field on the biomass, growth and calcification of the giant clam Tridacna maxima

Nutrients were added separately and combined to an initial concentration of 10 μM (ammonium) and/or 2 μM (phosphate) in a series of experiments carried out with the giant clam Tridacna maxima at 12 microatolls in One Tree Island lagoon, Great Barrier Reef, Australia (ENCORE Project). These nutrient concentrations remained for 2 to 3 h before returning to natural levels. The additions were made every low tide (twice per day) over 13 and 12 mo periods for the first and second phase of the experiment, respectively. The nutrients did not change the wet tissue weight of the clams, host C:N ratio, protein content of the mantle, calcification rates or growth rates. However, ammonium (N) enrichment alone significantly increased the total population density of the algal symbiont (Symbiodinium sp.: C = 3.6 · 10⁸ cell clam⁻¹, N = 6.6 · 10⁸ cell clam⁻¹, P = 5.7 · 10⁸ cell clam⁻¹, N + P = 5.7 · 10⁸ cell clam⁻¹; and C = 4.1 · 10⁸ cell clam⁻¹, N = 5.1 · 10⁸ cell clam⁻¹, P = 4.7 · 10⁸ cell clam⁻¹, N + P = 4.5 · 10⁸ cell clam⁻¹, at the end of the first and second phases of the experiment, respectively), although no differences in the mitotic index of these populations were detected. The total chlorophyll a (chl a) content per clam but not chlorophyll a per cell also increased with ammonium addition (C = 7.0 mg chl a clam⁻¹, N = 13.1 mg chl a clam⁻¹, P = 12.9 mg chl a clam⁻¹, N + P = 11.8 mg chl a clam⁻¹; and C = 8.8 mg chl a clam⁻¹, N = 12.8 mg chl a clam⁻¹; P = 11.2 mg chl a clam⁻¹, N + P = 11.3 mg chl a clam⁻¹, at the end of the first and second phases of the experiment, respectively). The response of clams to nutrient enrichment was quantitatively small, but indicated that small changes in inorganic nutrient levels affect the clam–zooxanthellae association. Received: 2 June 1997 / Accepted: 9 June 1997