Dynamics of the phycotoxin domoic acid: accumulation and excretion in two commercially important bivalves

Batch cultures of the toxigenic diatomNitzschia pungens Grunow f.multiseries Hasle were fed to blue mussels (Mytilus edulis) and deep sea Atlantic scallops (Placopecten magellanicus) to elucidate conditions under which domoic acid (DA) was accumulated and excreted (depurated). Mussels accumulated the toxin to a maximum level of 13 g g^-1, at rates of 0.21 to 3.7 g h^-1 g^-1, dry weight. Accumulation efficiency (the proportion of accumulated DA to estimated net uptake) ranged from 1–5%. The highest filtration rate of 1.71 h^-1 occurred at concentrations of 4–8 × 10⁶ Nitzschia cells 1^-1 with no formation of pseudofeces. Depuration rates between fed and starved mussels over a 2 h test period were the same. The depuration rate of domoic acid was about 17% d^-1 and did not account for the low uptake efficiencies, so it is suggested that most of the DA is lost from mussels in the solution during the feeding process. Domoic acid accumulation in mussels was dependent on the amount of toxin available, which in turn was a function of the density and growth phase of theNitzschia population. Changes in filtration rate withNitzschia concentration and depuration rate with time can account for the DA levels of mussels collected during toxic episodes in Cardigan Bay, Prince Edward Island, Canada in 1988 and 1989.Scallops accumulated DA (0.39–1.3 g h^-1 g^-1, more slowly than mussels, however, accumulation efficiencies ranged from 5–100%. Filtration rates remained relatively low and constant at 0.081 h^-1. Scallops retained domoic acid longer than mussels, a fact which must be considered in the marketing of whole scallops for human consumption.     

Effects of food and silt on filtration,respiration and condition of the freshwater mussel Hyridella menziesi (Unionacea: Hyriidae): implications for bioaccumulation

The effect of exposure to different concentrations of food and suspended silt on filtration, respiration and condition were studied in the freshwater mussel Hyridella menziesi. Using a milk solids-based food and kaolin to simulate silt, mussels were maintained at different combinations of food and silt concentrations for 3 weeks. Between treatments mean filtration rates ranged from 0.97–1.66 l g^–1 h^–1, and respiration from 0.50–1.35 mg O₂ g^–1 h^–1. Silt (non-volatile suspended solids up to 35 mg l^–1) failed to have a significant effect on filtration rate or condition, but with increasing food levels (volatile suspended solids up to 35 mg l^–1) filtration rate was reduced, and condition was reduced at the lowest food concentration (<5 mg l^–1). Respiration showed a food × silt interaction between treatment blocks. When food was low respiration increased with increasing silt concentrations, and when silt was low (<5 mg l^–1) respiration increased with increasing food concentrations. The observed effects of food and silt on filtration, respiration and condition are discussed in terms of their potential for affecting contaminant bioaccumulation. In low-food situations (i.e., <5 mg l^–1), if mussels are pumping large volumes of water, contaminant uptake rates could be enhanced, whereas abundant food would result in lower pumping rates and lower uptake rates. Changes in metabolism with food concentration have implications for contaminant elimination, and changes in biochemical composition associated with changing condition could affect the tissue distribution and retention of contaminants.     

The role of the blue mussel Mytilus edulis in the cycling of nutrients in the Oosterschelde estuary (The Netherlands)

The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured in situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant positive correlation, and the average C:N ratio of the fluxes was 9.4. There was a high release of phosphate, nitrate, ammonium and silicate from the mussel bed into the water column. The effluxes of dissolved inorganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements with individual nutrient excretion rates showed that excretion by the mussels contributed 31–85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17–94% of the ammonium flux from the mussel bed. The mussels did not excrete silicate or nitrate. Mineralization of biodeposition on the mussel bed was probably the main source of the regenerated nutrients.From the in situ observations net budgets of N, P and Si for the mussel bed were calculated. A comparison between the uptake of particulate organic N and the release of dissolved inorganic N (ammonium + nitrate) showed that little N is retained by the mussel bed, and suggested that denitrification is a minor process in the mussel bed sediment. On average, only 2/3 of the particulate organic P, taken up by the mussel bed, was recycled as phosphate. A net Si uptake was observed during phytoplankton blooms, and a net release dominated during autumn. It is concluded that mussel beds increase the mineralization rate of phytoplankton and affect nutrient ratios in the water column. A comparison of N regeneration by mussels in the central part of the Oosterschelde estuary with model estimates of total N remineralization showed that mussels play a major role in the recycling of nitrogen.     

Amino-acid absorption by developing herring eggs

¹⁴C-glycine absorption by eggs of the herringClupea harengus from a 2 µM solution at 15°C depends on the stage of embryonic development. Unidirectional¹⁴C-glycine influx rates are small at early stages: 0.6 ± 0.1 and 0.5 ± 0.1 pmoles egg^–1 h^–1 in embryos 5 h and 28 h after fertilization, respectively. They increase drastically about 51 h after fertilization (prior to blastopore closure) to 3.7 ± 0.9 pmoles egg^–1 h^–1. Glycine uptake steadily continues to increase almost until hatching (maximum values = 18.8 ± 2.7 pmoles egg^–1 h^–1), decreasing slightly prior to hatching. Distribution ratios (radioactivity µl^–1 of egg volume: radioactivity µl^–1 ambient medium) exceed the equilibrium ratio of 1 between 51 h and 78 h after fertilization, reaching values of 4.7 two days prior to hatching, thus suggesting the presence of a transport mechanism capable of transferring the amino acid against the concentration gradient. Curves for concentration-dependent¹⁴C-glycine and¹⁴C--aminoisobutyric acid absorption are very similar; they consist of a linear portion at higher concentrations and a saturable component, indicating a mediated uptake process. Calculations performed by means of aminoacid absorption rates and O₂ uptake data suggest that herring eggs scarcely obtain nutritional benefits from absorption of dissolved amino acids in natural spawning areas.     

Respiration, nutrient excretion and filtration rate of tropical freshwater mussels and their contribution to production and energy flow in Lake Kariba, Zimbabwe

The productivity and ecological role of benthos in man-made Lake Kariba was assessed through the use of P/B-ratios and by measuring the metabolism (respiration, N and P excretion) of the most abundant mussel species (Aspatharia wahlbergi, Corbicula africana and Caelatura mossambicensis) in laboratory experiments. For A. wahlbergi also filtration rate was estimated.The annual production of benthos for the populated 0–12 m interval was estimated at 11.0 g m ^–2 yr^–1 (shellfree dry weight) of which mussels contributed for 8.81 g (80%), snails 2.16 g (20%) and insects 0.03 g (0.3%) respectively. The most important mussel species in the lake were Caelatura mossambicensis (4.97 g m^–2 yr^–1) and Corbicula africana (3.33 g). The dominant snail species was Melanoides tuberculata (1.63 g). For the total lake, also including deeper unpopulated bottoms, the annual production of benthos was 2.70 g m^–2 yr^–1 (shell-free dry weight).Respiration and excretion varied with temperature displaying a bell-shaped relationship. Metabolic rates in Aspatharia wahlbergi increased about 5× between 16.5 °C and the maximum at 34.0 °C and then decreased again at 39.0 °C, when the mussels showed signs of severe stress. Metabolism in Corbicula africana had a lower optimum with fairly constant activity between 18.6 and 29.2 °C, rapidly decreasing above this temperature.The average respiration, nutrient excretion and water filtration rates for mussels in Lake Kariba at 25.2 °C were estimated to about 0.6 mg O₂ 85 µg NH₄–N, 1.5 µg PO₄–P and 0.51 water filtered h^–1 g^–1 shellfree dry weight. This gives that a volume corresponding to about the total epilimnion of the lake is filtered by the mussels annually. Further, mussels can be estimated to remineralise % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGymaiaac+% cacaaI0aaaaa!3A2B!\[1/4\] of the total load of phosphate, and 8 times the total load of nitrogen every year. The population needs 3.5 × 10⁴ tons of organic carbon for its maintainance, which indicates that about 5% of the annual phytoplankton production is channeled through mussels. We conclude that the mussels, rather than being an important food source for fish, seem to play a large role in the nutrient dynamics of Lake Kariba.     

In situ measurements of uptake of inorganic carbon and nitrogen by the aquatic liverworts Jungermannia vulcanicola Steph. and Scapania undulata (L.) Dum. in an acid stream,Kashiranashigawa, Japan

In situ uptake of inorganic carbon and nitrogen by the aquatic liverworts Jungermannia vulcanicola Steph. and Scapania undulata (L.) Dum. was measured in an acid stream, Kashiranashigawa, Japan. The uptake activities were similar in the both species. The activities were highest at the tip of shoots, and decreased gradually towards the base. Carbon uptake at the tip in the light was 10.4 × 10^–4 for J. vulcanicola and 8.1 × 10^–4 g C g dry wt^–1 h^–1 1 for S. undulata. Ammonium was effectively incorporated into the shoots, and the uptake activity at the tip was between 1.9 × 10^–5 and 5.8 × 10^–5 g N g dry wt^–1 h^–1. Nitrate uptake was smaller than ammonium uptake. The ratio of dark to light uptake in ammonium uptake experiments was larger than that in carbon uptake experiments. These results suggest that the liverworts use ammonium as a major nitrogen source, and that ammonium uptake was less dependent on light than carbon uptake.     

Uptake of sulphate,taurine, cysteine and methionine by symbiotic and free-living dinoflagellates

Sulphate uptake by Amphidinium carterae, Amphidinium klebsii and Gymnodinium microadriaticum grown on artificial seawater medium with sulphate, cysteine, methionine or taurine as sulphur source occurred via an active transport system which conformed to Michaelis-Menten type saturation kinetics. Values for K _m ranged from 0.18–2.13 mM and V _max ranged from 0.2–24.2 nmol · 10⁵ cells^–1 · h^–1. K _m for symbiotic G. microadriaticum was 0.48 mM and V _max was 0.2 nmol · 10⁵ cells^–1 · h^–1. Sulphate uptake was slightly inhibited by chromate and selenate, but not by tungstate, molybdate, sulphite or thiosulphate. Cysteine and methionine (0.1 mM), but not taurine, inhibited sulphate uptake by symbiotic G. microadriaticum, but not by the two species of Amphidinium. Uptake was inhibited 45–97% under both light and dark conditions by carbonylcyanide 3-chlorophenylhydrazone (CCCP); under dark conditions sulphate uptake was 40–60% of that observed under light conditions and was little affected by 3-(3,4-dichlorophenyl) 1,1-dimethylurea (DCMU).The uptake of taurine, cysteine and methionine by A. carterae, A. klebsii, cultured and symbiotic G. microadriaticum conformed to Michaelis-Menten type saturation kinetics. K _m values of taurine uptake ranged from 1.9–10 mM; for cysteine uptake from 0.6–3.2 mM and methionine from 0.001–0.021 mM. Cysteine induced a taurine uptake system with a K _m of 0.3–0.7 mM. Cysteine and methionine uptake by all organisms was largely unaffected by darkness or by DCMU in light or darkness. CCCP significantly inhibited uptake of these amino acids. Thus energy for cysteine and methionine uptake was supplied mainly by respiration. Taurine uptake by A. carterae was independent of light but was inhibited by CCCP, whereas uptake by A. klebsii and symbiotic G. microadriaticum was partially dependent on photosynthetic energy. Taurine uptake by cultured G. microadriaticum was more dependent on photosynthetic energy and was more sensitive to CCCP. Cysteine inhibited uptake of methionine and taurine by cultured and symbiotic G. microadriaticum to a greater extent than in the Amphidinium species. Methionine did not greatly affect taurine uptake, but did inhibit cysteine uptake. Taurine did not affect the uptake of cysteine or methionine.     

Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters

Field studies to examine the in situ assimilation and production of ammonium (NH₄ ⁺) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing ¹⁵N-NH₄ ⁺ as a tracer showed the bacterial population taking up 0.041–0.128 g-atoms Nl^–1d^–1, which was 8–25% of total NH₄ ⁺ uptake rates. The large bacterial uptake of NH₄ ⁺ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using ³H-leucine and -adenine were l.0gCl^–1 d^–1 before the bloom and 16.2 g Cl^–1 d^–1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH₄ ⁺ was found to supply 35–60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20–50% of the total amino acids taken up were mineralized to NH₄ ⁺. Bacterial production of NH₄ ⁺ was occurring simultaneously to its uptake and contributed 27–55% of total regenerated NH₄ ⁺ in surface waters. Using a variety of ¹⁵N-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.     

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO ₃ ^– deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO ₃ ^– uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO ₃ ^– additions equivalent to 30 kg N ha^–1 year^–1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used ¹⁵N to determine the kinetic parameters of NO ₃ ^– uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO ₃ ^– –N m^–2 and determined the kinetic parameters of NO ₃ ^– and NH ₄ ⁺ uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO ₃ ^– fertilization (range = non-detectable to 33 nmol NO ₂ ^– g^–1 h^–1). Maximal rates of NO ₃ ^– uptake in overstory trees also were low, ranging from 0.2 to 1.0 mol g^–1 h^–1. In seedlings, the mean V _max for NO ₃ ^– uptake in fine roots (1 mol g^–1 h^–1) was approximately 30 times lower than the V _max for NH ₄ ⁺ uptake (33 mol g^–1 h^–1). Our results suggest that A. saccharum satisfies its N demand through rapid NH ₄ ⁺ uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO ₃ ^– .     

Diel variation in concentration,assimilation and respiration of dissolved free amino acids in relation to planktonic primary and secondary production in two eutrophic lakes

Concentration of dissolved free amino acids (DFAA) and assimilation of the 5 most abundant DFAA (glutamic acid, serine, glycine, alanine and ornithine) were measured at 3-h intervals over 27 h in two Danish, eutrophic lakes. The carbon flux of the amino acid assimilation was compared with the major routes of carbon flux, including primary production, bacterial production and zooplankton grazing. In Frederiksborg Slotssø, the mean DFAA concentration was 275 nM with distinct peaks (up to 783 nM) 3 h after sunrise. Assimilation rates of the 5 amino acids amounted on the average to 2.03 µg Cl^–1 h^–1, but high values up to 7.41 µg Cl^–1 h^–1 occurred 3 h after sunrise and at midnight. The mean turnover time of the amino acid pools was 3.2 h. In Lake Mossø, the mean DFAA concentration was 592 nM with peak of 1 161 nM at dusk. The assimilation rate averaged 0.44 µg Cl^–1 h^–1, and the mean turnover time of the amino acid pools was 39 h. In Lake Mossø, similar turnover times of glutamic acid and serine were determined from the ¹⁴C-amino acid tracer technique and Michaelis-Menten uptake kinetics, indicating that the tracer technique gave reliable values of the actual assimilation. The average respiration percentages of the assimilated amino acids were 45% in Frederiksborg Slotssø and 51% in Lake Mossø. Extracellular organic carbon (EOC) released from the phytoplankton contributed DFAA to the water. In Lake Mossø, 81% of the ambient EOC pool was <700 daltons and 9.3% of the EOC was DFAA. This corresponded to about 2.4% of the DFAA pool. Bacterial productivity, determined by means of frequency of dividing cells and ³⁵S-SO₄ dark uptake techniques gave similar results and constituted 4.5 and 3.7 µg Cl^–1 h^–1 in Frederiksborg Slotssø and Lake Mossø, respectively. The bacterial productivity suggested that DFAA were essential substrates to the bacteria, especially in Frederiksborg Slotssø. The zooplankton biomass in Frederiksborg Slotssø was six times larger than that in Lake Mossø, but cladocerans were dominant in both lakes. The zooplankton grazing probably was an important regulatory factor for the bacterial productivity.     

Uptake and accumulation of zinc in juvenile rainbow trout,Salmo gairdneri

Synopsis The toxicity of zinc to rainbow trout was determined and the 72 h median lethal concentration was found to be 2.00 mg l^–1 in freshwater, hardness 7.50 mg l^–1 as calcium. An insignificant increase in zinc concentration of internal tissues occurred in fish exposed to 1.52 mg l^–1 in freshwater for 72 h. However, there was a significant uptake of zinc by gills and the body surface. Fish exposed to 10 mg l^–1 zinc for 72 h in two-thirds sea water showed significant zinc uptake by liver, rectum and muscle, when compared to control fish. Drinking rate decreased from 1.43 to 0.26 ml kg^–1 h^–1 when zinc sulphate was added to freshwater. Trout adapted to two-thirds sea water showed no decrease in drinking, about 7 ml kg^–1 h^–1 when zinc was added to the water.     

Grazing and assimilation rates of natural populations of planktonic rotifersKeratella cochlearis,Keratella quadrata andKellicottia longispina in a eutrophic lake (Aydat,France)

The filtering rates of¹⁴C labelledChlamydomonas sp. by 3 dominant species of rotifers were studied in eutrophic Lake Aydat. They varied from 4 to 53 µl ind^–1 h^–1 forKeratella cochlearis, from 2 to 56 µl.^–1h^–1 forKeratella quadrata and from 3 to 52 µl ind^–1 h^–1 forKellicottia longispina. Their maximum assimilation efficiency was 32%. At the measured grazing rates, these populations could clear the water in less than two days during July. In Lake Aydat, the rotifers community could play an important role in the regulation of seasonal succession of phytoplankton and bacteria.     

Evidence for amino-acid: proton cotransport in Ricinus cotyledons

During germination and early growth of the castor-bean (Ricinus communis L.), protein in the endosperm is hydrolyzed and the amino acids are transferred into the cotyledons and then via the translocation stream to the axis of the growing seedling. The cotyledons retain the ability to absorb amino acids after removal of the endosperm and hypocotyl, exhibiting rates of transport up to 70 mol g^-1 h^-1. The transport of L-glutamine was not altered by KCl or NaCl in low concentrations (0–20 mM). High concentrations of KCl (100 mM) inhibited transport, presumably by decreasing the membrane potential. An increase in the pH of the medium bathing the cotyledons was observed for 10 min following addition of L-glutamine but not with D-glutamine, which is not transported. The rate of proton uptake was dependent on the concentration of L-glutamine in the external solution. Inhibitors and uncouplers of respiration (azide, 2, 4-dinitrophenol, carbonyl cyanide phenylhydrazone and N-ethylmaleimide) inhibited both L-glutamine uptake and L-glutamine-induced proton uptake. Amino acids other than L-glutamine also caused a transient pH rise and the rate of proton uptake was proportional to the rate of amino-acid uptake. The stoichiometry was 0.3 protons per amino acid transported. Addition of sucrose also caused proton uptake but the alkalisation by sucrose and by amino acids were not additive. Nevertheless, when sucrose was added 60 min after providing L-glutamine at levels saturating its uptake system, a rise in pH was again observed. The results were consistent with amino-acid transport and sucrose transport in castor-bean cotyledons both occurring by a proton cotransport in the same membrane system but involving separate carriers.     

Influence of water activity on Streptococcus diacetylactis metabolism

During the cheese-making process, water activity (a_w) is one of the essential environmental parameters acting on bacterial growth and metabolic pathways. The influence of a_w on Streptococcus diacetylactis growth and lactic acid production was studied. The specific growth rate was linearly related to water availability in the milk medium. The cell behaviour was quite different above and below a_w=0.95, which can be considered a limiting value. Below this value, the lactic acid production reached 1.4–6.1 mg·g^–1, whereas the specific productivity was 2.0–2.6 mg·10^–10 cells·h^–1. Changes in the consumption of lactose and amino acids during the different growth phases was completely modified by decreasing the water availability in the medium. Correspondence to: N. Cochet     

Direct uptake of nitrogen by Pfiesteria piscicida and Pfiesteria shumwayae, and nitrogen nutritional preferences

The rates of uptake of a range of forms of nitrogenous nutrients were measured in cultures of Pfiesteria piscicida and Pfiesteria shumwayae maintained at varying physiological states. The measured rates of dissolved N uptake under some conditions approached the rates of N uptake that are achieved through phagotrophy. Rates of dissolved N uptake by P. piscicida contributed <10% of the cellular N of flagellated cells feeding on algae, but were equal to or greater than phagotrophic N acquisition in cells recently removed from fish cultures. Specific N uptake rates (V, h⁻¹) were higher for cells that were maintained on algal prey for long periods (months) than those that were grown with live fish. However, rates of N uptake on a cellular basis for cells grown on or recently removed from fish were comparable to those maintained on algal prey, likely reflecting differences in the sizes of cells of different physiological condition. Preferences for form of N generally followed a decreasing trend of amino acids > urea > NH₄⁺ > NO₃⁻. Nitrate consistently was not a preferred form of N. Although Pfiesteria spp. are often found in eutrophic environments, the relationship between Pfiesteria spp. and nutrient availability is likely to be primarily indirect, mediated through the production of various prey on which Pfiesteria spp. feed. These findings also confirm, however, that when dissolved N concentrations are elevated, they can contribute to the supplemental nutrition of these cells, and thus may provide a significant source of N to Pfiesteria spp. in nature.     

Extracellular enzymes in a small polyhumic lake: origin,distribution and activities

Seven extracellular enzymes were studied in a small polyhumic lake. Phosphatase, glucosidase and leucine-aminopeptidase were found to contribute strongest to the regeneration of nutrients and processing of polymeric organic substrates. Phosphatase activity varied between 10–800 nmol l^–1 h^–1 with highest activity in May–June. Glucosidase and leucine-aminopeptidase varied between 5–130 nmol l^–1 h^–1 with maxima in early and late summer. Phosphatase and leucine-aminopeptidase are stratified significantly in the water column. About 60% of leucine aminopeptidase could be attributed to bacterioplankton, whereas phosphatase were found to be 50–70% free dissolved in the water.     

The Effects of Food Concentration and Quality on the Feeding Rates of Three Size Classes of the Greenshell<Superscript>TM</Superscript> Mussel, <Emphasis Type="Italic">Perna canaliculus</Emphasis>

In order to calibrate carrying capacity models, investigations were conducted into the effects of food concentration and food quality on the feeding rates of small (25–50 mm), medium (60–85 mm) and large (90–115 mm) Greenshell mussels (Perna canaliculus). Experimental diets varying from 3.3 to 6.0 μg l⁻¹ chlorophyll a concentration and 12–25% organic content were fed to mussels housed in individual flow through chambers. Not surprisingly, this study found that the main factor affecting feeding rates is mussel size. Small mussels were observed to maintain a constant filtration rate of approximately 20 mg h⁻¹ irrespective of food concentration or quality, whereas mussels of greater than 60 mm length had more variable filtration rates between 30 and 80 mg h⁻¹. The filtration rates of these large mussels were also observed to increase positively with organic content, and showed no sign of levelling out, even at the highest organic content tested (25%). Highest rejection rates (50–70 mg h⁻¹) were observed when the organic content of the available seston was low, suggesting that P. canaliculus are able to selectively reject organic material, thereby organically enriching their diet. It appears that the organic content of the seston is the primary determinant of the net efficiency with which food is selected from the available seston by the mussel. The present study shows that P. canaliculus of all sizes are capable of adapting their feeding behaviour to compensate for changes in the food supply, which may occur over relatively short time periods, in the culture environment.     

Net release of dissolved organic matter by the scleractinian coral Acropora pulchra

The net production of dissolved organic matter (DOM) and dissolved combined and free amino acids (DCAA and DFAA, respectively) by the hermatypic coral Acropora pulchra was measured in the submerged condition, and the production rates were normalized to the coral surface area, tissue biomass, and net photosynthetic rates by zooxanthellae. When normalized to the unit surface area, the production rates of dissolved organic carbon and nitrogen (DOC and DON, respectively) were 37 and 4.4 nmol cm^− 2 h^− 1, respectively. Comparing with the photosynthetic rate by zooxanthellae, which was measured by ¹³C-tracer accumulation in the soft tissue of the coral colony, the release rate of DOC corresponded to 5.4% of the daily net photosynthetic production. The tissue biomass of the coral colony was 178 µmol C cm^− 2 and 23 µmol N cm^− 2, indicating that the release of DOC and DON accounted for 0.021% h^− 1 and 0.019% h^− 1 of the tissue C and N, respectively. The C:N ratios of the released DOM (average 8.4) were not significantly different from those of the soft tissue of the coral colonies (average 7.7). While DFAA did almost not accumulate in the incubated seawater, DCAA was considerably released by the coral colonies at the rate of 2.1 nmol cm^− 2 h^− 1 on average. Calculating C and N contents of the hydrolyzable DCAA, it was revealed that about 20% and 50%–60% of the released bulk DOC and DON, respectively, were composed of DCAA.     

Influence of temperature rise on kinetic parameters during batch propagation of Kluyveromyces fragilis in cheese whey under ambient conditions

Three 5 l working volume fermenters were used to investigate the growth of the yeast Kluyveromyces fragilis in acid cheese whey under ambient temperature in order to assess the specific growth rate and yield, the lactose and oxygen uptake rates during the various phases of batch culture, the effect of increasing temperature on the various kinetic parameters, and the need for a cooling unit for single cell production batch systems. The initial dissolved oxygen in the medium was 5.5 mg l^–1 and the pH was maintained at 4.5. The observed lag phase, specific growth rate and maximum cell number were 4 h, 0.2 h^–1 and 8.4 × 10⁸ cells ml^–1, respectively. About 99% of the lactose in cheese whey was utilized within 20 h, 85% during the exponential growth phase. The specific lactose utilization rates by K. fragilis were 0.20 × 10^–12, 1.457 × 10^–12, 0.286 × 10^–12 and 0.00 g lactose cell^–1 h^–1, for the lag, exponential, stationary and death phases, respectively. The dissolved oxygen concentration in the medium decreased as the cell number increased. The lowest oxygen concentration of 1.2 mg l^–1 was observed during the stationary phase. The volumetric oxygen transfer coefficient was 0.41 h^–1 and the specific oxygen uptake rates were 0.32 × 10^–12, 2.14 × 10^–12, 0.51 × 10^–12 and 0.003 × 10^–12 mg O₂ cell^–1 h^–1, for the lag, exponential, stationary and death phases, respectively. The maximum temperature recorded for the medium was 33 °C, indicating that a cooling unit for batch production of single cell protein at ambient temperature is not needed for this type of bioreactor. The increase in medium temperature affected the cell growth and the lactose and oxygen uptake rates.