Incorporation of organic aggregates by marine mussels

Two marine mussels, Geukensia demissa (Dillwyn) and Mytilus edulis (L.) collected in 1990 in Old Silver Beach, Falmouth, Massachusetts, incorporated nitrogen when fed ¹⁵N-labelled organic aggregates produced from dissolved organic nitrogen released by the brown sea-weed Fucus vesiculosis. Uptake of ¹⁵N on the aggregate diet was linear over the course of 24 h, and unincorporated ¹⁵N was eliminated from the gut after 48 h. Both species of mussels incorporated approximately five times more N when they were fed organic aggregates than when they were fed either ¹⁵N-labelled dissolved organic material (DOM) or particulate detritus, both of which were also derived from the seaweed. Nitrogen uptake was greatest in controls fed the diatom Thalassiosira weissflogii; mussels fed phytoplankton incorporated seven times more nitrogen than those fed aggregates. However, aggregates could supply an estimated 7 to 14% of the N requirements for both mussels, whereas DOM or particulate detritus could only supply 1 to 3%. These data provide evidence that a food web pathway exists from primary producer to released dissolved organic nitrogen to microbial organic aggregate to metazoan consumer, and, further, that it can be more important in a detrital food web than either particulate detritus or DOM.     

Feeding biology of a predatory and a facultatively predatory nematode (Enoploides longispiculosus and Adoncholaimus fuscus)

This paper reports on the feeding biology of a predatory and of a facultatively predatory nematode, Enoploides longispiculosus and Adoncholaimus fuscus, respectively. Both species represent genera which are common and abundant in the littoral of the North Sea and in adjacent estuaries. Observations on the foraging behaviour of both species are given, and for the former species, a range of prey from its natural habitat is identified. Respiration was determined using a polarographic oxygen electrode technique and compared to consumption determined as predation rates on the monhysterid nematode Diplolaimelloides meyli. The daily C-loss due to respiration accounted for 15% of the measured C-consumption in E. longispiculosus and for 111% in A. fuscus, proving the observed feeding rates in the latter species to have been inadequate for the maintenance of its aerobic metabolism. Daily respiration rates at an average environmental temperature were 219 ng C ind⁻¹d⁻¹ for adults of A. fuscus and 21.9 ng C ind⁻¹ d⁻¹ for adults of E. longispiculosus. Using radiotracer techniques, no uptake of bacterial cells or of organic matter in the dissolved phase was demonstrated for E. longispiculosus. In A. fuscus, however, a significant drinking of label in the dissolved or volatile fraction occurred; bacterial cells were taken up at a level insignificant to the nematode's daily C-ration. It is concluded that E. longispiculosus has a fairly strict predatory feeding strategy, while A. fuscus gains a majority of C from additional foraging strategies, among which the uptake of dissolved material and scavenging on macrofauna carcasses (as reported in the literature) may be of particular importance. Received: 28 August 1998 / Accepted: 8 March 1999     

Studies on the transport of sugars in the holothurian Holothuria scabra

Experiments using ¹⁴C sugars were carried out on the holothurian Holothuria scabra Jäger, in order to assess the role of its perivisceral fluid in the translocation and transportation of dissolved organic materials. The results obtained indicate that the perivisceral fluid plays a significant part. Rates of ¹⁴C glucose, fructose and sucrose uptake were followed in major tissues such as the alimentary canal, haemal system, respiratory tree and body wall. The monosaccharides are absorbed more intensively by digestive and haemal systems, the disaccharide by respiratory tree and body wall. It is, therefore, presumed that tissues in H. scabra selectively absorb sugars depending on their metabolic activity.     

The significance of microbial carbon in the nutrition of the deposit feeding polychaete Nereis succinea

A partial carbon budget was calculated for a population of the deposit feeding polychaete Nereis succinea (Frey and Leuckart) for a North Carolina, USA salt marsh in order to determine if the ingestion and assimilation of microbial carbon was sufficient to meet the carbon requirement. Carbon required by the population was estimated by calculating annual production, 2.1 g C m^-2, and respiration, 9.4 g. There was no net release of dissolved organic carbon. Annual consumption of microbial carbon (as determined by ATP) was estimated to be 5.2 g m^-2. Assimilation efficiency of heterotrophic, detrital microbes was estimated to be 57%. If this value is used for all the microbial carbon, then total assimilation was 3.0 g C m^-2, or about one-fourth the carbon requirement. N. succinea was able to assimilate carbon from sterile plant detritus which suggests that some of the carbon needed to balance the budget may come from direct uptake of the plant substrate. Other possible additional sources of carbon include consumption of meiofauna and uptake of dissolved organic matter.     

Experimental evidence for direct nutrient assimilation by the lophophore of articulate brachiopods

The uptake of inorganic and non-nutritive organic molecules has been compared with uptake of nutritive molecules by the articulate brachiopod Terebratalia transversa (Sowerby). Only minimal uptake of Na₂ ¹⁴CO₃ and ¹⁴C-urea was observed, while ¹⁴C-glucose was concentrated extensively. After administration of a dilute solution of ¹⁴C-glucose over timed intervals, whole organ counts and autoradiographs showed that labelled material was accumulated along the exposed ciliated epidermal tissue of the lophophore and mantle and concentrated along the peritoneal lining of the coelom even before appearing in the gastrointestinal tract. The presence or absence of bacteria had little discernible effect upon extent and rate of uptake. The uptake experiments suggest that the lophophore not only creates an inhalent and exnalent current as is common in other filter-feeders, but also appears to be adapted for extraction of dilute nutrients in seawater. This ability of the lophophore to extract nutrients may help explain the evolutionary trend of the lengthening of the articulate lophophore and the reduction of the intestine to a short blind-ended gut.This work was supported by National Science Foundation Grant GB-20067.     

Microbial uptake of dissolved organic matter in Mcmurdo Sound,Antarctica

The distribution and activity of bacterioplankton, and the turnover of dissolved organic matter (DOM) were examined in McMurdo Sound, Antarctica. On the eastern side of the Sound, bacteria averaged 6.5×10⁸ l^-1, and turnover rates of dissolved adenosine triphosphate, D-glucose and l-leucine averaged 16, 116 and 124 h, respecitvely. These molecules as well as thymidine were taken up maximally from 0° to 5°C and near-maximally from -1.5° to 0°C, indicating bacterial adaptation to rapid turnover of dissolved organic matter at the ambient temperature. On the west side of the Sound, bacteria averaged only 0.65×10⁸ l^-1, and turnover times for adenosine triphosphate, D-glucose and lleucine averaged 59, 20454, and 3070 h, respectively. Total microbial adenosine triphosphate (an indicator of total microbial biomass) and chlorophyll a were also much lower at the western than at the eastern side stations. Moreover, no primary production could be detected at one western side station (New Harbor). Thus, in McMurdo Sound, the western side is highly oligotrophic, but the eastern side has an abundant active bacterioplankton, comparable to that of temperate coastal waters.Communicated by N. Holland, La Jolla     

Uptake of organic material by aquatic invertebrates. VI. Role of epiflora in apparent uptake of glycine by marine crustaceans

Soft-bodied marine invertebrates from most invertebrate phyla are capable of taking up amino acids from seawater. Marine crustaceans were originally reported not to accumulate amino acids. Artemia salina, Limnoria tripunctata, Tigriopus californicus, and Corophium acherusicum were examined for their ability to accumulate C¹⁴-labelled glycine from dilute solution in seawater. In initial experiments with Artemia and Limnoria the organisms were found to become very radioactive. When animals were preincubated in streptomycin or in a mixture of antibiotics, uptake of the C¹⁴-glycine was drastically reduced. Microorganisms associated with the arthropods and in the culture media were found capable of accumulating C¹⁴-labelled glycine. Efforts to demonstrate removal of amino acids from seawater by colorimeteric determination of material remaining in the medium were unsuccessful. The microorganisms responsible for the uptake are apparently associated with the exoskeleton of the organisms and not the gut. There is probably no substantial contribution to the nutrition of these crustaceans by the epiflora. Preincubation in the antibiotic mixture depressed the uptake of C¹⁴-glucose and C¹⁴-acetate by about an order of magnitude. It is concluded that there is no evidence for accumulation or assimilation of small organic compounds by the small crustaceans examined.     

Uptake of dissolved organics by marine bacteria as a function of fluid motion

A mass transfer analysis predicts that fluid motion can increase the assimilation of dissolved organics by attached compared to free-living microorganisms under certain conditions. To test this we examined the effect of advective flow and fluid shear on the uptake of model compounds (leucine and glucose) by natural assemblages of heterotrophic bacteria, collected from Roosevelt Inlet, Delaware Bay (USA), in 1989. We found that [³H]leucine uptake by cells held in fluid moving at 20 to 70 m d^–1 was eight times larger than uptake by cells at a velocity of 3 m d^–1. This effect was only observed at low leucine concentrations (ca. 1 nM), when uptake was likely not saturated. When we added leucine at concentrations expected to saturate leucine uptake (ca. 11 nM), fluid motion past cells did not affect uptake. Fluid flow past bacteria did not increase [³H]glucose uptake, and laminar shear rates of 0.5 to 2.1 s^–1 did not increase either glucose or leucine uptake by suspended bacteria. These results indicate that fluid motion increases bacterial uptake of certain lowmolecular-weight dissolved organics only when the microorganism exists in an advective flow field. As predicted from a mass transfer model, fluid shear rates in natural systems are too low to affect bacterial uptake of such compounds.     

A kinetic and autoradiographic study of the direct assimilation of amino acids and glucose by organs of the musselMytilus edulis

The uptake of amino acids and glucose dissolved in sea water by different parts of common mussels (Mytilus edulis, L.) is studied from the first minutes up to 1 week. Autoradiography and donble countings on¹⁴C and²H labelled samples show that, in the first hour, the gills and mantle can concentrate several hundred times the dissolved nutritive molecules from very dilute solutions, whereas the digestive tract is not involved significantly during the first hours. An important -amylasic activity has been detected in the gills. The gill epithelium shows a strong positive reaction with mixtures used for the histochemical detection of chymotrypsin. This suggests that the digestion of small particles as well as the absorption of dissolved food might be initiated on the surface of the palleal-gill areas, and completed later in the hepatic caeca.     

Fluctuations and interactions of bacterial activity in sandy beach sediments and overlying waters

Fluctuations and interactions of the following microbiological variables and sediment properties were investigated on samples from sandy beaches of the Baltic Sea: bacterial number and biomass, net uptake and respiration rate of ¹⁴C-glucose (U), concentration of natural free dissolved glucose and fructose, actual uptake rate and turnover time of glucose, sand-grain size and shape, water and organic matter content of the sediment. Spearman rank correlation analysis demonstrated significant relationships between cell number, biomass and actual uptake rate of glucose. The concentration of natural glucose varied with sand-grain shape, and the uptake rates of glucose were inversely correlated with the water content of the sediment. In the overlying water, cell number and/or biomass were significantly correlated with both concentration and uptake rates of glucose. Partial correlation analysis, however, indicates that, in the water overlying the sediment at least one of the standing crop variables (bacterial number or biomass) is independently variable with uptake activity. The sediment bacterial standing crop does not necessarily reflect metabolic activity. Various significant interactions were noted between mainly sediment properties (water content, organic matter content, grain size) and variables in the water above (cell number, biomass, concentration and uptake rates of glucose).Publication No. 206 of the Joint Research Program at Kiel University (Sonderfor-schungsbereich 95 der Deutschen Forschungsgemeinschaft).     

Comparison between the carbon-14 and oxygen consumption method for the determination of the activity of heterotrophic bacterial populations

The activity of the heterotrophic microbial population in the saline Lake Grevelingen (The Netherlands) and the Mediterranean Etang Salses Leucate (France) was determined by measuring the oxygen consumption rate, and the uptake of ¹⁴C-labelled glycollate, pyruvate and an amino acid mixture. The maximum uptake rate of the applied organic compounds in Lake Grevelingen was generally less than 10% of the carbon mineralization rate calculated from the oxygen consumption experiments. Only for pyruvate and glycollate higher values were found of about 30 to 40% with one exceptionally high value for pyruvate of 149%. However, these higher percentages were found in winter, when the activity of the heterotrophic microbial population was very low. In Etang Salses Leucate higher maximum uptake rates of the ¹⁴C-labelled compounds were found, relating this uptake to the oxygen consumption rate. Yet the maximum uptake rate is still always lower than 35% of the carbon mineralization calculated from the oxygen uptake rate. Taking into account that maximum uptake rates were considered, the results demonstrate that the uptake of ¹⁴C-labelled organic compounds represents a serious underestimation of the activity of the bacterial population in situ. The extent of the underestimation depends on the water type. It was concluded that the determination of the heterotrophic activity by measuring oxygen consumption rates offers a better insight into the carbon mineralization process in natural waters than the uptake experiments with ¹⁴C-labelled substrates.Communication no. 228 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

Studies on the mechanism of biosynthesis of wax esters in Euchaeta norvegica

Preparations of Euchaeta norvegica catalyse the incorporation of (1-¹⁴C) hexadecanol, (9,10-³H₂) oleic acid, (U-¹⁴C) glucose, (U-¹⁴C) alanine and (U-¹⁴C) aspartic acid into wax esters. Both the fatty alcohol and fatty acid moieties of the wax esters are labelled, indicating de novo biosynthesis. A limited interconversion of fatty acid and fatty alcohol has been demonstrated. Incorporation of glucose into wax esters is diminished by prior starving of E. norvegica; it is also reduced in the presence of ecdysterone. Formation of wax esters from glucose is inhibited by increased oxygen tensions, rotenone and dinitrophenol, and is relatively insensitive to decreased oxygen tensions and cyanide. The data are discussed with particular reference to the reasons why wax esters are biosynthesised by calanoids.     

Use of tritiated substrates in the study of heterotrophy in seawater

An improved method is described for the study of heterotrophic utilization of dissolved organic substances by marine microorganisms. The method is based on the use of ³H-labelled organic substrates of very high specific activity, rather than the conventionally used ¹⁴C-labelled substrates. Direct measurement of the rate of tracer uptake at near ambient concentration can thus be made instead of extrapolation using the Michaelis-Menten equation. The method also permits comparison between the rates of tracer uptake in sub-samples exposed to different physico-chemical conditions (temperature, light, pollutants, etc.) without the necessity of determining the ambient substrate concentration. The method was applied to the determination of D-glucose uptake by nearshore and pelagic natural microbial populations, and was found to be sensitive and convenient.     

Phytoplankton exudate release as an energy source for the growth of pelagic bacteria

The release of dissolved organic carbon (DOC) from phytolankton during photosynthesis, and the utilization of this carbon by planktonic bacteria, was studied using ¹⁴CO₂ and selective filtration. Natural sea water samples from a coastal area of the Northern Baltic Sea were incubated in the laboratory for detailed studies, and in situ for estimation of annual dynamics. In a laboratory incubation (at +1°C) the concentration of ¹⁴C-labelled dissolved organic carbon increased for about 2 h and then reached a steady state, representing about 0. 1% of the total DOC. Labelled organic carbon in the phytoplankton and bacterial fractions continued to increase almost linearly. The continuous increase in the bacterial fraction is thought to represent almost instantaneous utilization of the DOC released from the phytoplankton during photosynthesis. As an annual average, in 4 h in situ incubations, about 65% of the labelled organic carbon was found in the phytoplankton fraction (>3 m), about 27% in the bacterial fraction (0.2 to 3 m) and the remaining 8% as DOC (<0.2 m). Large variations in these percentages were recorded. The measured annual primary production was 93 g C m^-2 (March to December), and the estimated bacterial production due to phytoplankton exudates 29 g C m^-2. This represents a release of DOC of about 45% of the corrected annual primary production of 110 g C m^-2 (assuming a bacterial growth efficiency of 0.6).     

Decomposition of urea associated with photosynthesis of phytoplankton in coastal waters

Decomposition of urea in seawater was studied in Mikawa Bay, a shallow eutrophic bay on the southern coast of central Japan. The urea concentration in seawater ranged from 1.3 to 5.9 μg-at. N/1 and comprised 12 to 40% of the dissolved organic nitrogen. Using ¹⁴C labelled urea, the rate of CO₂ liberation from urea and the incorporation rate of urea carbon into the particulate organic matter were determined. For the surface samples, high rates of CO₂ liberation from urea as well as the incorporation of urea carbon into the particulate organic matter were observed in the light, while much lower rates were obtained in the dark. Incubation experiments with exposure to different light intensities revealed that the rate of CO₂ liberation from urea and the incorporation of urea carbon into particulate organic matter changed with light intensity, showing a pattern similar to that of photosynthesis. The highest liberation and incorporation rates were observed at 12,000 lux. Incubation in light and in dark produced marked decreases and increases, respectively, in urea and ammonia, while no appreciable changes were observed for nitrate and nitrite. It is suggested that urea decomposition associated with photosynthetic activity of phytoplankton is one of the major processes of urea decomposition, and that it plays a significant role in the nitrogen supply for phytoplankton in coastal waters.     

Respiration,photosynthesis and carbon metabolism in planktonic ciliates

Release of¹⁴C-labelled carbon dioxide from uniformly labelled cells was used to measure respiration by individual ciliates in 2-h incubations in 1989 and 1990. In a strictly heterotrophic ciliate,Strobilidium spiralis (Leegaard, 1915), release of labelled carbon dioxide was equivalent to ca. 2.8% of cell C h^–1 at 20°C, and there was no difference between rates in the dark and light. In the chloroplast-retaining ciliatesLaboea strobila Lohmann, 1908,Strombidium conicum (Lohmann, 1908) Wulff, 1919 andStrombidium capitatum (Leegaard, 1915) Kahl, 1932, release of labelled carbon dioxide was less in the light than in the dark in experiments done at 15°C. InL. strobila release of radiolabel as carbon dioxide was equivalent to ca. 2.4% of cell C h^–1 in the dark but ca. 1% at 50µE m^–2 s^–1, an irradiance limiting to photosynthesis. InS. conicum release of radiolabel as carbon dioxide was equivalent to ca. 4.4% of cell C h^–1 in the dark, but at an irradiance saturating to photosynthesis (250 to 300µE m^–2 s^–1) there was no detectable release of labelled carbon dioxide. InS. capitatum release of radiolabel as carbon dioxide was equivalent to ca. 4.3% of cell C h^–1 in the dark but at an irradiance saturating to photosynthesis was ca. 2.4% of cell C h^–1. These data, combined with data from photosynthetic uptake experiments, indicate that¹⁴C uptake underestimates the total benefit of photosynthesis by 50% or more in chloroplastretaining ciliates.Contribution no. 7510 from the Woods Hole Oceanographic Institution     

Origin and Distribution of Suspended Organic Matter As Inferred From Carbon Isotope Composition in A Mediterranean Semi-Enclosed Marine System

Abstract

The origin and distribution of suspended organic matter, the trophic features and the stable carbon isotopic composition of particulate organic carbon (POC) were studied monthly in a Western Mediterranean semi-enclosed basin. Sampling stations were selected as a function of wind-exposure and the degree of vegetation cover and then compared with an adjacent unvegetated site. the predominant vegetation was seagrass (Posidonia oceanica and Cymodocea nodosa) and Caulerpa prolifera. Water samples were analyzed for total suspended matter (inorganic and organic fractions), photosynthetic pigments (chlorophyll-a and phaeopigments), dissolved organic carbon, particulate organic carbon and their isotopic composition. Temperature and salinity were also measured at the same sampling sites within range of Mediterranean limits. the suspended organic matter concentration was 1.77 ± 1.55 mg l^?1; the chlorophyll-a concentration was low (0.35 ± 0.24 μg l^?1); the disolved organic carbon concentration was 2,140 ± 2,010 μg l^?1; the particulate organic carbon concentration was 212 ± 106 μg l^?1 and the isotopic composition was 18.77 ± 2.51%_°. There were significant temporal differences except for phaeopigments, POC and its POC isotopic composition, and there were no spatial differences other than for δ¹³C. This picture highlighted a general seasonal trend and trophical features similar to adjacent sea.

Spatial differences in δ¹³C showed that the source of suspended organic matter was different between stations as that between sources and wind-hydrodynamic constraints. In     

A method to determine in situ zooplankton grazing rates on natural particle assemblages

In situ zooplankton grazing rates on natural particle assemblages were stimated by measuring zooplankton uptake of labelled autotrophic (with Na¹⁴CO₃) and heterotrophic (with [methyl-³H]-thymidine) particulate matter in 1-h incubations in clear, Plexiglas, Haney chambers. The in situ grazing rates are in the same range as those measured for zooplankton in the laboratory using standard particle counting techniques. A negative selection coefficient for ³H-labelled particles indicated a lower filtration efficiency or avoidance of these particles by zooplankton.     

Zooxanthellae providing assimilatory power for the incorporation of exogenous acetate in Heteroxenia fuscescens (Cnidaria: Alcyonaria)

The soft coral Heteroxenia fuscescens (Ehrb.) and its isolated zooxanthellae (endosymbiotic dinoflagellates) were investigated with particular regard to uptake and utilization of exogenously supplied ¹⁴C-acetate in the light and in the dark. The incorporation of ¹⁴C from ¹⁴C-acetate into the host tissue and into the zooxanthellae was consistently much higher in the light than in the dark. The incorporated ¹⁴C-acetate was rapidly metabolized by the host and algae and was recovered from different assimilate fractions. The major proportion of radiocarbon from metabolized ¹⁴C-acetate was located in host tissue. The CHCl₃-soluble fraction composed of diverse lipids showed the strongest ¹⁴C-labelling. Zooxanthellae isolated prior to incubation accounted for about 80% of the acetate incorporation recorded for zooxanthellae in situ (in vivo). It is concluded from a comparison of acetate incorporation and conversion under light and dark conditions that most of the lipid reserve of the host tissue originates from fatty acids, which are synthesized within the algal symbionts and are then translocated to the heterotrophic partner via extrusion. The acetate units needed for lipid synthesis are obtained by absorption of free acetate from dissolved organic matter (DOM) in the seawater as well as by photosynthetic assimilation of inorganic carbon. Thus, in H. fuscescens, lipogenesis is operated as a light-driven process to which the zooxanthellae considerably contribute assimilatory power by performing fatty acid synthesis and translocation of lipid compounds to their intracellular environment (host cell). A metabolic scheme is proposed to account for the different pathways of carbon conversion observed in H. fuscescens. The incubations took place in August 1980 and the analytical part from October 1980 to January 1984.     

Uptake of inorganic carbon and internal carbon cycling in symbiont-bearing benthonic foraminifera

Incorporation rates of inorganic carbon and its distribution between the organic matter and the skeleton have been measured using ¹⁴C tracer techniques on two species of symbiont-bearing benthonic foraminifera in the Gulf of Elat: Amphistegina lobifera (a perforate species) and Amphisorus hemprichii (an imperforate species). Under constant experimental conditions, incorporation rates of the radiotracer become linear with time after several hours in A. hemprichii and after one day in A. lobifera. A. lobifera showed a lag time of 24 h for skeletal incorporation, whereas in A. hemprichii uptake into the skeleton started within 2 h. Pulse-chase incubations in radioactive seawater, followed by unlabelled incubations, demonstrate transfer of photosynthetically acquired ¹⁴C into the skeleton of A. lobifera. No such transfer was found in A. hemprichii. The total ¹⁴C uptake by A. lobifera increased during the first 24 h of cold chase incubation. This increase suggests the existence of an internal inorganic carbon pool that was lost (probably evaporated) during the analysis of pulse incubations. However, during the following chase incubations, the ¹⁴C in this pool was incorporated mainly into the skeleton and retained during analysis, causing the increase in the total uptake. No such increase was found in A. hemprichii. Additional ¹⁴C uptake experiments on other species of the genera Operculina, Heterostegina and Borelis suggest that the differences in pathways for incorporation of carbon between A. lobifera and A. hemprichii can be generalized to the perforate and imperforate foraminiferal groups. In perforate species, respired carbon originally taken up through photosynthesis is partly recycled into the skeleton. In imperforate species such a transfer has not been demonstrated. Perforate species seem to have a large internal inorganic carbon pool which serves mainly for calcification and possibly also for photosynthesis, while imperforate species may take up carbon for calcification directly from seawater or have a very small inorganic carbon pool.