Observations of heterotrophic activity on photoassimilated organic matter

The natural heterotrophic utilization of released and particulate photoassimilated compounds by natural phytoplankton populations was followed during the summer of 1975 in the Kiel Bight (Western Baltic Sea). Phytoplankton exudates (2 to 21% of primary production) seem to represent an important substrate for heterotrophic bacteria: uptake rates between 8 and 17,5%/h were observed. Samples incubated with antibiotics indicated that the heterotrophic uptake of phytoplankton exudates during photosynthesis could lead to underestimation of the release rate. Production of 14_{CO 2}was used as an estimate of the mineralization of particulate photoassimilated matter. The mineralization rates measured were in the range of 6 to 20% after 20 h. In order to confirm the quantitative measurements, micro-autoradiographic examinations of the samples were performed.     

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.     

Influence des conditions hivernales sur les productions phyto- et zooplanctoniques en Méditerranée Nord-Occidentale. IV. Distribution verticale des taux d'adénosine triphosphate et de production primaire

This paper describes a method which utilizes ¹⁴C-labelled glucose for measuring the assimilation and mineralization of dissolved organic compounds in the sea. Incubation is carried out in 500 ml conic flasks, in darkness, at a temperature similar to that of the in-situ sampling levels. Direct trapping of CO₂ respired is performed in a scintillation vial, with an efficiency of 92% over a period of 4 h. The hyamin, utilized to trap the CO₂, excites the scintillating liquid in the presence of CO₂; the respiration data must then be corrected by a factor of 0.62. When HA Millipore filters are used for filtration, they quench assimilation values and the results must be corrected by a factor of 1.06. The reproducibility of the described method is good. Some parameters have been calculated: total heterotrophic activity, assimilation rate, respiration rate, bacterian yield, and the turn-over time of substrate in the water.     

Biodegradation of aromatic hydrocarbon in marine sediments of three North Sea oil fields

On a survey around three North Sea oil fields in 1980 the biodegradation of the hydrocarbons ¹⁴C-naphthalene and ¹⁴C-benzo[a]pyrene was measured. An amino acid mixture was taken as reference substrate for total heterotrophic activity. The rate of uptake of naphthalene was found to increase with decreasing distance from the oil platform at all fields with maximum values at stations 0.5 miles (ca 0.8 km) from the centre of oil activity. Data on naphthalene uptake parallel GC-MS data for aromatic hydrocarbons and allow an estimation of the actual uptake velocity of naphthalene in the sediment to be made. A significant mineralisation of benzo[a]pyrene could only occasionally be detected. The respiration of amino acids did not show any pattern related to the oil field activities.     

Pyruvate-utilizing bacteria as potential contributors to the food web in the Arabian Gulf

Water samples from the Kuwaiti coasts of the Arabian Gulf were used for counting and isolating bacteria capable of growth on low molecular weight organic compounds known to be released by picocyanobacteria. The compounds tested were potassium acetate, sodium pyruvate, fumaric acid, succinic acid, sodium citrate and glycerol. For comparison, the bacterial numbers on glucose (a conventional carbon source) and Tween 80 and crude oil (unconventional carbon sources), as sole sources of carbon and energy were also determined. Sodium pyruvate was, in most cases, the carbon and energy source most commonly utilized by the cultivable surface water bacteria. The most common cultivable bacterial genera on the test carbon sources were Pseudoalteromonas, Vibrio, Cobetia and Roseobacter. Less common genera were Rhodococcus, Pseudomonas and Bacillus. Quantification of heterotrophic bacteria associated with cultures of local picocyanobacterial strains, originally isolated from the Gulf surface water, also revealed that the carbon source most commonly utilized by cultivable bacteria was sodium pyruvate. However, a different bacterial composition was identified, with Alcanivorax, Stappia and Marinobacter as the major heterotrophic genera. All heterotrophic bacteria that grew on sodium pyruvate could also grow on β-alanine, as sole sources of carbon and energy. From this study we suggest that the Arabian Gulf bacteria utilizing picocyanobacterial low molecular weight compounds, particularly pyruvate may potentially contribute to the food web in this aquatic system. The experiments comply with the current laws of Kuwait.     

Production primaire pélagique de l'Etang de Berre en 1977 et 1978. Comparaison avec le milieu marin (Méditerranée nord-occidentale)

Measurements of primary production and photosynthetic efficiency were carried out in the brackish lake “Etang de Berre” near Marseilles (France), which is diluted by the Durance River, and in the area of Carry-le-Rouet (Mediterranean Sea) about 25 km off the Rhône River outlet. Primary production (¹⁴C) estimations were made in Etang de Berre from December 1977 to November 1978. The carbon uptake rates ranged between 38 and 1 091 mg C m^-3 d^-1, with an average surface value of 256 mg; water-column carbon-uptake rates ranged between 240 and 2 310 mg C m^-2 d^-1, with an average of 810, representing 290 g C m^-2 per year and 45 000 tons per year of photosynthetized carbon for the whole lake. Gross photosynthetic production measured by the method of Ryther was studied over a 2 yr period. The values obtained from marine water (Carry-le-Rouet) ranged from 23 to 2 337 mg C m^-2 d^-1, with a weighted average of 319, representing about 110 g C m^-2 per year. The values in brackish water (Etang de Berre) ranged from 14 to 1 778 mg C m^-2 d^-1, with a weighted average of 682, representing 250 g C m^-2 per year and 38 400 tons per year of photosynthesized carbon for the whole lake. The values derived from both methods of primary production measurements are approximately similar. Net production (computed from biomass estimations by Utermöhl's method) was compared with gross photosynthetic production. The net production in marine water did not display significant variations: most values were usually near zero. On the other hand, net production in brackish water exhibited a number of clear variations compared with concentrations of gross photosynthetic production during the whole 2 yr period. This large difference between estimations of gross and net production may be due to grazing, which is high in Etang de Berre, but slower and more constant in seawater. The ratios of primary production: chlorophyll a and gross photosynthetic production: biomass were also studied. In Etang de Berre, the former ratio ranges between 0.57 and 3.75, with an average of 1.44; this is similar to previously reported values. The ratio gross production: biomass in Etang de Berre varies between 0.3 and 4.2, with an average of 1.27, also confirming previous data. The very high values calculated for marine waters in the present study may result from an under-estimation of biomass.     

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.     

Production organique primaire dans un milieu saumâtre eutrophe (Etang de Berre). Effets d'une forte dilution (dérivation des eaux de la Durance)

¹⁴C primary production measurements were made over a period of 5 years (1965–1969, inclusive) in the brackish lake Etang de Berre, near Marseilles (France). The diversion of the River Durance into the Etang de Berre took place during this period (March 1966) and introduced an important modification into the organic production ecosystem, mainly through increased and variable freshening, accompanied by substantial nutrient input. The seasonal distribution of production rates displayed 3 bloom periods: the first (short and slight) in spring, the second (the most important as regards intensity and duration) in summer, and the third in autumn (October). Before the diversion of the river in 1965, the carbon-uptake rates in the lake ranged between 25 mg/m²/day in winter and 800 mg/m²/day in summer-autumn, the mean value for the year being 150 g C/m², which represents 2.5×10⁴ tons of photosynthesized carbon for the whole lake. After the diversion, more than 3000 mg C/m² day were measured; for 1968, the inclusive uptake rate was 384 g C/m², representing 6×10⁴ tons of synthesized carbon for the whole lake. Nevertheless, noticeable variations occurred from one year to another. From the annual nutrient input of phosphate to the Etang de Berre through the inflow of Durance waters, the quantity of potentially synthesizable elements has been calculated, in terms of carbon, according to the normal P:C ratio of organic substances; this quantity is called R. The difference between measured production, P, and R gives a measure of the regenerated production. This portion of production represented about 80% of the total production before 1968 but only 16% in 1969, a year of maximum fresh-water inflow. This phenomenon could be due to modifications of the ecophysiology of the phytoplankton resulting from the considerable freshening. With increasing nutrient load, eutrophication first occurs, then still greater dilution results in inhibition of production.     

Carbon fixation and analysis of assimilates in a coral-dinoflagellate symbiosis

Freshly collected pieces of the hermatypic coral Acropora cf. scandens containing dinoflagellate endosymbionts (presumably Gymnodinium microadriaticum) were allowed to assimilated ¹⁴C from H¹⁴CO ₃ ^- in the light and in the dark. Time-dependent carbon uptake resulted in intense ¹⁴C-labelling of ethanol-soluble as well as of insoluble assimilates. About forty ¹⁴C-labelled assimilates have been identified. Polymeric (ethanol-insoluble) compounds achieve about 30% of total radiocarbon incorporation after 60 min incubation. Kinetics of ¹⁴C-labelling of single assimilates are analyzed. Percentages of typical photosynthates in the soluble fraction undergo characteristic time-dependent changes. Lipids proved to be the main accumulation products of carbon assimilation by incorporating more than 50% of ¹⁴C after 60 min photosynthesis. The data indicate that low-molecular weight photosynthates such as ¹⁴C-glycerol and ¹⁴C-glucose are rapidly converted to constituents of the polymeric fraction(s) of the coral. Besides peptides, polysaccharides, and lipophilic substances, considerable amounts of ¹⁴C are confined to skeletal CaCO₃ of the coral. The results are discussed with respect to trophic and metabolic interrelationships between the autotrophic dinoflagellates and the A. cf. scandens tissues.     

Post-bloom grazing byCalanus glacialis,C. finmarchicus andC. hyperboreus in the region of the Polar Front,Barents Sea

The plankton community in the Polar Front area of the Barents Sea was investigated during a cruise from 14 to 28 July 1987. The colonial algaePhaeocystis pouchetii andDinobryon pellucidum dominated the phytoplankton. Depth integrated carbon assimilation rates varied from 190 to 810 mg C m^–2 d^–1. A high carbon:chlorophyll ratio (which varied from 123 to 352) prevailed at the three stations investigated, which may relate to facultative heterotrophic behaviour byD. pellucidum. The herbivorous zooplankton community was dominated byCalanus glacialis, C. finmarchicus, andC. hyperboreus. Maximum zooplankton biomass was found in the same depth strata as phytoplankton chlorophyll maximum. The herbivorous copepod populations did not display consistent day-night vertical migration patterns. Phytoplankton consumption rates of the various life stages were estimated from the turnover rate of plant pigments in the gut. The gut defecation rate constant (R) varied from 0.014 to 0.027 min^–1 at 0°C in copepodites (Stage II to adult female) ofC. glacialis, independent of developmental stage.Calanus spp. community carbon ingestion rates calculated from particulate carbon:chlorophyll ratios, were 10, 65 and 400% of daily phytoplankton carbon fixation rates at Stations 1, 2 and 3, respectively.     

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     

Spatial variation of growth and gonadal developments of Maurolicus muelleri in the Norwegian Sea and in a Norwegian fjord

The epibenthic megafauna of the high-Arctic Northeast Greenland shelf was investigated by means of seafloor photography and Agassiz trawl catches. At 54 stations in water depths between 40 and 770 m, sequences of color slides, each depicting about 1 m² of the seafloor, were obtained along photographic transects of about 100 to 600 m length. The photographs were quantitatively analyzed for abundance of epibenthic organisms identified by comparison with specimens collected from trawl catches. Megabenthic biomass was estimated by multiplying density values with averge body mass figures. For five dominant brittle star species, the population oxygen uptake and, thus, organic carbon mineralization potential were approximated by applying individual respiration rates of average-sized specimens to density figures. Multivariate analyses of the megabenthic species distribution revealed a distinct depth zonation. Shallow shelf banks (<150 m), characterized by coarse sediments, many stones and boulders as well as negative bottom water temperatures, housed a rich epifauna (30 to 340 ind m^–2, 1.8 to 10.5 g AFDW m^–2), strongly dominated (80 to 98% by numbers) by the brittle stars Ophiocten sericeum and Ophiura robusta. The oxygen uptake by brittle stars ranged from 0.4 to 95 mol O₂ m^–2 h^–1 (i.e., assuming a respiratory quotient of 0.8, an organic carbon mineralization of 0.1 to 21.9 mg C m^–2 d^–1). At the bank flanks sloping to the shelf troughs (100 to 580 m), finer sediments prevailed, stones were rare, and bottom water temperatures were positive due to the inflow of Atlantic water. Compared to bank sites, total epibenthic abundances as well as carbon mineralization by brittle stars were roughly ten times and total biomass about four times smaller. In deep shelf depressions as well as at the continental slope (200 to 770 m), stones were completely lacking, and sediments very fine. Epibenthic standing stock and carbon mineralization were one to two orders of magnitude lower than on the banks. The estimation of brittle star oxygen uptake indicates that a considerable portion of the organic carbon produced in the polynya and partitioned to the benthos may be remineralized by epibenthic bank assemblages.     

Growth and herbivory by heterotrophic dinoflagellates in the Southern Ocean,studied by microcosm experiments

Growth and herbivory of heterotrophic dinoflagellates (Gymnodinium sp.) from the Weddell Sea and the Weddell/Scotia Confluence were studied in 1988 in 100-liter microcosms. The microcosms were screened through 200-µm or 20-µm mesh nets and incubated for 12 d at 1 °C under artificial light. Mean cell volume of dinoflagellates was 1 000 to 1 500µm³, and that of their phytoplankton prey 360 to 430µm³. Dinoflagellate growth rate followed a Holling type II functional response, with a maximum growth rate of 0.3 d^–1 and half-saturation food concentrations of 1.0µg chlorophylla l^–1, 50µg C l^–1, or 1 500 cells ml^–1. Carbon budgets based on¹⁴CO₂ assimilation and biomasses of phytoplankton and heterotrophic dinoflagellates suggested a balance between phytoplankton grazing loss and dinoflagellate consumption, assuming a dinoflagellate carbon conversion efficiency of 40%. Applying this to the functional response yielded estimates of maximum ingestion rate (0.8µg Cµg^–1 C d^–1, or 6 pg C dinoflagellate^–1 h^–1) and maximum clearance (0.8 to 1.2 × 10⁵ body volumes h^–1, or 80 to 120 nl ind.^–1 h^–1). The microcosm experiments suggested that heterotrophic dinoflagellates may contribute significantly to maintenance of low phytoplankton biomass in the Southern Ocean.     

Nutrition of the temperate Australian soft coral Capnella gaboensis

We examined the ability of Capnella gaboensis Verseveldt, 1977 (Coelenterata: Octocorallia: Alcyonacea: Nephtheidae) to utilize heterotrophic food sources, and the importance of heterotrophic nutrition and photosynthesis in its diet, by using preserved material and histological sections of field-collected specimens and by means of laboratory experiments in which coral branches were fed with ¹⁴C-labelled food of different sizes. The study was conducted from April 1982 to August 1984. C. gaboensis receives nutrition from the photosynthesis of its symbiotic zooxanthellae, Symbiodinium sp., and from heterotrophic sources. Up to 10% of the algal photosynthate was translocated to the animal-host tissues. The contribution of translocated carbon from the zooxanthellae to the daily respiratory carbon requirement of the animal was estimated to be well below 50% in all seasons except in the summer of 1983–1984, indicating that the coral must rely on additional sources of nutrition (i.e., heterotrophy) for most, if not all, of the year. Field (Sydney Harbour: 33°50S; 151°15E) and laboratory observations and experiments indicated that this coral probably feeds upon zooplankton, small particulate matter and dissolved organic matter.     

Fate of herbicides and nonylphenol in soil–plant–water systems amended with contaminated sewage sludge

We studied the fate of sludge spiked with ¹⁴C-labelled diuron, glyphosate and nonylphenol applied to the soil by the way of contaminated sewage sludge in the soil-plant-water system. Here we show that the mineralization of the chemicals in mixture is reduced by 40–80% by comparison with a direct soil contamination. The persistence of the chemicals in soils is increased in the presence of sludge. We showed also that the chemicals present in the sludge are mobile and partly transferred to soil leachates and plant seedlings. These results allow postulating that these compounds may induce an ecotoxicological impact on the soil ecosystem.     

Sedimentation and benthic mineralization of organic detritus in a Norwegian fjord

The supply of particulate material to the sea-bed as well as the oxygen consumption and the redox potential of the sea-bed were measured during a one-year period (1979/1980) at 60 and 90 m depth in the inner part of a west Norwegian fjord, Fanafjorden. At both sites, uniform sedimentation rates of total particulate material (825 and 885 g m^-2 yr^-1, respectively) and particulate inorganic material (576 and 616 g m^-2 yr^-1, respectively) were found. The sedimentation rates of particulate organic carbon (96 and 107 g m^-2 yr^-1, respectively) and particulate organic nitrogen (10 and 12 g m^-2 yr^-1, respectively) were low in winter, higher in summer and autumn, with maxima in May/June, reflecting similar maxima in the phytoplankton biomass in the area, with 6 to 8 wk delay. The oxygen consumption of the sea-floor was lowest in winter/spring and highest in summer. Thirtytwo and 38 g C m^-2 yr^-1 (respiration quotient=0.85) were metabolized by the sediment at 60 and 90 m, respectively. The simultaneous measurements of sedimentation rates and sediment oxygen uptake throughout a whole year demonstrated that the benthic mineralization is governed by the sedimentation over a longer time-scale, but that seasonal imbalances do occur. A box-model of the flux of particulate organic carbon to the sediment surface is presented, and includes the relevant processes and some quantitative estimates.     

Diel variation of TCO2 in the upper layer of oceanic waters reflects microbial composition,variation and possibly methane cycling

Six diel TCO₂ cycles determined by infrared (IR) photometry from five drift stations occupied between 24 February and 16 March 1979 in the mixed layer of the northwestern Caribbean Sea are examined. Comparison of TCO₂ variation with coincident salinity and O₂ variation demonstrated that TCO₂ often co-varied with these independently measured variables. During five diel cycles TCO₂ variation was characterized by nocturnal production and diurnal consumption. The inverse, diurnal production of CO₂, occurred downstream from Misteriosa Bank, whose corals apparently contributed to a water mass having a twofold increase of POC and a sixfold larger population of heterotrophic nanoplankters. For the five diel studies carried out in waters with balanced or nearly blanced heterotrophic and phototrophic components of the nanoplankton, CO₂ consumption at constant salinity always occurred between 06.00 and 09.00 hrs. Net uptake often continued through 15.00 hrs, but not always in the absence of significant salinity changes. At constant salinity net O₂ evolution never exceeded 0.5 mol l^-1 h^-1 while net CO₂ uptake consistently averaged 3 mol l^-1 h^-1 for an apparent net production of 36 mg C m^-3 h^-1, which greatly exceeds the O₂ changes and open ocean ¹⁴C estimates from the literature. Diurnal consumption was apparently balanced by nocturnal production of CO₂ so that no significant net daily change in TCO₂ was observed. Departures from theoretical PQ and RQ and the possibility of nocturnal variations in formaldehyde and carbonate alkalinity imply that chemotrophs, both methane producers and methane oxidizers, play a significant role in CO₂ cycling. This could be through the metabolism of the nonconservative gases CH₄, CO, and H₂, and a link between chemotrophy and phototrophy through these gases is hypothesized. These open system measurements were subject to diffusion and documentable patchiness, but temporal TCO₂ changes appear to indicate the net direction of microbiological activity and join a growing body of literature showing dynamic variation in CO₂ and O₂ that exceeds estimates by ¹⁴C bottle assays of carbon fixation.     

Different proxies for the reactivity of aquatic sediments towards oxygen: A model assessment

Information on benthic carbon mineralization rates is often derived from the analysis of oxygen microprofiles in sediments. To enable a direct comparison of different sediment environments, it is often desirable to characterize sediments by a single proxy that expresses their “reactivity” towards oxygen. For this, there are three commonly used proxies: the oxygen penetration depth (OPD), the oxygen flux at the sediment-water interface (DOU), and the maximum volumetric oxygen consumption rate (R_max). The OPD can be directly determined from the oxygen depth profile, while the DOU is usually obtained by a linear fit to the oxygen gradient either in diffusive boundary layer. The oxygen consumption rate R_max requires the fitting of a reactive-transport model to the data profile. This article shows that the OPD alone is a suboptimal proxy, because it shows a strong dependence on the half-saturation constant K_s, and secondly, because it is sensitive to the particular re-oxidation conditions right above the oxic-anoxic interface. Similarly, the volumetric oxygen consumption rate R_max is rather strongly dependent on the kinetic model formulation employed. To show this we fitted three different (Bouldin, Blackman and Monod) kinetics to the same oxygen data profiles. When fitting these models, the R_max values obtained differed by 20% for exactly the same oxygen profile. Accordingly, if one reports R_max values, it is crucial to specify the kinetic model alongside. Overall, DOU emerges as sediment reactivity proxy which is the least model dependent.     

Absorption of volatile fatty acids from ambient water by the lugwormArenicola marina

Absorption of volatile fatty acids (propionate, acetate, butyrate, and formiate) from the ambient water by the lugwormarenicola marina was demonstrated using¹⁴C-labelled compounds and by gas-liquid chromatography. Uptake was investigated in the presence of antibiotics. The rate of uptake was highest with propionate (v_max 0,58 mol g^-1 w wt · h, K_t 0.75 mol l^-1). Butyrate, acetate, and formiate were absorbed at lower rates in that order. Absorption of propionate, butyrate, and acetate showed saturation kinetics. In the presence of butyrate and acetate, propionate absorption was inhibited (maximum inhibition 78%). These results indicate that a carrier is involved in the absorption of propionate. Following absorption, the volatile fatty acids were rapidly metabolized. The largest proportion of label was incorporated into amino acids, with the pattern of distribution of radioactivity being specific for each volatile fatty acid.     

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.