Oxygen/sulfide regime and nematode fauna associated with Arenicola marina burrows: new insights in the thiobios case

Profiles of oxygen and sulfide around the burrows of the lugworm, Arenicola marina, from a North Sea tidal flat were examined with microelectrodes, and the steep gradients were related to the microdistribution of nematodes. Around the tail shaft free oxygen penetrated only 2 mm into the burrow wall, coinciding with a bright zone sharply limited by the ambient black sediment. Contrastingly, in normal bottoms of the tidal flat (controls) only the surface of the bright zone was supplied with free oxygen. Here, the dark colouration coincided with the presence of free hydrogen sulfide. Around the tail shaft the nearest free hydrogen sulfide was detected 6 mm from the burrow wall leaving several millimetres of black sediment without measurable free sulfide. We discuss how these divergencies may relate to the stability of the oxygen/sulfide gradients and the course of time involved in their formation. A total of 54 nematode species were identified. Based on non-metric Multidimensional Scaling Ordination, four nematode assemblages corresponded to four microhabitats of the A. marina burrow: the funnel, the feeding pocket, the tail shaft and the feacal cast. The tail shaft assemblage (oxic plus partly anoxic zones) was similar to that of the anoxic zone of the control sediment. It was dominated by the most abundant nematode in the present study, Metalinhomoeus biformis (mean abundance in tail shaft 202 indx10 cm^-3). Adults of common nematode species from sulfidic microhabitats had a significantly higher length/diameter ratio than those inhabiting the oxic zone of the control sediment (p<0.001). The chemical recordings and metric analysis indicate that these slender nematodes around the A. marina tail shaft and in the reduced horizons of the reference sites represent thiobiotic assemblages, as compared to the shorter and stouter oxybiotic species characterising the assemblages from the surface zone and (partly) the funnel.     

Stromatolitoid microbial nodules from Bermuda —a special micro habitat for meiofauna

In Bermuda, stromatolitoid microbial nodules are found in dense groups in seagrass beds and on subtidal sandy or rocky bottoms. They develop between January and the beginning of August and may reach 15 cm in diameter and 6 cm in height. Nodules grow on top of the sediment surface and consist of convex interwoven mats of the cyanobacterium Phormidium corium and fine sediment particles trapped in these mats. Nodules were studied in the field and laboratory (in 1989–1992) with respect to their structure, microbial community, sediment chemistry and associated meiofauna. Vertical profiles taken with microelectrodes showed steep gradients of oxygen and sulfide. While free oxygen was only detected in the upper 2 mm, the sulfide concentrations increased with depth and reached maximal 1250 moll^-1 inside the nodules. On the basis of microscopic observations and sediment chemistry, vertical sections through a nodule reveal four distinct layers: (A) an oxic green surface layer of growing cyanobacteria mats, (B) an anoxic yellow-white laminated layer of sediment particles, mucus and unpigmented cyanobacteria sheaths, (C) an anoxic sulfidic (sulfide <50 moll^-1) interface where colour changes from yellow to gray and (D) an anoxic sulfidic (sulfide can increase to 1250 moll^-1) gray core composed of sediment and decomposing cyanobacteria. The following meiofauna taxa were found in the nodules: Ciliata, Turbellaria, Gnathostomulida, Gastrotricha, Nematoda, Kinorhyncha, Polychaeta, Ostracoda and Harpacticoida. The distribution of the meiofauna was significantly different (p<0.05) within the four layers. The highest density of individuals was found in the sulfidic interface and core. Nematodes represented the dominant group in general. Thiobiotic organisms, such as the Gnathostomulida, Solenofilomorphidae, and Stilbonematinae were primarily found in the anoxic sulfidic layers of the nodules.     

Effects of sulfide exposure history and hemolymph thiosulfate on oxygen-consumption rates and regulation in the hydrothermal vent crab Bythograea thermydron

The hydrothermal vent crab Bythograea thermydron is exposed to high environmental concentrations of sulfide and low levels of oxygen for extended periods of time. It has previously been shown that hydrogen sulfide is oxidized to the relatively non-toxic thiosulfate (S₂O ₃ ^2– ), which accumulates in the hemolymph. Hemolymph thiosulfate levels in freshly captured crabs vary significantly among crabs from different hydrothermal vent sites as well as between crabs from different microhabitats within the same site. Hemolymph thiosulfate concentrations were not significantly different between crabs captured at the same site 6 mo apart. Hemolymph thiosulfate concentrations ranged from 66 mol 1^–1 in a crab captured at a site with relatively low sulfide venting, to 3206 mol 1^–1 in an individual that was netted from an active smoker vent with much higher sulfide exposure. The differences in hemolymph thiosulfate between sites and the stability of hemolymph thiosulfate in crabs captured at the same site at different times suggest that sulfide exposure is significantly different between sites and that this exposure may not vary significantly over the course of a few months. B. thermydron experimentally exposed to sulfide had high levels of thiosulfate in their hemolymph and increased abilities to regulate oxygen consumption in conditions of low oxygen. This enhancement of regulatory abilities suggests that the previously demonstrated increased hemocyaninoxygen (Hc–O₂) affinity due to elevated thiosulfate may be adaptive in vivo. Average oxygen-consumption rates were much higher in crabs experimentally exposed to sulfide than in unexposed crabs. Crabs injected with isosmotic thiosulfate did not have increased oxygen-consumption rates as did the sulfide-exposed individuals, but did show a similar reduction in P _c (the critical partial pressure of oxygen at which crabs can no longer regulate oxygen consumption). This suggests that it is the sulfide exposure and/or detoxification rather than the elimination of thiosulfate that causes the increase in metabolic rate. Thiosulfate diffuses into dead crabs and into live crabs exposed to 15 mmol S₂O ₃ ^2- l^–1, indicating substantial permeability, and yet live crabs are able to eliminate thiosulfate when incubated in sea water containing 1.5 mmol S₂O ₃ ^2- l^–1, suggesting a process that has an active component.     

Sulfide-oxidizing bacteria in the burrowing echinoid,Echinocardium cordatum (Echinodermata)

Symbiotic filamentous bacteria thrive in the intestinal caecum of the deposit-feeding echinoid Echinocardium cordatum. Specimens of E. cordatum were collected at Wimereux (Nord Pas-de-Calais, France) in 1991. Their symbiotic bacteria build nodules by forming multilayered mats around detrital particles that enter the caecum. The morphological features of the bacteria are those of Thiothrix, a sulfide-oxidizing genus. The filaments, which may form rosettes, are sheathed and made by a succession of hundreds of rod-shaped bacteria which store elemental sulfur in the presence of external sulfide. Live bacteria are restricted to the outer layers of the nodules. Their sulfide-oxidizing activity was investigated, using a Biological Oxygen Monitor, by measuring the O₂-consumption when reduced sulfur compounds are provided. They oxidize thiosulfate and sulfide. Optimal sulfide oxidation occurs at intermediary pO₂ (100 to 160 M O₂l^-1). Spectrophotometry has confirmed that the sulfur content of the filamentous symbiotic sulfideoxidizing bacteria depends on the presence of external sulfide. This is the first report of symbiotic intradigestive Thiothrixspp.-like bacteria; it lengthens the list of symbioses between sulfide-oxidizing bacteria and invertebrates from sulfide-rich habitats.     

Sedimentation and sulfate reduction under a mussel culture

The sedimentation and dissimilatory sulfate reduction under a blue-mussel culture were quantified in order to gain information on the environmental impact of intense mussel farming. The sedimentation rate (3 g C·m^-2·d^-1) under a culture is nearly three times higher than at a nearby reference station. A build-up of sediment rich in organic material and sulfide takes place under the mussels. At 15°C the sulfate reduction rate was 30.5 mmol SO ₌ ⁴ ·m^-2·^-1 in the upper 10 cm of the mussel sediment. The increase in sedimentation under a mussel culture and the consequent effects should be considered when establishing mussel farms.     

Oxygen dependent sulfide detoxification in the lugwormArenicola marina

The lugwormArenicola marina L. oxidizes entering sulfide to thiosulfate. After 8 h of normoxic incubations with sulfide concentrations of 0.2 to 1.0 mmoll^-1 thiosulfate in the coelomic fluid amounted up to about 4 mmoll^-1 whereas sulfite concentrations were 100-fold lower and no accumulation of sulfate in the coelomic fluid was found. The sulfide oxidation was highly oxygen dependent. An increase of oxygen partial pressure ( ) in the medium was followed by enhanced thiosulfate production and by a decrease of sulfide concentration in the coelomic fluid. Under normoxia, the sulfide oxidation rate was sufficient to compensate the influx of sulfide into the coelomic fluid when the sulfide concentration in the medium was below 0.33 mmoll^-1. When external sulfide was raised beyond this level, sulfide up to 5 moll^-1 in the coelomic fluid appeared. Succinate in the body wall tissue was low as long as no sulfide appeared in the coelomic fluid, indicating the maintenance of an aerobic metabolism. The oxidation of sulfide to thiosulfate was localized in the mitochondria of the body wall tissue. The oxygen consumption of mitochondria was stimulated by the addition of sulfide. The mitochondrial sulfide oxidation rate depended on the amount of mitochondrial protein and followed a Michaelis-Menten kinetic. An apparentK _m of 0.68±0.29 moll^-1 and aV _max of 41.9±22.3 nmol min^-1 mg^-1 protein was calculated. Sulfide was stoichiometrically oxidized to thiosulfate with 1 mol sulfide consuming 1 mol oxygen. Sulfide oxidation was not inhibited by sulfide concentrations as high as 100 moll^-1. At low concentrations of cyanide or azide, when respiration without sulfide was already inhibited, sulfide oxidation could still be stimulated, tentatively indicating the existence of an alternative terminal oxidase. Specimens examined in the present study were collected near St. Pol de Leon, France, from 1989 to 1992.     

Etude des eaux rouges dues à la prolifération des bactéries photosynthétiques sulfo-oxydantes dans l'étang du Prévost,lagune saumâtre méditerranéenne

Dystrophic crises or malaígues with red water, were observed in summer, in Mediterranean brackish lagoons near Montpellier, France. During July 1976, the photosynthetic sulfide-oxidizing bacteria causing this phenomenon were isolated from the Prévost lagoon. The most important genera isolated from this red water wereChromatium, Thiocystis andThiocapsa (Chromatiaceae): the predominant species wasThiocapsa roseopersicina. Water and sediment samples from the same lagoon were collected during the winter season (October 1976 and January 1977) and were enriched with organic matter. The samples were incubated in aquaria under artificial light (800 to 1000 lux) at room temperature (ca. 22°C). Eight to 10 days later, red water developed which was similar to that observedin situ in summer. These red waters develop under conditions of anaerobiosis and H₂S production. Such conditions are necessary for the growth of Chromatiaceae. Excessive deposists of organic matter in the lagoon water lead to increased oxygen consumption and overproduction of hydrogen sulfide, which can be oxidized by photosynthetic sulfideoxidizing bacteria. This bacteria bloom (causing the red water) removes the hydrogen sulfide, thus re-establishing aerobiotic conditions in the water of the Prévost lagoon.     

Burrow environment and coelomic fluid characteristics of the echiuran worm Urechis caupo from populations at three sites in northern California

The burrow microhabitat and physiology of the echiuran worm Urechis caupo at a high-density site (Elkhorn Slough, California, USA), were investigated from 1987 to 1990 to determine physical and chemical conditions, worm density and distribution, and coelomic fluid characteristics such as heme composition, pO₂, pH, and coelomocyte volume. During tidal exposure, worm burrows at this site exhibited, on average, 52% air-saturated water, 11M burrow-water sulfide, 85 g/mg wet weight sediment sulfide, and salinity and osmolalities similar to those of seawater. These conditions are compared to those of another California site, Bodega Bay, which had slightly lower oxygen concentrations, but higher water and sediment-sulfide levels. A more limited comparison to a third site, Princeton Harbor, California, is included. Worms from Bodega Bay, the higher sulfide site, had greater concentrations of hematin, a non-globin heme compound contained in the coelomocytes, and exhibited a greater tolerance to sulfide in the laboratory. These data are consistent with the hypothesis that hematin is a sulfide-detoxifying agent that may enhance survival of U. caupo in the sulfide-rich mudflat environment.     

Impact of the polychaete Capitella sp. I on microbial activity in an organic-rich marine sediment contaminated with the polycyclic aromatic hydrocarbon fluoranthene

Polychaetes belonging to the genus Capitella are often present in high numbers in organic-rich sediments polluted with, e.g., oil components, and Capitella spp. may have a great impact on the biogeochemistry of these sediments. We examined the influence of Capitella sp. I on microbial activity in an organic-rich marine sediment contaminated with the polycyclic aromatic hydrocarbon, fluoranthene. Capitella sp. I were added to microcosms (10 000 ind m⁻²) and the impact of a pulse-sedimentation of fluoranthene-contaminated sediment (3 mm layer) was studied for a period of 12 d after sedimentation. The sediment oxygen uptake and total sediment metabolism (TCO₂ production) increased in cores with worms (71 to 131%), whereas the anaerobic activity, measured as sulfate reduction rate 12 d after sedimentation, was lower compared to cores without worms. The effect of fluoranthene on sulfate reduction was most pronounced in the presence of worms, with a 34% reduction versus 16% in cores without worms. The reduced sulfur pools in cores with worms were smaller than in cores without worms, suggesting that the reduced anaerobic activity was caused by increased oxidation of the sediment, which may favor O₂ and other electron-acceptors (e.g. NO₃ ⁻, Fe³⁺, Mn⁴⁺) in organic matter decomposition. The sediment oxygen uptake and TCO₂ production did not show significant changes due to fluoranthene treatment, indicating that these parameters were either less sensitive to fluoranthene stress or recovered more rapidly (i.e. within 48 h) than sulfate reduction rates. Bioturbation by Capitella sp. I altered the depth profile of fluoranthene such that fluoranthene was found in deeper sediment layers (down to 2 cm) where diffusional loss and microbial breakdown probably are reduced relative to surface layers. In cores without worms, fluoranthene was found down to 1 cm, with 75% remaining in the upper 5 mm. Received: 5 December 1996 / Accepted: 11 February 1997     

Bioturbation effects of the amphipod Corophium volutator on microbial nitrogen transformations in marine sediments

Microcosms containing different densities of Corophium volutator, ranging from 0 to 6000 ind m^-2, were incubated in a flow-through system. Benthic fluxes of CO₂, O₂, NO₃ ^- and NH₄ ⁺ were measured regularly. Thirteen days after setup the microcosms were sacrificed and sediment characteristics, pore water NO₃ ^-, NH₄ ⁺ and exchangeable NH₄ ⁺ concentrations, and potential nitrification activity were measured. The presence of C. volutator increased overall mineralization processes due to burrow construction and irrigation. The amphipods increased the ratio CO₂/O₂ fluxes from 0.73 to 0.86 in microcosms inhabited by 0 and 6000 ind m^-2, respectively. Burrow ventilation removed NH₄ ⁺ from the sediment, which was nitrified in the oxic layer and transported NO₃ ^- to the burrow sediment, where denitrification potential was enhanced. Nitrification and total denitrification rates (denitrification of NO₃ ^- coming from the overlying water and of NO₃ ^- generated within the sediment) were calculated and discussed. Bioturbation by C. volutator increased both nitrification and denitrification, but denitrification was stimulated more than nitrification. Denitrification of NO₃ ^- coming from the overlying water was stimulated 1.2- and 1.7-fold in microcosms containing 3000 and 6000 ind m^-2 relative to control microcosms. The presence of C. volutator (6000 ind m^-2) stimulated nitrogen removal from the system, as dinitrogen, 1.5-fold relative to non-bioturbated microcosms. C. volutator individuals used in our study were collected from Norsminde Fjord, Denmark, in 1990.     

Aerobic nitrogenase activity measured as acetylene reduction in the marine non-heterocystous cyanobacterium Trichodesmium spp. grown under artificial conditions

Aerobic nitrogenase activity in the marine non-heterocystous cyanobacterium Trichodesmium spp. NIBB 1067, isolated off the Izu Peninsula, Japan in 1983 and grown under artificial conditions, was assayed by the acetylene reduction method. This strain exhibited acetylene reduction activity under aerobic conditions when cells had been grown in the medium free of combined nitrogen. Activity was markedly enhanced by light, and dependent on the growth phase being higher during the exponential growth phase and lower during the late linear and stationary growth phases. Since typical colony formation occurred during the last growth phase, the present results contradict the idea that N₂-fixation depends on colony formation. The photosynthesis inhibitor DCMU at 10^-6 M inhibited light-dependent acetylene reduction completely. Acetylene reduction by Trichodesmium spp. was tolerant of O₂ as strongly as that in the heterocystous cyanobacteria. Even at a partial pressure of oxygen (p_{O 2}) of 3 atm, the activity still remained as high as half of the maximum. It was almost under anaerobic conditions. Maximum activity was obtained at pO₂ of ca. 0.1 atm.     

The association of N2-fixing bacteria with sea urchins

Dinitrogen fixation associated with bacteria in the gastrointestinal tract of sea urchins appears to be a widespread phenomenon: sea urchins from the tropics (Diadema antillarum, Echinometra lacunter, Tripneustes ventricosus), the temperature zone (Strongylocentrotus droebachiensis) and the arctic (S. droebachiensis) exhibited nitrogenase activity (C₂H₂ reduction). Pronounced seasonal variation was found in nitrogenase activity of temperate sea urchins feeding on kelp (Laminaria spp.) and eelgrass (Zostera marina). The mean monthly nitrogenase activity was inversely correlated with the nitrogen content of the sea urchin's food, which varied up to fivefold over the course of a year. The highest rate of nitrogenase activity recorded for a temperate sea urchin during the 14 month sampling period was 11.6g N fixed g wet wt^-1 d^-1, with a yearly mean activity of 1.36 g N fixed g wet wt^-1 d^-1. Studies with ¹⁵N confirmed the C₂H₂ reduction results and showed incorporation of microbially-fixed nitrogen into S. droebachiensis demonstrating that N₂ fixation can be a source of N for the sea urchin. Laboratory experiments indicated that part of the sea urchin's (S. droebachiensis) normal gastrointestinal microflora is responsible for the observed nitrogenase activity.     

Interrelationships between sulfate reducing and methane producing bacteria in coastal sediments with intense sulfide production

Coastal sediments receiving different amounts of organic carbon through sedimentation were investigated with respect to sulfate reduction and methanogenic activity. Sampling was carried out at sediment temperatures of 7° and 15°C. Sulfate-reducing and methanogenic bacteria were found at all depths. Sulfate reduction decreased with depth and the highest sulfide concentrations were found a few centimeters below the sediment surface, up to 15 mM at 15°C and pH 7.1. In the same segments a maximum in the methane concentration was also found, 0.91 mM. The high sulfide concentration inhibited the methane formation from acetate but not from carbon dioxide. In the organic rich sediment sulfate reduction was limited by the diffusion of SO ₄ ⁺ into the sediment and methane production from acetate by sulfide diffusion out of the sediment. When electron acceptor concentration limits sulfate reduction, thermodynamic calculations show that the utilization of electron donors more reduced than acetate is favored. In the sediment with the high carbon-input, acetate predominated at 15°C whereas in the low carbon-input sediment hardly any short chain organic acids were detected. The possibility of a shift in sulfate reduction from acetate oxidation to acetate production is discussed.     

Diurnal patterns in photosynthetic capacity and depth-dependent photosynthesis-irradiance relationships inSynechococcus spp. and larger phytoplankton in three water masses in the Northwest Atlantic Ocean

The photosynthetic characteristics of prokaryotic phycoerythrin-rich populations of cyanobacteriaSynechococcus spp. and larger eukaryotic algae were compared at a neritic frontal station (Pl), in a warm-core eddy (P2), and at Wilkinson's Basin (P3) during a cruise in the Northwest Atlantic Ocean in the summer of 1984.Synechococcus spp. numerically dominated the 0.6 to 1 m fraction, and to a lesser extent the 1 to 5 m size fractions, at most depths at all stations. At P2 and P3, all three size categories of phytoplankton (0.6 to 1 m, 1 to 5 m, and >5 m) exhibited similar depth-dependent chages in both the timing and amplitude of diurnal periodicities of chlorophyllbased and cell-based photosynthetic capacity. Midday maxima in photosynthesis were observed in the upper watercolumn which damped-out in all size fractions sampled just below the thermocline. For all size fractions sampled near the bottom of the euphotic zone, the highest photosynthetic capacity was observed at dawn. At all depths, theSynechococcus spp.-dominated size fractions had lower assimilation rates than larger phytoplankton size fractions. This observation takes exception with the view that there is an inverse size-dependency in algal photosynthesis. Results also indicated that the size-specific contribution to potential primary production in surface waters did not vary appreciably over the day. However, estimates of the percent contribution ofSynechococcus spp. to total primary productivity in surface waters at the neritic front were significantly higher when derived from short-term incubator measurements of photosynthetic capacity rather than from dawn-to-duskin situ measurements of carbon fixation. The discrepancy was not due to photoinhibitory effects on photosynthesis, but appeared to reflect increased selective grazing pressure onSynechococcus spp. in dawn-to-dusk samples. Low-light photoadaptation was evident in analyses of the depth-dependency ofP-I parameters (photosynthetic capacity,P _max; light-limited slope, alpha;P _max alpha,I _k ; light-intensity beyond which photoinhibition occurs,I _b ) of the > 0.6 m communities at all three stations and was attributable to stratification of the water column. There was a decrease in assimilation rates andI _k with depth that was associated with increases in light-limited rates of photosynthesis. No midday photoinhibition ofP _max orI _b was observed in any surface station. Marked photoinhibition was detected only in the chlorophyll maximum at the neritic front and below the surface mixed-layer at Wilkinson's Basin, where susceptibility to photoinhibition increased with the depth of the collected sample. The 0.6 to 1 m fraction always had lower light requirements for light-saturated photosynthesis than the > 5 m size fraction within the same sample. Saturation intensities for the 1 to 5 m and 0.6 to 1 m size fractions were more similar whenSynechococcus spp. abundances were high in the 1 to 5 m fraction. The > 5 m fraction appeared to be the prime contributor to photoinhibitory features displayed in mixed samples (> 0.6 m) taken from the chlorophyll maxima. InSynechococcus spp.-dominated 0.6 to 1 and 1 to 5 m size fractions, cellular chlorophylla content increased 50- to 100-fold with depth and could be related to increases in maximum daytime rates of cellularP _max at the base of the euphotic zone. Furthermore, the 0.6 to 1 m and > 5 m fractions sampled at the chlorophyll maximum in the warm-core eddy had lower light requirements for photosynthesis than comparable surface samples from the same station. Results suggest that photoadaptation in natural populations ofSynechococcus spp. is accomplished primarily by changing photosynthetic unit number, occuring in conjuction with other accommodations in the efficiency of photosynthetic light reactions.     

Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

Nitrogen fixation by blue-green algae associated with the siphonous green seaweed Codium decorticatum: effects on ammonium uptake

Nitrogen fixation (acetylene reduction) at rates of up to 1.2 g N₂ g dry wt^-1 h^-1 was measured for the siphonous green seaweed Codium decorticatum. No nitrogenase activity was detected in C. isthmocladum. The nitrogenase activity was light sensitive and was inhibited by the addition of DCMU and triphenyl tetrazolium chloride. Additions of glucose did not stimulate nitrogen fixation. Blue-green algae (Calothrix sp., Anabaena sp., and Phormidium sp.) were implicated as the organisms responsible for the nitrogenase activity. They occurred in a reduced microzone within the C. decorticatum thallus where nitrogen fixation was optimized. Nitrogen fixation did not affect the kinetic constants for ammonium uptake in C. decorticatum (K_s=12.0 M, V_max=13.4 mol NH₃ g dry wt^-1 h^-1) determined using the perturbation method. Nevertheless, C. decorticatum thalli which fixed nitrogen had internal dissolved nitrogen concentrations which were over 1.4 times higher than in non-fixing thalli. This suggests that if C. decorticatum does derive part of its nitrogen requirement from the blue-green algae which it harbors, the transfer does not involve competition between this process and the uptake of ambient ammonium.     

Metabolic and blood characteristics of the hydrothermal vent tube-worm Riftia pachyptila

Specimens of the hydrothermal vent pogonophoran Riftia pachyptila Jones were collected by submersible at a depth of 2 600 m at the 21°N hydrothermal vent site on the East Pacific Rise (20°50N, 109°06W) in April and May of 1982. The worms were maintained in pressurized aquaria for up to 45 d for metabolic studies. Consumption of O₂ was regulated down to low P_{O 2} (oxygen partial pressure) values; O₂ consumption rates were 0.63 and 1.12 mol g^-1 wet wt h^-1 at 2.5° and 8°C, respectively; such rates were comparable to those previously measured for other pogonophorans. Intact specimens of R. pachyptila (including bacterial symbionts) did not consume significant amounts of CH₄ from the environment. The respiratory quotients, in the absence of added sulfide, indicated that metabolism was mainly heterotrophic. High rates of uptake of dissolved amino acids were recorded for one specimen. The total [CO₂] in the vascular blood and the Hb-containing coelomic fluid were high. Under anaerobic conditions, there were equilibrium distributions of pH, total [CO₂] and sulfide concentrations between the vascular blood and the coelomic fluid, apparently because these metabolites were readily exchanged between the two compartments. The vascular blood bound neither CH₄ nor H₂. However, sulfide was reversibly bound by both the vascular blood and coelomic fluid; because this binding depended strongly on pH (with a maximum at about 7.5), HS^- was probably the molecular species bound. Under anaerobic, but not aerobic conditions, the trophosome bound substantial amount of sulfide; thus, the high concentrations of sulfide in the trophosome may have resulted mainly from sulfide bound to sulfide oxidases under anaerobic conditions. The coelomic fluid had a relatively low buffering capacity (2.2 mmol CO₂pH^-1).     

Corophium multisetosum (Amphipoda: Corophiidae) in Canal de Mira,Portugal: Some factors that affect its distribution

The population ofCorophium multisetosum (L.) from Canal de Mira, Ria de Aveiro, Portugal, was sampled seasonally from December 1985 to September 1986, as part of a larger survey of the benthic invertebrate macrofauna. Its distribution along the channel exposes the species to a range of salinities from freshwater to above 30. A principal components analysis, using the physical and chemical parameters of the sediment as variables and sampling stations as operational taxonomic units, indicated that abundance is negatively correlated with salinity, depth, and the occurrence of sediments rich in particles below 125µm and rich in organic matter. Abundance is positively correlated with temperature. The distribution of the species does not seem to be affected by the occurrence of sediment grades between 125 and 1 000µm. Paired-choice salinity experiments indicated thatC. multisetosum prefers salinities within the range 2.5 to 10. In multi-choice experiments concerning sediment grade, amphipods did not show any significant preference within the 125 to 500µm range, although the 125µm grade was chosen less frequently. The influence of temperature on the overall distribution ofC. multisetosum is discussed.     

Contrasting effects of sulfide and thiosulfate on symbiotic CO2-assimilation of Phallodrilus leukodermatus (Annelida)

Experiments were conducted in order to specify the reductants responsible for the carbon dioxide fixation of the symbiotic sulfur bacteria in the gutless marine oligochaete Phallodrilus leukodermatus (Annelida) from shallow calcareous sediments in Bermuda. Carbon dioxide-uptake rates were suppressed by S⁼ and stimulated by S₂O₃ ⁼. Individuals which hosted bacteria containing reserve energy substances maintained a high short-term CO₂-uptake activity, while bacteria in worm homogenates and in worms treated with an antibiotic (Baypen) did not show any significant metabolic activity. Absolute uptake rates in P. leukodermatus were usually considerably higher than those reported for other animals harbouring prokaryotic sulfuroxidizing symbionts. Utilization of thiosulfate rather than sulfide is compatible with the preferred occurrence of the worms around the redox discontinuity layer and has been confirmed in other thiobiotic animals. Sulfur stored in the symbiotic bacteria appears to be oxidized to sulfate and be excreted when the worms are held under energy-limited conditions. The data emphasize the complexity of the possible metabolic pathways involved in the oxidation of reduced-sulfur compounds by bacterial symbionts in marine invertebrates.     

Regulation of growth in Laminaria digitata: use of in-vivo nitrate reductase activities as an indicator of nitrogen limitation in field populations of Laminaria spp.

The seasonal variation in growth rate of a population of Laminaria digitata (Huds.) Lamour growing at Arbroath, Scotland was studied between August 1981 and September 1982, and was found to follow the biphasic annual cycle typical of this genus. Growth rates were maximum (0.3 cm cm^-1 mo^-1) in early June and minimum (0.05 cm cm^-1 mo^-1) between September and January. An analysis of the relationship between the seasonal changes in environmental factors (inorganic nitrogen concentrations, irradiance and temperature) with those of growth rate and the accumulation or mobilisation of cellular reserves of carbohydrates and nitrate, indicated that growth was nitrogen-limited between June and October and light-limited (with a possible co-involvement of temperature) for the remainder of the year. These conclusions were supported by the seasonal changes in the ratio of actual: potential in-vivo nitrate reductase activities in L. digitata, thus confirming the suitability of this technique for monitoring the occurrence of nitrogen limitation in Laminaria spp. The seasonal changes in blade nitrate reductase activities closely followed those of growth rate, with maximum activities [0.3 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] being present in late May and minimum levels [0.01 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] occurring between November and March. The correlation observed between nitrate reductase activities and growth rate is consistent with the ability of Laminaria spp. to store excess inorganic nitrogen, available during winter and early spring, as NO ₃ ^- , and with the requirement to conserve enzyme protein during the summer period of nitrogen limitation.