Kinetics of hydrogen sulfide oxidation by immobilized autotrophic and heterotrophic bacteria in bioreactors

Summary The kinetics of H₂S oxidation in bioreactors with separately packed autotrophic Thiobacillus thioparus CH11 and heterotrophic Pseudomonas putida CH11 were evaluated. The reaction rates were determined to be first-order below 20 ppm, zero-order above 60 ppm, and fractional-order in the intermediate concentration ranges for the Thiobacillus thioparus CH11 bioreactor, and first-order below 35 ppm, zero-order above 80 ppm, and fractional-order in the intermediate concentration ranges for the Pseudomonas putida CH11 bioreactor. The saturation constants for H₂S by Thiobacillus thioparus CH11 and Pseudomonas putida CH11 were calculated to be 30.3 ppm and 44.2 ppm, respectively.     

ORGANIC CARBON SUPPLEMENTATION OF STERILIZED MUNICIPAL WASTEWATER IS ESSENTIAL FOR HETEROTROPHIC GROWTH AND REMOVING AMMONIUM BY THE MICROALGA CHLORELLA VULGARIS1

Heterotrophic growth of the microalga Chlorella vulgaris Beij. in synthetic as well as sterilized municipal wastewater of a nonindustrialized city was measured. The city wastewater contained high levels of ammonium and nitrate, medium levels of phosphate, and low levels of nitrite and organic molecules and could not support heterotrophic growth of C. vulgaris. Evaluation of 11 known carbon sources for this microalga that were added to standard synthetic wastewater containing the same levels of nitrogen and phosphorus as the municipal wastewater revealed that the best carbon sources for heterotrophic growth were Na‐acetate and d ‐glucose. These provided the highest growth rates and the largest removal of ammonium. Growth increased with concentration of the supplement to an optimum at 0.12 M Na‐acetate. This carbon source was consumed completely within 10 d of incubation. Higher concentrations inhibited the growth of C. vulgaris. The microalgal populations under heterotrophic growth conditions were one level of magnitude higher than that under autotrophic growth conditions that served as a comparison. No growth occurred in the dark in the absence of a carbon source. Na‐acetate was superior to d ‐glucose. In municipal wastewater, when Na‐acetate or d ‐glucose was added, C. vulgaris significantly enhanced ammonium removal under heterotrophic conditions, and its capacity was equal to ammonium removal under autotrophic growth conditions. This study showed that sterilized wastewater can be treated by C. vulgaris under heterotrophic conditions if supplemented with the appropriate organic carbon source for the microalgae.     

川中丘陵区冬水田种植模式转旱作土壤呼吸组分特征及碳平衡

冬水田-水稻是川中丘陵区传统的稻田种植模式,冬水田种植模式转变是实现多熟种植及机械化的重要途径。为探究冬水田-水稻种植模式转旱作过程中作物季及休闲期土壤呼吸速率及其组分构成,试验设置冬水田-水稻转旱作(FTD)、冬水田-水稻(FR)和冬闲田-玉米(FM)3种不同种植模式,采用根排除法和静态明箱-气相色谱法原位取样测定作物季及季后休闲期土壤呼吸及其组分,并通过测算净生态系统生产力(NEP)进而判断冬水田-水稻转旱作过程的农田系统碳汇强度。结果表明:(1)FTD显著提高了土壤总呼吸速率及其自养和异养呼吸速率,从而提高了其累积排放量(P<0.05)。与FR相比,FTD的土壤总呼吸及其自养和异养呼吸的累积排放量分别提高了13.14倍、11.32倍和15.56倍(P<0.05);与FM相比,FTD的土壤总呼吸及其自养和异养呼吸的累积排放量分别提高了70.56%、40.83%和115.47%(P<0.05)。(2)与FR和FM相比,FTD均降低了土壤呼吸及其组分的温度敏感性(Q₁₀),且土壤总呼吸的温度敏感性介于异养呼吸和自养呼吸之间。(3)FR,FM和FTD的净生态系统生产力(NEP)均为正值,其数值分别为7911.66 kg/hm²,5667.89 kg/hm²和1583.46 kg/hm²,均表现为大气CO₂的碳汇,但与FR与FM相比,FTD显著降低了其净生态系统生产力,呈现出较弱的碳汇。     

EFFICIENCY OF GROWTH AND NUTRIENT UPTAKE FROM WASTEWATER BY HETEROTROPHIC,AUTOTROPHIC, AND MIXOTROPHIC CULTIVATION OF CHLORELLA VULGARIS IMMOBILIZED WITH AZOSPIRILLUM BRASILENSE1

Heterotrophic growth of microalgae presents significant economic advantages over the more common autotrophic cultivation. The efficiency of growth and nitrogen, phosphorus, and glucose uptake from synthetic wastewater was compared under heterotrophic, autotrophic, and mixotrophic regimes of Chlorella vulgaris Beij. immobilized in alginate beads, either alone or with the bacterium Azospirillum brasilense. Heterotrophic cultivation of C. vulgaris growing alone was superior to autotrophic cultivation. The added bacteria enhanced growth only under autotrophic and mixotrophic cultivations. Uptake of ammonium by the culture, yield of cells per ammonium unit, and total volumetric productivity of the culture were the highest under heterotrophic conditions when the microalga grew without the bacterium. Uptake of phosphate was higher under autotrophic conditions and similar under the other two regimes. Positive influence of the addition of A. brasilense was found only when light was supplied (autotrophic and mixotrophic), where affinity to phosphate and yield per phosphate unit were the highest under heterotrophic conditions. The pH of the culture was significantly reduced in all regimes where glucose was consumed, similarly in heterotrophic and mixotrophic cultures. It was concluded that the heterotrophic regime, using glucose, is superior to autotrophic and mixotrophic regimes for the uptake of ammonium and phosphate. Addition of A. brasilense positively affects the nutrient uptake only in the two regimes supplied with light.     

Autotrophe Gewebekulturen von Ruta graveolens und deren 14CO2-Markierungsprodukte

Summary It is possible to obtain autotrophic callus cultures by inhibiting cell respiration. During a first passage of four weeks the cultures synthesized chlorophyll on an agar-medium with a minimum of organic substances such as sugar, amino acids and vitamins. In the second passage these cultures were kept on the same medium but were aerated with a mixture of 99% N₂ and 1% CO₂. In the third and last passage the medium contained only mineral substances and the same mixture of N₂ and CO₂ was used for aeration. This pure mineral medium was supplemented with the Hoagland's solution.These autotrophic callus cultures were grown for about two years under these conditions and showed a growth quotient of ten.Three different groups of tissues were taken for the ¹⁴CO₂-fixation. The first group was grown for four weeks on a heterotrophic medium and aerated with O₂. This is the socalled respirating group. The second and third group were both aerated with the mixture of N₂/CO₂ but they were grown on different mediums. One of these groups was grown on a heterotrophic medium for four weeks: these are heterotrophic photosynthesizing tissues. The third group was grown on a pure mineral medium, and these are the autotrophic photosynthesizing callus tissues.Respirating tissues are different from photosynthesizing cultures in respect to the quantity of light-induced CO₂-fixation.The thin-layer chromatograms reveal the difference between heterotrophic and autotrophic tissues. In the light dependent ¹⁴CO₂-incorporation the difference is in the amounts of the labelled amino acids glycine and serine. In the dark dependent incorporation the difference is found in the amount of the labelled amino acid aspartic acid. The more autotrophic these tissues are, the higher the level of the CO₂-fixation in these amino acids is.

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Mit Hilfe der Deutschen Forschungsgemeinschaft.     

Thawing permafrost increases old soil and autotrophic respiration in tundra: Partitioning ecosystem respiration using δ13C and ∆14C

Ecosystem respiration (R_eco) is one of the largest terrestrial carbon (C) fluxes. The effect of climate change on R_eco depends on the responses of its autotrophic and heterotrophic components. How autotrophic and heterotrophic respiration sources respond to climate change is especially important in ecosystems underlain by permafrost. Permafrost ecosystems contain vast stores of soil C (1672 Pg) and are located in northern latitudes where climate change is accelerated. Warming will cause a positive feedback to climate change if heterotrophic respiration increases without corresponding increases in primary production. We quantified the response of autotrophic and heterotrophic respiration to permafrost thaw across the 2008 and 2009 growing seasons. We partitioned R_eco using Δ¹⁴C and δ¹³C into four sources–two autotrophic (above – and belowground plant structures) and two heterotrophic (young and old soil). We sampled the Δ¹⁴C and δ¹³C of sources using incubations and the Δ¹⁴C and δ¹³C of R_eco using field measurements. We then used a Bayesian mixing model to solve for the most likely contributions of each source to R_eco. Autotrophic respiration ranged from 40 to 70% of R_eco and was greatest at the height of the growing season. Old soil heterotrophic respiration ranged from 6 to 18% of R_eco and was greatest where permafrost thaw was deepest. Overall, growing season fluxes of autotrophic and old soil heterotrophic respiration increased as permafrost thaw deepened. Areas with greater thaw also had the greatest primary production. Warming in permafrost ecosystems therefore leads to increased plant and old soil respiration that is initially compensated by increased net primary productivity. However, barring large shifts in plant community composition, future increases in old soil respiration will likely outpace productivity, resulting in a positive feedback to climate change.     

Investigation of mixotrophic,heterotrophic, and autotrophic growth of Chlorella vulgaris under agricultural waste medium

Growth of Chlorella vulgaris and its lipid production were investigated under autotrophic, heterotrophic, and mixotrophic conditions. Cheap agricultural waste molasses and corn steep liquor from industries were used as carbon and nitrogen sources, respectively. Chlorella vulgaris grew remarkably under this agricultural waste medium, which resulted in a reduction in the final cost of the biodiesel production. Maximum dry weight of 2.62 g L^?1 was obtained in mixotrophic growth with the highest lipid concentration of 0.86 g L^?1. These biomass and lipid concentrations were, respectively, 140% and 170% higher than autotrophic growth and 300% and 1200% higher than heterotrophic growth. In mixotrophic growth, independent or simultaneous occurrence of autotrophic and heterotrophic metabolisms was investigated. The growth of the microalgae was observed to take place first heterotrophically to a minimum substrate concentration with a little fraction in growth under autotrophic metabolism, and then the cells grew more autotrophically. It was found that mixotrophic growth was not a simple combination of heterotrophic and autotrophic growth.     

Glycollate production and excretion byAlcaligenes eutrophus

Autotrophic cultures of the facultative chemolithotrophAlcaligenes eutrophus have been found to excrete glycollate. This excretion was greatly stimulated by the incorporation of up to 20% (v/v) oxygen in the hydrogen used for gassing. The stimulatory effect of oxygen was prevented by the addition of 10% (v/v) CO₂ to the gassing mixture. Glycollate excretion only in the presence of oxygen was increased by the addition of 2-pyridyl-hydroxymethane sulphonic acid (HPMS), an inhibitor of glycollate oxidation, indicating that glycollate formation itself was stimulated by oxygen. No glycollate excretion by cultures grown heterotrophically on pyruvate was detected, either in the absence or presence of HPMS, under heterotrophic or autotrophic conditions.Extracts from autotrophic cells showed phosphoglycollate phosphatase and glycollate oxidoreductase activities, which were considerably lower in extracts prepared from pyruvate- or fructose-grown (heterotrophic) cells. The increase in activity of both enzymes upon cell transfer from heterotrophic to autotrophic growth was prevented by chloramphenicol and resembled the induction ofd0ribulose 1,5-diphosphate carboxylase under the same conditions.Abbreviations DTE dithioerythritol - EDTA ethylenediamine tetraacetate - HPMS 2-pyridyl-hydroxymethane sulphonie acid - RuDP d-ribulose 1,5-diphosphate     

THE OXIDATION OF HYDROGEN SULFIDE BY BEGGIATOA

Knowledge of the conditions in which Beggiatoa is capable of autotrophic nutrition is incomplete. It is not known whether sulfur-free trichomes from heterotrophic cultures are able to return to the utilization of H₂S-oxidation. Devices were developed which permitted the supply of pure cultures of Beggiatoa, previously cultivated heterotrophically, with H₂S, O₂, and CO₂. Development in media devoid of organic nutrients was achieved, and subculturing under autotrophic conditions could be repeated indefinitely. The strains used behaved differently with respect to their tendency to grow autotrophically. The ability to dispense with organic substrates corresponds to the place in the groups to which they had previously been assigned. All the strains multiplied better when, under otherwise equal conditions, the inorganic medium was supplemented with acetate, very low concentrations of which were effective. This result may, however, be due to the selection of varieties by the isolation procedure. The mixotrophic tendency of our strains may not be a general feature of the genus. There are indications that the wider forms of Beggiatoa tend more toward autotrophic growth than the narrower ones.     

Control of the glutathione S-transferase and mas1' promoter-driven GUS activity in auxin heterotrophic and autotrophic tobacco calli by exogenous 2,4-d-induced ethylene

Auxin autotrophic and heterotrophic lines of tobacco calli may differ not only in their indoleacetic acid (IAA) synthetizing abilities and sensitivities to exogenous auxins, but also in their gene expression patterns. Auxin autotrophic callus tissues from the leaf protoplasts of transgenic Nicotiana tabacum SR1 plants involving mas1′::GUS gene fusion were generated and the growth of cultures was compared with that of the heterotrophic lines of the same transgenic tissues on MS medium containing different concentrations of IAA or 2,4‐d . The mas1′::GUS gene fusion expression was investigated, together with the glutathione S‐transferase activities (GST, EC 2.5.1.18) in auxin autotrophic and heterotrophic tobacco calli. Both the mas1′ promoter and GST gene promoters contain ocs or ocs‐like elements, responsible for both auxin and ethylene/wound inducibility. The mas1′ promoter exhibited a much higher expression activity in the heterotrophic cultures growing on IAA than in the autotrophic ones, but in contrast with the natural auxin, the mas1′::GUS activity decreased at elevated 2,4‐d concentrations in the heterotrophic tissues and increased with increasing 2,4‐d concentrations in the autotrophic lines. The induction of GST activity by different exogenous auxin concentrations was much higher in the autotrophic lines, especially in the case of 2,4‐d . Higher concentrations of external 2,4‐d resulted in increased ethylene production, which displayed different kinetics in the two types of calli. The ethylene‐inducing 2,4‐d concentrations increased the growth of the heterotrophic, but decreased that of the autotrophic lines. Blocking the ethylene receptors and hence the signal perception by 2,5‐norbornadiene (NBD) in the heterotrophic tissues increased the 2,4‐d ‐induced mas1′ promoter and GST activities, suggesting that the gaseous hormone counteracted the auxin response pathway. This was not found in the autotrophic tissues, where NBD decreased the mas1′‐driven GUS activity. The GST activities were slightly decreased, or almost independent of the action of ethylene. It is suggested that the cross‐talk between the auxin‐ and ethylene‐induced signal transduction pathways may differ in the auxin autotrophic and heterotrophic lines.     

Hydrocarbons in green and blue-green algae

Liquid column chromatography and thin-layer chromatography were used to determine the total content of hydrocarbons and gas chromatography was used to evaluate composition of hydrocarbons in green algae (Chlorella kessleri, C. vulgaris, Chlorella sp.,Scenedesmus acutus, S. acuminatus, S. obliquus) and the blue-green alga (Spirulina platensis) cultivated under autotrophic or heterotrophic conditions. InC. kessleri cultivated under heterotrophic conditions the content of hydrocarbons was found to be about 10^-2 % (per dry mass), whereas under autotrophic conditions it was about 10^-3 % (per dry mass). The highest content of hydrocarbons was detected in species of the genusScenedesmus cultivated autotrophically (10^-1 %). Heptadecane and hexacosane were found as major alkanes, 1-heptadecene was detected among alkenes.     

Analysis of bacteria communities in an up-flow fixed-bed (UFB) bioreactor for treating sulfide in hydrocarbon wastewater

An up-flow fixed-bed (UFB) bioreactor with patented functional polyurethane foam (FPUF) carriers was used to treat sulfide in hydrocarbon wastewater. Community compositions of autotrophic and heterotrophic bacteria were analyzed by polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE). DGGE results showed that a relatively stable bacterial community composed of heterotrophic and autotrophic bacteria formed in the bioreactor by the end of experiment, which ensured 92–100% sulfide removal efficiencies. Furthermore, autotrophic genera of Thiobacillus and Thiomonas, as well as those of the heterotrophic genus of Acinetobacter survived and exhibited high sulfide oxidation activity under all three operational conditions. Different special genera were also observed under each operational condition, such as the halophilic genus of Nesterenkonia. In addition, a new genus of sulfide oxidation bacteria was found in the bioreactor, which had the ability to synthesize cytoplasm from organic compounds. These genera have wide applications for the treatment of sulfide in hydrocarbon wastewater.     

Sequential heterotrophic/autotrophic cultivation – An efficient method of producing Chlorella biomass for health food and animal feed

Sequential heterotrophic/autotrophic cultivation method was investigated for production of high concentration of Chlorella biomass with high cellular protein and chlorophyll contents. By using autotrophic growth medium, which contains glucose as organic carbon source, for heterotrophic culture, the protein and chlorophyll contents of the cells could be increased by simply illuminating the culture broth and aerating with CO₂-enriched air at the end of the heterotrophic culture. A system was then constructed for continuous sequential heterotrophic/autotrophic production of algal biomass. The system was composed of the conventional mini-jar fermentor for the heterotrophic phase and a tubular photobioreactor for the autotrophic phase. The exhaust gas from the heterotrophic phase was used for aeration of the autotrophic phase in order to reduce the CO₂ emission into the atmosphere. With this system, it was possible to produce high Chlorella biomass concentration (14 g L-¹) containing 60.1% protein and 3.6% chlorophyll continuously for more than 640 h. During the steady state, about 27% of the CO₂ produced in the heterotrophic phase was re-utilized in the autotrophic phase. When the tubular photobioreactor was replaced with a 3.5-L internally illuminated photobioreactor, the productivity increased from 2 g L-¹ d-¹ to 4 g L-¹ d-¹. However, the chlorophyll content of the cells was lower due to the lower light supply coefficient of the photobioreactor. This revised version was published online in August 2006 with corrections to the Cover Date.     

Indirect partitioning of soil respiration in a series of evergreen forest ecosystems

A simple estimation of heterotrophic respiration can be obtained analytically as the y-intercept of the linear regression between soil-surface CO₂ efflux and root biomass. In the present study, a development of this indirect methodology is presented by taking into consideration both the temporal variation and the spatial heterogeneity of heterotrophic respiration. For this purpose, soil CO₂ efflux, soil carbon content and main stand characteristics were estimated in seven evergreen forest ecosystems along an elevation gradient ranging from 250 to 1740 m. For each site and for each sampling date the measured soil CO₂ efflux (R _S) was predicted with the model R _S = a × S _C + b × R _D ± ε, where S _C is soil carbon content per unit area to a depth of 30 cm and R _D is the root density of the 2–5 mm root class. Regressions with statistically significant a and b coefficients allowed the indirect separation of the two components of soil CO₂ efflux. Considering that the different sampling dates were characterized by different soil temperature, it was possible to investigate the temporal and thermal dependency of autotrophic and heterotrophic respiration. It was estimated that annual autotrophic respiration accounts for 16–58% of total soil CO₂ efflux in the seven different evergreen ecosystems. In addition, our observations show a decrease of annual autotrophic respiration at increasing availability of soil nitrogen. Section Editor: A. Hodge     

The regulation of hydrogenase formation as a differentiating character of strains of Paracoccus denitrificans

Paracoccus denitrificans strains Stanier 381 (DSM 65), Morris (DSM 413), and Vogt 11 (DSM 415) and eleven newly isolated strains were compared with respect to the localization of hydrogenase and its regulation. In all strains hydrogenase was found to be membrane-bound and not able to reduce pyridine nucleotides.The enzyme was inducible in strain 381 and was found only in cells grown with hydrogen as the sole hydrogen donor; in cells grown under mixotrophic or heterotrophic conditions the hydrogenase activity was zero.In all other strains hydrogenase was constitutive and was present in cells grown under autotrophic, mixotrophic and heterotrophic conditions. Under the latter conditions the specific hydrogenase activity was even higher than under mixotrophic conditions.     

Metabolic regulation of pyruvate kinase isolated from autotrophically and heterotrophically grown Paracoccus denitrificans

Pyruvate kinase (ATP: pyruvate phosphotransferase (EC 2.7.1.40) was partially purified from both autotrophically and heterotrophycally grown Paracoccus denitrificans. The organism grown under heterotrophic conditions contains four times more pyruvate kinase than under autotrophic conditions. The enzyme isolated from both sources exhibited sigmoidal kinetics for both phosphoenolpyruvate (PEP) and ADP. The apparent M _m for ADP and PEP in the autotrophic enzyme were 0.63 mM ADP and 0.25 mM PEP. The effect of several low molecular weight metabolites on the pyruvate kinase activity was investigated. Ribose-5-phosphate, glucose-6-phosphate and AMP stimulated the reaction at low ADP levels; this stimulation was brought about by an alteration in the apparent K _m for ADP. The pyruvate kinases differ in their response to adenine nucleotides, but both preparations seem to be under adenylate control. The results are discussed in relation to the role of pyruvate kinase as a regulatory enzyme in P. denitrificans grown under both autotrophic and heterotrophic conditions.Non-Common Abbreviations PEP phosphoenolpyruvate - R-5-P ribose-5-phosphate - G-6-P glucose-6-phosphate - F-6-P fructose-6-phosphate - 3-PGA 3-phosphoglycerate     

The stoichiometry of nitrogen and phosphorus spiralling in heterotrophic and autotrophic streams

1. Nutrient spiralling provides a conceptual framework and a whole‐system approach to investigate ecosystem responses to environmental changes. We use spiralling metrics to examine how the coupling of nitrogen and phosphorus uptake varies between streams dominated by either heterotrophic (i.e. bacteria‐dominated) or autotrophic (algal‐dominated) microbial communities. 2. Algae generally exhibit greater capacity to store nutrients than bacteria because of differences in cellular structures. These differences led us to hypothesise that the uptake of N and P in heterotrophic ecosystems should have reduced stoichiometric variation in response to changes in supply N : P compared to autotrophic ecosystems when assimilation dominates nutrient uptake. 3. To test this hypothesis, we used an array of serial nutrient additions in several streams in the South Fork Eel River watershed in Northern California. In one set of experiments, N and P were added alone and simultaneously in separate experiments to two small, heterotrophic streams to assess uptake rates and interactions between nutrient cycles. In a second set of experiments, N and P were added simultaneously at a range of N : P in one heterotrophic and one autotrophic stream to assess differences in uptake responses to changes in supply N : P. 4. Results of these experiments suggest two important conclusions. First, increased N supply significantly shortened P uptake lengths, while P addition had little impact on N uptake in both streams, indicating that uptake of non‐limiting nutrients is tightly coupled to the availability of the limiting element. Second, changes in P uptake and uptake ratios (U_N : U_P) with increased supply N : P supported our hypothesis that heterotrophic streams are more homeostatic in their responses to changes in nutrient supply than autotrophic streams, suggesting that physiological controls on nutrient use scale up to influence ecosystem‐scale patterns in nutrient cycling.     

Linkage of microbial kinetics and bacterial community structure of MBR and hybrid MBBR–MBR systems to treat salinity‐amended urban wastewater

Three pilot‐scale bioreactors were started up and operated under salinity‐amended urban wastewater feeding. The bioreactors were configured as membrane bioreactor and two different hybrid, moving bed biofilm reactor‐membrane bioreactor and operated with a hydraulic retention time of 9.5 h, a solid residence time of 11.75 days and a total solids concentration of 2500 mg L^?1. The three systems showed excellent performance in suspended solids, BOD₅, and COD removal (values of 96–100%, 97–99%, and 88–90%, respectively), but poor nitrogen removal (values of 20–30%). The bacterial community structure during the start‐up phase and the stabilization phase were different, as showed by β‐diversity analyses. The differences between aerobic and anoxic biomass—and between suspended and attached biomass—were higher at the start‐up phase than at the stabilization phase. The start‐up phase showed high abundances of Chiayiivirga (mean values around 3–12% relative abundance) and Luteimonas (5–8%), but in the stabilization phase, the domination belonged to Thermomonas (3–14%), Nitrobacter (3–7%), Ottowia (3–11.5%), and Comamonas (2–6%), among others. Multivariate redundancy analyses showed that Thermomonas and Nitrosomonas were positively correlated with fast autotrophic kinetics, while Caulobacter and Ottowia were positively correlated with fast heterotrophic kinetics. Nitrobacter, Rhodanobacter, and Comamonas were positively correlated with fast autotrophic and heterotrophic kinetics. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1483–1495, 2017     

Nutritional diversity of symbiotic ascidians in a Fijian seagrass meadow

In a coastal lagoon of Dravuni Island, Fiji, at least six species of compound ascidians, some of them harboringProchloron as symbiotic algae, were found in aSyringodium-dominated seagrass meadow. Based on their heterotrophic (filrer feeding rates) and autotrophic (photosynthetic) activities, carbon gain of the ascidians was categorized into two groups: (i) supported by heterotrophic metabolism; and (ii) supported by both heterotrophic and autotrophic metabolisms.Didemnum molle, Lissoclinum bistratum andLissolinum voeltzkowi belong to the latter group, and the relative contribution of the autotrophic process was a significant portion of their carbon gain (52–74%). These symbiotic ascidians were found in light microhabitats, while the heterotrophic species occupied shady environments rich in suspended organic materials, such as the sheath surface of the seagrass.     

Net heterotrophy in Faroe Islands clear-water lakes: causes and consequences for bacterioplankton and phytoplankton

1. Five oligotrophic clear‐water lakes on the Faroe Islands were studied during August 2000. Algal and bacterial production rates, community respiration, and CO₂ saturation were determined. In addition, we examined the plankton community composition (phytoplankton and heterotrophic nanoflagellates) and measured the grazing pressure exerted by common mixotrophic species on bacteria. 2. High respiration to primary production (6.6–33.2) and supersaturation of CO₂ (830–2140 μatm) implied that the lakes were net heterotrophic and that the pelagic heterotrophic plankton were subsidised by allochthonous organic carbon. However, in spite of the apparent high level of net heterotrophy, primary production exceeded bacterial production and the food base for higher trophic levels appeared to be mainly autotrophic. 3. We suggest that the observed net heterotrophy in these lakes was a result of the oligotrophic conditions and hence low primary production in combination with an input of allochthonous C with a relatively high availability. 4. Mixotrophic phytoplankton (Cryptomonas spp., Dinobryon spp. and flagellates cf. Ochromonas spp.) constituted a large percentage of the plankton community (17–83%), possibly as a result of their capacity to exploit bacteria as a means of acquiring nutrients in these nutrient poor systems.