Extracellular Organic Carbon Research Articles

Effect of excess CO2 on semi-continuous microalgae systems: Carbon biofixation

Microalgae-based CO2 fixation is a promising solution for achieving carbon neutrality. However, the low rate of carbon fixation is a significant issue due to gas transfer. A closed cultivation system was employed to overcome this challenge, incorporating headspace micro-positive pressure and elevated total CO2 (TCO2) levels to minimize CO2 loss and improve CO2 mass transfer. Microalgae could thrive and maintain a higher level of system pH within the TCO2 range of 0.12–0.49 mol CO2/L absorbent, and high TCO2 levels over 0.36 mol CO2/L absorbent were the optimal range. The semi-continuous microalgae system removed up to 8.72 ± 0.85 g CO2/L/day. Simultaneously, it exhibited phosphate and nitrate removal rates of 50.95 ± 10.71 and 182.14 ± 30.71 mg/L/day, respectively. The study found that 80% of the removed carbon was fixed as organic biomass. An increased level of TCO2 resulted in the accumulation of more lipids within the cells. Moreover, microalgae released carbon fixation products into the extracellular environment, and the dissolved organic carbon fixation rate reached as high as 14.58 ± 1.89 mmol/L/day. These findings suggest that microalgae use multiple pathways for carbon fixation, including biomass organic carbon accumulation, extracellular organic carbon secretion, and inorganic carbon precipitation. Improving carbon fixation rates may benefit the industrial application of microalgae technology, which aims to mitigate carbon emissions.

Responses of Cyanobacterial Crusts and Microbial Communities to Extreme Environments of the Stratosphere.

How microbial communities respond to extreme conditions in the stratosphere remains unclear. To test this effect, cyanobacterial crusts collected from Tengger Desert were mounted to high balloons and briefly exposed (140 min) to high UV irradiation and low temperature in the stratosphere at an altitude of 32 km. Freezing and thawing treatments were simulated in the laboratory in terms of the temperature fluctuations during flight. Microbial community composition was characterized by sequencing at the level of DNA and RNA. After exposure to the stratosphere, the RNA relative abundances of Kallotenue and Longimicrobium increased by about 2-fold, while those of several dominant cyanobacteria genera changed slightly. The RNA relative abundances of various taxa declined after freezing, but increased after thawing, whereas cyanobacteria exhibited an opposite change trend. The DNA and RNA relative abundances of Nitrososphaeraceae were increased by 1.4~2.3-fold after exposure to the stratosphere or freezing. Exposure to stratospheric environmental conditions had little impact on the total antioxidant capacity, photosynthetic pigment content, and photosynthetic rate, but significantly increased the content of exopolysaccharides by 16%. The three treatments (stratospheric exposure, freezing, and thawing) increased significantly the activities of N-acetyl-β-D-glucosidase (26~30%) and β-glucosidase (14~126%). Our results indicated cyanobacterial crust communities can tolerate exposure to the stratosphere. In the defense process, extracellular organic carbon degradation and transformation play an important role. This study makes the first attempt to explore the response of microbial communities of cyanobacterial crusts to a Mars-like stratospheric extreme environment, which provides a new perspective for studying the space biology of earth communities.

Performance and mechanism of carbon dioxide fixation by a newly isolated chemoautotrophic strain Paracoccus denitrificans PJ-1

CO2 is regarded as a major contributor to the global warming. CO2 utilization is promising to reduce the CO2 emissions. Currently, the biofixation of CO2 using chemoautotrophs has markedly gain interest in CO2 utilization. In this study, a newly isolated chemoautotroph, Paracoccus denitrificans PJ-1, was used for the biofixation of CO2 under anaerobic condition. Experimental results revealed that Paracoccus denitrificans PJ-1 achieved a high carbon fixation rate (13.25 mg·L−1·h−1) which was ∼10 times faster than the previous reported chemotrophic bacteria using thiosulfate as electron donor. The best CO2 fixation activity of Paracoccus denitrificans PJ-1 was achieved at the pH value of 9.0 and CO2 concentration of 20 vol%. Meanwhile, a high CO2 fixation yield of 106.03 mg·L−1 was reached. The presence of oxygen was adverse to the biofixation, indicating that strain PJ-1 was more suitable for CO2 fixation in anaerobic environments. Carbon mass balance analysis revealed that the carbon from CO2 was mainly fixed into the extracellular organic carbon rather than the biomass. GC-MS analysis and cbbL gene test revealed that Paracoccus denitrificans PJ-1 fixed CO2 through the Calvin-Benson-Bassham cycle and mainly converted CO2 to oxalic acid and succinic acid. Overall, the excellent CO2 fixation capacity of Paracoccus denitrificans PJ-1 suggests that it had potential for CO2 utilization.

Reservoir: The Importance of Extracellular Organic Carbon Released by Phytoplankton

Reservoir: The Importance of Extracellular Organic Carbon Released by Phytoplankton

Production of Heterotrophic Bacterioplankton in a Large Meso-Eutrophic Reservoir: The Importance of Extracellular Organic Carbon Released by Phytoplankton

The spatial distribution and seasonal dynamics of heterotrophic bacterioplankton production have been studied and the value of autochthonous sources of substrates for bacteria has been evaluated in the pelagic zone of the meso-eutrophic Rybinsk Reservoir (the Upper Volga). During the vegetation period, the bacterial production ranges from 32 to 1352 (on average 444 ± 44) mg C/(m2 × day). The total input of organic carbon from the processes of extracellular production of phytoplankton, viral lysis of prokaryotic cells, and feeding of protists provides 9–64% (32 ± 3% on average) of the daily carbon demand for heterotrophic bacterioplankton.

Light Regimes Shape Utilization of Extracellular Organic C and N in a Cyanobacterial Biofilm.

ABSTRACTAlthough it is becoming clear that many microbial primary producers can also play a role as organic consumers, we know very little about the metabolic regulation of photoautotroph organic matter consumption. Cyanobacteria in phototrophic biofilms can reuse extracellular organic carbon, but the metabolic drivers of extracellular processes are surprisingly complex. We investigated the metabolic foundations of organic matter reuse by comparing exoproteome composition and incorporation of 13C-labeled and 15N-labeled cyanobacterial extracellular organic matter (EOM) in a unicyanobacterial biofilm incubated using different light regimes. In the light and the dark, cyanobacterial direct organic C assimilation accounted for 32% and 43%, respectively, of all organic C assimilation in the community. Under photosynthesis conditions, we measured increased excretion of extracellular polymeric substances (EPS) and proteins involved in micronutrient transport, suggesting that requirements for micronutrients may drive EOM assimilation during daylight hours. This interpretation was supported by photosynthesis inhibition experiments, in which cyanobacteria incorporated N-rich EOM-derived material. In contrast, under dark, C-starved conditions, cyanobacteria incorporated C-rich EOM-derived organic matter, decreased excretion of EPS, and showed an increased abundance of degradative exoproteins, demonstrating the use of the extracellular domain for C storage. Sequence-structure modeling of one of these exoproteins predicted a specific hydrolytic activity that was subsequently detected, confirming increased EOM degradation in the dark. Associated heterotrophic bacteria increased in abundance and upregulated transport proteins under dark relative to light conditions. Taken together, our results indicate that biofilm cyanobacteria are successful competitors for organic C and N and that cyanobacterial nutrient and energy requirements control the use of EOM.

The Contribution of Attached Bacteria to Microcystis Bloom: Evidence From Field Investigation and Microcosm Experiment

ABSTRACTThe research of the relationships among free-living bacteria, attached bacteria and different algal species can provide valuable evidence for understanding the reason of algal bloom development under nutrient limitations. Significantly positive relationships between chlorophyll a and attached bacteria in Microcystis-bloom ponds and between chlorophyll a and free-living bacteria in other algal bloom ponds were observed. Microcosm experiments further confirmed the synchronized growth of Microcystis and attached bacteria as well as Pediastrum and free-living bacteria. Carbon and bacteria stimulation experiments suggested that extracellular organic carbon from Microcystis was more complex, inducing specific attached bacterial communities and markedly high extracellular enzymes, which can strongly fuelled Microcystis development. On the contrary, dissolved organic carbon from Pediastrum was simple and suitable for free-living bacteria, which can made a distinct effect on Pediastrum growth depending on the degree of nutrients uptake by free-living bacteria. Based on the above results, it can be proposed that the reason of Microcystis bloom breakout in Chinese shallow lakes should be attributed to the close nutrient complementary relationship between Microcystis and specific attached bacteria mediated by dissolved organic carbon and extracellular enzymes at the development of bloom, which might provide scientific support for the research of Microcystis bloom control.

Characteristics and primary productivity of East Antarctic pack ice during the winter-spring transition

Microbial communities have evolved mechanisms to allow them to survive within the challenging and changing pack ice environment. One such mechanism may be the exudation of photosynthetically-derived organic carbon into various extracellular pools. During the 2nd Sea Ice Physics and Ecosystems eXperiment (SIPEX-2), East Antarctic pack ice productivity and subsequent carbon allocation were quantified, together with physico-biogeochemical characteristics (29 September–28 October, 2012). Mean ice thickness ranged between 0.80 and 2.16 m, and typically exhibited a warm ice interior with weak temperature gradients. All stations, with one exception, were layered with granular (mean: 78%), columnar (mean: 15%), and mixed granular/columnar (mean: 4%) ice. Highest ice brine-volume fractions were at the ice–water interface, but all ice had high brine-volume fractions conducive for brine percolation (mean: 15%). Dissolved inorganic nutrient concentrations in the brine were scattered around theoretical dilution lines (TDLs), with some values of nitrate and nitrite, ammonium and silicic acid falling below TDLs, indicating nutrient depletion. Bulk ice dissolved organic carbon was low (mean: 64µmolkg−1), but most samples showed enrichment in relation to TDLs. Microbial biomass (bacterial and algal) was low, and generally showed maxima in the sea-ice interior. Bottom ice algal communities were dominated by pennate diatom species (mean: 86% of total cell abundance). 14C-total primary productivity (14C-TPP) ranged from <0.01 to 2.22mgC (mgchl a)−1d−1 (<0.01 to 3.03mgCm−2d−1). The relative contribution of 14C-total extracellular organic carbon (14C-TEOC) to 14C-TPP decreased over the observational period (range: 44–21%), with the remaining proportion being 14C-particulate organic carbon. 14C-TEOC composition was dominated by low molecular weight 14C-extracellular dissolved organic carbon (mean: 61%), with the remaining proportion allocated to 14C-colloidal organic carbon. Production of 14C-extracellular polymeric substances was not detected at any station.

Extracellular organic carbon dynamics during a bottom-ice algal bloom (Antarctica)

Antarctic fast ice provides a habitat for diverse microbial communities, the biomass of which is mostly dominated by diatoms capable of growing to high standing stocks, particularly at the icewater interface. While it is known that ice algae exude organic carbon in ecologically significant quantities, the mechanisms behind its distribution and composition are not well under- stood. This study investigated extracellular organic carbon dynamics, microbial characteristics, and ice algal photophysiology during a bottom-ice algal bloom at McMurdo Sound, Antarctica. Over a 2 wk period (November to December 2011), ice within 15 cm from the icewater interface was collected and sliced into 9 discrete sections. Over the observational period, the total concen- trations of extracellular organic carbon components (dissolved organic carbon (DOC) and total carbohydrates (TCHO) — the sum of monosaccharides (CHOMono) and polysaccharides (CHOPoly)) increased, and were positively correlated with algal biomass. However, when normalised to chlorophyll a, the proportion of extracellular organic carbon components substantially decreased from initial measurements. Concentrations of DOC generally consisted of <20% TCHO, typically dominated by CHOMono, which decreased from initial measurements. This change was coincident with improved algal photophysiology (maximum quantum yield) and an increase in sea-ice brine volume fraction, indicating an increased capacity for fluid transport between the brine channel matrix and the underlying sea water. Our study supports the suggestion that microbial exudation of organic carbon within the sea-ice habitat is associated with vertical and temporal changes in brine physicochemistry.

The interactions between Chlorella vulgaris and algal symbiotic bacteria under photoautotrophic and photoheterotrophic conditions

A Chlorella vulgaris ATCC 13482 culture was semi-continuously cultivated for 18 months in a 4-L photobioreactor and formed associated consortia with other symbionts. Three symbiotic bacterial strains were isolated on heterotrophic medium agar plates. Based on 16S rDNA analysis, they were found to show closest similarity to Pseudomonas alcaligenes, Elizabethkingia miricola and Methylobacterium radiotolerans. C. vulgaris was co-cultured with each bacterial strain, and it was found that the symbiotic bacterium Pseudomonas sp. had a growth-promoting effect on C. vulgaris while the other two inhibited algal growth. The interactions between C. vulgaris and Pseudomonas sp. were further investigated under different cultivation conditions. The co-culture resulted in 1.4 times greater algal cell concentration than that of C. vulgaris alone under photoautotrophic condition. In contrast, the algal cell concentration was lower in the co-culture compared with single algal culture when glucose was supplied in the medium (photoheterotrophic). Under both cultivation conditions, the number of Pseudomonas sp. increased at the beginning of experiment, and then decreased. However, the bacterial number decreased to almost zero under photoheterotrophic conditions, while the growth of bacteria went into a stationary phase under photoautotrophic conditions. The chlorophyll content in C. vulgaris cell was higher in co-culture than in single algal culture. Algal cells in photoautotrophic condition showed higher photosynthetic efficiency compared to those in photoheterotrophic condition. Extracellular organic carbon dissolved in the medium continuously increased under photoautotrophic condition. The mutualistic and competing relationships between C. vulgaris and symbiotic bacteria observed in this study could aid our understanding of algae–bacteria interactions in nature as well as broadening its practical applications.

Photosynthetic carbon allocation of an Antarctic sea ice diatom (Fragilariopsis cylindrus)

Antarctic sea ice provides an ephemeral but important habitat for algal productivity and is characterised by extreme physiochemical variations. In this study, we assess the ability of a sea ice diatom (Fragilariopsis cylindrus) to cope with physiochemical changes through examination of physiological status and allocation of 14C-incorporated organic carbon into particulate and extracellular fractions, using closed-bottle incubations over 49d. Carbon allocation was found to vary with growth stage and shifts in the physicochemical environment, in particular the carbonate system. Total extracellular organic carbon was comprised of at least 85% low molecular weight 14C-colloidal-organic carbon. The relative contribution of 14C-extracellular polymeric substances and 14C-total extracellular organic carbon to 14C-total primary production varied from lag to senescent growth phases, increasing from 0 to 5.7% and 32.9% to 69.5%, respectively. Carbon allocation into 14C-extracellular polymeric substances was correlated with a decline in CO2 availability and increased pH. Overall, the results demonstrate that carbon exudation may play an important role in adaptive algal physiology by buffering cells against biogeochemical shifts within brine channels, induced through photosynthetic activity.

Autotrophic Microbe Metagenomes and Metabolic Pathways Differentiate Adjacent Red Sea Brine Pools

In the Red Sea, two neighboring deep-sea brine pools, Atlantis II and Discovery, have been studied extensively, and the results have shown that the temperature and concentrations of metal and methane in Atlantis II have increased over the past decades. Therefore, we investigated changes in the microbial community and metabolic pathways. Here, we compared the metagenomes of the two pools to each other and to those of deep-sea water samples. Archaea were generally absent in the Atlantis II metagenome; Bacteria in the metagenome were typically heterotrophic and depended on aromatic compounds and other extracellular organic carbon compounds as indicated by enrichment of the related metabolic pathways. In contrast, autotrophic Archaea capable of CO2 fixation and methane oxidation were identified in Discovery but not in Atlantis II. Our results suggest that hydrothermal conditions and metal precipitation in the Atlantis II pool have resulted in elimination of the autotrophic community and methanogens.

Release of dissolved carbohydrates by Emiliania huxleyi and formation of transparent exopolymer particles depend on algal life cycle and bacterial activity

The coccolithophore Emiliania huxleyi plays a pivotal role in the marine carbon cycle. However, we have only limited understanding of how its life cycle and bacterial interactions affect the production and composition of dissolved extracellular organic carbon and its transfer to the particulate pool. We traced the fate of photosynthetically fixed carbon during phosphate-limited stationary growth of non-axenic, calcifying E. huxleyi batch cultures, and more specifically the transfer of this carbon to bacteria and to dissolved high molecular weight neutral aldoses (HMW NAld) and extracellular particulate carbon. We then compared the dynamics of dissolved carbohydrates and transparent exopolymer particles (TEP) between cultures of non-axenic and axenic diploid E. huxleyi. In addition, we present the first data on extracellular organic carbon in (non-axenic) haploid E. huxleyi cultures. Bacteria enhanced the accumulation of dissolved polysaccharides and altered the composition of dissolved HMW NAld, while they also stimulated the formation of TEP containing high densities of charged polysaccharides in diploid E. huxleyi cultures. In haploid E. huxleyi cultures we found a more pronounced accumulation of dissolved carbohydrates, which had a different NAld composition than the diploid cultures. TEP formation was significantly lower than in the diploid cultures, despite the presence of bacteria. In diploid E. huxleyi cultures, we measured a high level of extracellular release of organic carbon (34-76%), retrieved mainly in the particulate pool instead of the dissolved pool. Enhanced formation of sticky TEP due to bacteria-alga interactions, in concert with the production of coccoliths, suggests that especially diploid E. huxleyi blooms increase the efficiency of export production in the ocean during dissolved phosphate-limited conditions.

The use of antibiotics to reduce bacterioplankton uptake of phytoplankton extracellular organic carbon (EOC) in the Potomac River estuary

Phytoplankton production and accumulation of extracellular organic carbon (EOC) was tracked during diel intervals in microcosms by inhibiting bacterioplankton assimilation of EOC with streptomycin and kanamycin. Bacterioplankton production ( 3H-thymidine incorporation) and metabolism ( 14C-glucose incorporation) were monitored in samples collected from the Potomac River estuary to determine the effect of the antibiotics. Particulate (i.e., raw water) primary production and EOC (i.e., water passing through 1.0 μm glass fiber filter) production rates were monitored to determine the impact of antibiotics on phytoplankton. In preliminary experiments, neither streptomycin nor kanamycin alone significantly inhibited bacterioplankton activity compared to controls, but when both were present secondary production and metabolism were reduced up to 90%, and remained as such for 45 h. During field evaluations using a streptomycin and kanamycin mixture (50 μM each) particulate primary production and EOC production were not statistically different in control and antibiotic treated samples indicating that the antibiotics did not negatively influence phytoplankton production rates. In the presence of antibiotics dissolved free amino acids (DFAA) and, to a lesser extent, monosaccharides were significantly elevated compared to controls. This study demonstrates that streptomycin and kanamycin are capable of inhibiting bacterioplankton metabolism and uptake of dissolved organic carbon (DOC) in the samples tested so that the contribution of EOC to the DOC pool and to bacterioplankton metabolism could be measured and assessed.

Coupled photosynthesis and heterotrophic bacterial biomass production in a nutrient-limited wetland periphyton mat

Simultaneous measurements of photosynthesis (PS) and bacterial biomass production (BBP) were made on floating periphyton mats using a dual-isotope radioassay ( 14 C-bicarbonate and 3 H-L-leucine). Measurements were made across a gradient of light intensity in nutrient-limited periphyton, and under saturating light in a nutrient enrichment experiment. Mean PS in nutrient-limited periphyton followed standard light-saturation kinetics, but mean BBP showed little or no relationship to light. However, individual measurements of PS and BBP were correlated when data from all light levels were combined. Laboratory experiments revealed that both N and P enrichment altered the relationship between PS and BBP. N enrichment increased average PS and BBP, but weakened the correlation between these variables. P enrichment did not statistically increase PS or BBP, but did increase the dispersion of these data in bivariate space. Although the relationship between PS and BBP in the P enrichment was weaker than the relationship observed in the control, PS and BBP remained more tightly coupled during P enrichment than in N enrichment. Results of the present study suggest that N enrichment (i.e. induced P-limitation) may have decoupled PS and BBP by causing a competitive interaction for inorganic P. P enrichment, however, may have further facilitated a cooperative relationship, whereby bacterial production depended on the supply of either photosynthetically derived extracellular organic carbon or perhaps N derived from light-dependent N 2 fixation.

Production of extracellular organic carbon in the total primary production by freshwater benthic algae at the littoral zone and inflow river of Lake Biwa

(2006). Production of extracellular organic carbon in the total primary production by freshwater benthic algae at the littoral zone and inflow river of Lake Biwa. SIL Proceedings, 1922-2010: Vol. 29, No. 4, pp. 2021-2026.

Imbalance between phytoplankton production and bacterial carbon demand in relation to mucilage formation in the Northern Adriatic Sea

Spatial and temporal changes in phytoplankton production and bacterial C demand were investigated at four stations in the Northern Adriatic Sea over 3 years. The effect of the Po River plume was observed at the western stations; in particular, the northernmost one (B06) showed the highest values of primary production, both as hourly peaks (up to 14 mg C m − 3 h − 1 ) and daily water column integrated values (up to 740 mg C m − 2 day − 1 ), the southern station (C04) was only sporadically influenced and did not differ significantly from the easternmost ones (C12 and B13), where the lowest TPP values were recorded (around 1 mg C m − 3 h − 1 ). In this study the first in situ data are reported on short-term phytoplankton C extra cellular release in the Northern Adriatic Sea. At every station a considerable percentage of primary production (PER > 20% as an average, with peaks of up to 70%) was released as dissolved organic carbon. In particular, an association of fairly high PER (> 10%) and specific production ( P b > 10 mg C mg chl − 1 h − 1 ) was observed from spring to summer, when the mucilage phenomenon usually starts. This result might suggest the presence of an uncoupling between photosynthesis and growth, probably related with nutrient availability, which would be responsible for a high production of extra cellular organic carbon. Phytoplankton primary production and bacterial carbon production were closely related and bacterial C production accounted, on average, for a higher percentage of primary production than the values typically reported in the literature on aquatic environments. The flow of organic matter from phytoplankton to bacteria seems to satisfy the bacterial carbon demand during most of the spring and summer, at least in the upper water layers. However, during the summer, there is evidence that BCD sometimes exceeds the amount of C produced by phytoplankton. Neither phytoplankton nor bacterial production showed significant differences over the relevant years, and their absolute values did not change when comparing periods with or without mucilage. However, there were indications of an uncoupling between phytoplankton photosynthesis and growth and of a shift from an autotrophic to a heterotrophic metabolism, especially during the spring and summer period when mucilage might occur.

Benthic metabolism and nitrogen cycling in a subtropical east Australian estuary (Brunswick): Temporal variability and controlling factors

We examined temporal variability in benthic metabolism and nitrogen (N) cycling in the subtropical Brunswick Estuary, Australia from December 2000 to December 2002. Benthic metabolism was tightly coupled to the production of labile carbon (C) in the water column (phytodetritus) and temperature, both of which increased in summer, resulting in increased rates of benthic metabolism and a shift to sulfate reduction. C : N ratios of remineralized material showed a consistently low return of N relative to ‘‘Redfield’’-type material over the 2-yr study period (up to 84 : 1 and averaging 31 : 1). The highest remineralized C : N ratios occurred at a sediment CO2 productivity/ respiration of ;0.4, which corresponds to maximum respiration and maximum productivity, and also when watercolumn dissolved inorganic nitrogen concentrations were lowest, reflecting potential N limitation. The ‘‘missing N’’ was most likely assimilated by heterotrophic bacteria and autotrophic benthic microalgae (BMA). Extracellular organic carbon extruded by the BMA and subsequently decomposed may also account for some of the high remineralized C : N ratios. Net N2 effluxes were controlled by a complex interaction between the supply of NO from 2 3 the water column and nitrification, the supply and decomposition of labile C, benthic productivity, and macrofauna abundance. A N mass balance for the sediment over the 2-yr study period showed that a significant proportion of the mineralized nitrogen may have been removed from the microbial loop and passed up to the metazoan levels of the food chain. About 22% of the remineralized N was permanently removed via denitrification. This active competition for limited N resources between heterotrophs, autotrophs, and chemoautotrophs appears to be a mechanism by which N-limited oligotrophic subtropical estuaries tightly recycle and conserve N. The metabolism of benthic communities is a central component of the nutrient cycling and overall productivity of shallow coastal ecosystems. Much of the carbon decomposition in shallow coastal ecosystems occurs in bottom sediments through heterotrophic-mediated processes (Hammond et al. 1985), and the associated release of nutrients can provide a large proportion of the nitrogen and phosphorus required by pelagic communities (Boynton and Kemp 1985; Cowan and Boynton 1996). Bottom sediments can also be a sink for nutrients through the burial of carbon (C), nitrogen (N), and phosphorus; the loss of carbon through CO2 efflux; and the loss of N through denitrification. Denitrification (the reduction of NO through NO to nitrous oxide [N2O] and 22 32

Vertical planktonic structure in the central basin of Lake Baikal in summer 1999, with special reference to the microbial food web

Planktonic microbial interactions in the central basin of Lake Baikal were examined on a summer day in 1999. The subsurface maxima of bacterial abundance and chlorophyll concentration were recorded at the same depth, whereas the vertical distribution of heterotrophic nanoflagellates was the inverse of those of bacteria and picophytoplankton. Release of extracellular organic car-bon (EOC) from phytoplankton was estimated by the NaH14CO3 method as 2.4 µg C l−1 day−1. Bacterial production (4.3 µg C l−1 day−1), estimated in a bottle incubation experiment using size-fractionated water samples, exceeded the EOC released. Thus, other supplying sources of organic matter are needed for the bacterial production. Grazing (2.6 µg C l−1 day−1) was also estimated in the experiment and accounted for 60% of the bacterial production. This is the first report on the microbial food web in the central basin of Lake Baikal.

Physiology of diatom Skeletonema costatum (Grev.) Cleve photosynthetic extracellular release: evidence for a novel coupling between marine bacteria and phytoplankton

The photosynthesis of cellular materials by phytoplankton is accompanied by release of organic molecules from the algal cells into the water. The patterns of carbon fixation in particulate and dissolved pools were investigated in Skeletonema costatum cultured under 12 h light/12 h dark cycles. The short-term production (1–15 min) of particulate organic carbon (POC) and extracellular organic carbon (EOC) compounds was studied by measuring the uptake of 14C-labelled sodium bicarbonate and its subsequent incorporation and release into organic compounds. Slightly modified traditional 14C radiotracer protocols were used, including separation by electrophoresis and thin-layer chromatography and detection by autoradiography. Results indicated that there was a distinct difference between radiolabelled compounds in the POC and EOC pools. Several metabolites found in the EOC pool were not present in the POC pool, indicating the active release of these products from the cells into the ambient water during short-term incubations, and indicating that inorganic carbon fixation pathways in marine autotrophs might be partly extracellular.