The ecology of a subterranean isopod, Caecidotea tridentata

1. The blind isopod Caecidotea tridentata is the dominant invertebrate in a simple community of subterranean organisms inhabiting the local, shallow aquifer under Konza Prairie, Kansas, U.S.A.
2. The ecology of this karst aquifer was explored by sieving water from a spring, collecting water samples for analysis, and manipulating sediments and invertebrates in the laboratory. The size frequency distribution was stable and gravid females were present throughout the year, suggesting continuous reproduction.
3. The number of isopods collected per litre was negatively correlated with discharge and the abundance of bacteria, but positively related to the numbers of an amphipod ( Bactrurus hubrichti ). This may be explained if both the isopods and the amphipods track the saturated zone below the top of the water table; at lower discharge, the top of the water table is closer to the spring outlet. The suspended bacteria wash out in greater numbers with high discharge.
4. When isopods and carbon were added in a pairwise experimental design to sediments collected from the aquifer, the isopods significantly stimulated numbers and activity of planktonic and sedimentary bacteria, while carbon had no effect. This suggests that invertebrates can affect the microbes locally in an aquifer.     

Spatio-temporal patterns of microbial communities in a hydrologically dynamic pristine aquifer

Seasonal patterns of groundwater and sediment microbial communities were explored in a hydrologically dynamic alpine oligotrophic porous aquifer, characterized by pronounced groundwater table fluctuations. Rising of the groundwater level in consequence of snow melting water recharge was accompanied by a dramatic drop of bacterial Shannon diversity in groundwater from H' = 3.22 ± 0.28 in autumn and winter to H' = 1.31 ± 0.35 in spring and summer, evaluated based on T-RFLP community fingerprinting. Elevated numbers of bacteria in groundwater in autumn followed nutrient inputs via recharge from summer rains and correlated well with highest concentrations of assimilable organic carbon. Sterile sediments incubated to groundwater in monitoring wells were readily colonized reaching maximum cell densities within 2 months, followed by a consecutive but delayed increase and leveling-off of bacterial diversity. After 1 year of incubation, the initially sterile sediments exhibited a similar number of bacteria and Shannon diversity when compared to vital sediment from a nearby river incubated in parallel. The river bed sediment microbial communities hardly changed in composition, diversity, and cell numbers during 1 year of exposure to groundwater. Summing up, the seasonal hydrological dynamics were found to induce considerable dynamics of microbial communities suspended in groundwater, while sediment communities seem unaffected and stable in terms of biomass and diversity.     

Using Boreholes as Windows into Groundwater Ecosystems

Groundwater ecosystems remain poorly understood yet may provide ecosystem services, make a unique contribution to biodiversity and contain useful bio-indicators of water quality. Little is known about ecosystem variability, the distribution of invertebrates within aquifers, or how representative boreholes are of aquifers. We addressed these issues using borehole imaging and single borehole dilution tests to identify three potential aquifer habitats (fractures, fissures or conduits) intercepted by two Chalk boreholes at different depths beneath the surface (34 to 98 m). These habitats were characterised by sampling the invertebrates, microbiology and hydrochemistry using a packer system to isolate them. Samples were taken with progressively increasing pumped volume to assess differences between borehole and aquifer communities. The study provides a new conceptual framework to infer the origin of water, invertebrates and microbes sampled from boreholes. It demonstrates that pumping 5 m³ at 0.4–1.8 l/sec was sufficient to entrain invertebrates from five to tens of metres into the aquifer during these packer tests. Invertebrates and bacteria were more abundant in the boreholes than in the aquifer, with associated water chemistry variations indicating that boreholes act as sites of enhanced biogeochemical cycling. There was some variability in invertebrate abundance and bacterial community structure between habitats, indicating ecological heterogeneity within the aquifer. However, invertebrates were captured in all aquifer samples, and bacterial abundance, major ion chemistry and dissolved oxygen remained similar. Therefore the study demonstrates that in the Chalk, ecosystems comprising bacteria and invertebrates extend from around the water table to 70 m below it. Hydrogeological techniques provide excellent scope for tackling outstanding questions in groundwater ecology, provided an appropriate conceptual hydrogeological understanding is applied.     

Seasonal and spatial distribution of dissolved and particulate organic carbon and bacteria in the bank of an impounding reservoir on the Enns River, Austria

1. Interstitial bacterial abundance, production and ectoenzyme activity were investigated over an annual cycle in an Austrian river when infiltration of oligotrophic river water into a river-bank was artificially enhanced. These microbial parameters were related to porewater chemistry and the concentration of particulate (POC) and dissolved organic carbon (DOC).
2. Porewater chemistry reflected the hydrodynamic mixing of infiltrating river water with riparian groundwater. Seasonal fluctuations in the microbial parameters resulted mainly from changes in temperature and organic matter supply. Seasonal change in porewater chemistry in the river-bank was detectable laterally only within the first metre of the sediment and decreased rapidly with increasing distance from the sediment–water interface.
3. The DOC concentration decreased only slightly during lateral transport through the aquifer, while total organic carbon (TOC) concentration as well as abundance and activity of interstitial bacteria were reduced by up to one order of magnitude within the top metre of the sediment. Retention of incoming particulate matter structured the lateral distribution pattern of TOC concentration. The POC and not the DOC pool was the main source of carbon for interstitial bacteria and, therefore, the quality of POC determines the distribution of microbial metabolism within the riparian zone.     

Downstream increase in the abundance and heterotrophic activity of suspended bacteria in an intermittent calcareous headstream

SUMMARY. 1. Concentration of total bacteria, and heterotrophic activity (as turnover rate for glucose assimilation), in the water of an intermittent spring-fed calcareous headstream, increased with distance from the source to 1600 m.
2. This increase was gradual rather than stepwise and might be due to release of bacteria from submerged vegetation and the stream bed, and to input of terrestrial bacteria with groundwater.
3. The increment per unit length of stream was greatest in summer when aquatic vegetation was most abundant and discharge was low.
4. When the stream was partly dry, isolated peaks of suspended bacteria occurred and there was no regular downstream increase.
5. A lesser spring which upwelled into a natural basin consistently contained more suspended bacteria than the principal source, which was an artificial culvert.     

Seasonal trends in river biofilm storage products and electron transport system activity

SUMMARY. 1. Glycogen, poly-beta-hydroxyalkanoate (PHA) and electron transport system (ETS) activity levels were monitored seasonally in light- and dark-grown biofilms from a North Wales river.
2. A spring peak was evident in glycogen, PHA and ETS activity levels per cm² and per cell, with the exception of PHA per cell, which peaked in the winter.
3. There was a seasonal shift in the type of storage product found, perhaps reflecting seasonal changes in carbon sources.
4. The light-grown spring biofilm yielded the greatest amount of bio-film storage products per cm². This should be taken into account during caloric studies of river biofilms as a food source for grazing invertebrates.
5. A model is presented of factors influencing storage product deposition in river biofilms. The thickness and cell packing of river biofilms should be noted in future studies. Storage product deposition can occur deep within thick biofilms, irrespective of the river water chemistry.     

A stable carbon and nitrogen isotope study of lake food webs in Canada's Boreal Plain

1.  Stable carbon and nitrogen isotope and fish stomach content analyses were used to investigate food webs in five relatively undisturbed lakes on the Boreal Plain of Canada. Stable isotope analysis was also used to determine the importance of external and internal carbon sources.
2.  Overlap in the carbon and nitrogen signatures of primary producers made it difficult to determine unambiguously the feeding habits of many invertebrates. However, isotope analysis suggested that external carbon inputs were detectable in the aquatic food chains of the one lake with a short water residence time («1 year). In the other four lakes, with water residence times ≥1 year, autochthonous carbon was the only detectable carbon source in the food webs.
3.  Food webs in these lakes spanned a range of four to five trophic levels. Both invertebrates and fish appeared to eat a variety of food, often feeding at more than one trophic level.
4.  With the exception of one lake (SPH20), top predators in these lakes, northern pike ( Esox lucius ) and fathead minnows ( Pimephales promelas ), occupied similar trophic positions despite large differences in body size and trophic morphology. In SPH20, where there were two additional fish species, pike occupied a higher trophic position. However, all the top predators in each lake appeared to be omnivores and generalists.
5.  The prevalence of omnivory and the apparent generalist feeding habits of fish in these lakes suggest that organisms are flexible in their feeding habits and that these food webs will be resilient to disturbance.     

Numbers,diversity, and morphological characteristics of aerobic,chemoheterotrophic bacteria in deep subsurface sediments from a site in South Carolina

The aerobic, chemoheterotrophic bacteria indigenous to deep aquifers and other subsurface sediments (depths to 265 m) at a site in South Carolina were characterized by direct microscopy, enumeration of viable cells, analysis of colony morphologies on plates, and analysis of cell morphologies of isolated strains. Substantial numbers of viable bacteria (10^5‐10⁸/g) were present in all transmissive, aquifer sediments, and their numbers did not decrease with depth. Fewer bacteria (<10³/g) were detected in nontransmissive, confining layers. The highest viable counts were obtained on dilute media, but 10–50% of the bacteria in most aquifer sediments also grew rapidly on concentrated, nutrient‐rich media (indicating a high degree of metabolic flexibility). Most of the bacteria were mesophilic; relatively few psychrophiles or thermophiles were detected (<10³/g; in many cases, none). The bacterial flora was diverse (11–62 distinct colony types on enumeration plates of most aquifer sediments). Diversity did not decrease with depth, but the composition of the microflora (based on colony analysis) varied extensively from one geological formation to another. Almost 95% of the platable colonies that grew on enumeration plates contained nonstreptomycete bacteria, more than 80% of which were gram‐negative rods. Light microscopy of films released from aquifer sediments by flotation revealed the presence of dividing cells and microcolonies, thus implying that the in situ deep aquifer microflora was more metabolically active than that seen previously in shallow aquifers.     

Behavioural basis of intertidal zonation in Eurydice pulchra Leach

Eurydice pulchra Leach freshly collected at neap and spring tides exhibits an endogenous circatidal rhythm of swimming when kept without sand in constant conditions in laboratory aquaria. When provided with sand in the laboratory, tidally-phased swimming may occur spontaneously at some times of the lunar month, predominantly after the times of highest spring tides but not at neaps. In these cases, therefore, there appears to be clearly demonstrated for the first time in a marine animal a circasemilunar pattern of emergence from the substratum which is coupled to a circatidal rhythm of swimming; the isopods in the laboratory emerge from the sediment only when the swimming rhythm and emergence rhythm are at their maxima. In the field at neap tides some isopods emerge to swim at high tide, dependent upon their responses to water agitation, their feeding state and responses to light. At and just after spring tides in the field large numbers of the isopods swim at high tide, triggered by the endogenous circasemilunar rhythm of emergence which in the field is probably cued by the effects of increased water agitation. The main function of the endogenous circatidal rhythm of swimming appears to be to permit the isopods to return to their preferred zone in the sand before the fall of the tide. The coupled circasemilunar rhythm of emergence which induces greater swimming at spring tides reduces the risk of animals being stranded high on the shore during neap tides.     

An experimental study on the effects of salamander larvae on isopods in stream pools

SUMMARY 1. We used a combination of field surveys and field and laboratory experiments to examine the effects of predatory small-mouthed salamander larvae, Ambystoma barbouri (Kraus and Petranka), on the density and microhabitat use of benthic isopods, Lirceus fontinalis (Rafinesque).
2. A survey of four field stations yielded a significant negative relationship between the densities of salamander larvae and isopods.
3. A field experimental manipulation of salamander density confirmed that salamanders reduce isopod density and isopod use of open micro-habitat. Salamanders did not significantly affect the mean or coefficient of variation in isopod size.
4. A laboratory experiment showed that in the absence of refuges for isopods, salamanders were capable of eating large numbers of isopods; small isopods were particularly vulnerable to salamander predation.     

Microbial Populations of the River-Recharged Fredericton Aquifer

The City of Fredericton, New Brunswick, Canada obtains its water from the Saint John River-recharged Fredericton aquifer. Water quality improves as it flows from the river into the aquifer in a process called bank filtration. Microorganisms contribute to water quality improvements during bank filtration by removing organic carbon. In the Fredericton aquifer, microbially catalyzed reductive dissolution of manganese oxide minerals negatively affects water quality. Aerobic and anaerobic microorganisms were cultured from Fredericton aquifer production well water, from associated bedrock groundwater, and from Saint John River sediment core and sediment groundwater samples. Aerobes were the largest culturable groups in all samples. The ratio of aerobes to other microbial groups, i.e., those reducing nitrate, Mn^{4 +}, Fe^{3 +}, or sulfate, did not vary significantly along the water flow path from the river to the aquifer. Analysis of microbial community composition along the flow path indicated an essentially identical community except in the immediate vicinity of the aquifer. This is in agreement with the absence of macroscale redox zones in the sediment below the Saint John River as determined by groundwater geochemical data. Bacteria isolated from groundwater samples, identified by 16S ribosomal RNA gene sequencing, were α -, β -, γ -, and δ -Proteobacteria, Actinobacteria, and Firmicutes. In contrast to groundwater samples, the majority of bacteria isolated from sediments were γ -Proteobacteria. Numbers of manganese-reducing bacteria, including Aeromonas spp., were small, however Mn^{4 +} reduction ability was widespread in bacteria enriched and isolated with other electron acceptors. The diverse Fredericton aquifer microbial community likely uses manganese oxide minerals as a sink for electrons derived from organic carbon oxidation.     

Distribution of aerobic bacteria,protozoa, algae,and fungi in deep subsurface sediments

The distribution of microorganisms in deep subsurface profiles was determined at three sites at the Savannah River Plant, Aiken, South Carolina. Acridine orange direct counts (AODC) of bacteria were highest in surface soil samples and declined to the 10⁶ to 10⁷ per gram range in the subsurface, but then did not decline further with depth. In the subsurface, AODC values varied from layer to layer, the highest being found in samples from sandy aquifer formations and the lowest in clayey interbed layers. Sandy aquifer sediments also contained the highest numbers of viable bacteria as determined by aerobic spread plate counts (CFU) on a dilute heterotrophic medium. In some of these samples bacterial CFU values approached 100% of the AODC values. Viable protozoa (amoebae and flagellates, but no ciliates) were found in samples with high bacterial CFU values. A variety of green algae, phytoflagellates, diatoms, and a few cyanobacteria were found at low population densities in samples from two of the three boreholes. Low numbers of fungi were evenly distributed throughout the profiles at all three sites. Microbial population density estimates correlated positively with sand content and pore‐water pH, and negatively with clay content and pore‐water metal concentration. A large diversity of prokaryotic and eukaryotic microorganisms was found in samples with high population densities. A survey of bacterial strains isolated from subsurface samples revealed associations of gram‐positive bacteria with high clay sediments and gram‐negative bacteria with sandy sediments. The ability to deposit lipophilic storage material (presumably poly‐ß‐hydroxybutyrate) was found in a high proportion of isolates from sandy sediments, but only rarely in isolates from high clay sediments.     

Drift and upstream movement of invertebrates in a springbrook community ecosystem

Structure, drift, and upstream movement of populations of benthic macroinvertebrates, in particular Synurella dentata Hubricht and Lirceus fontinalis Raf., were examined within a temperate spring ecosystem. Chemical and physical aspects of the springbrook were also investigated and life histories of the gammarids and asellids noted.Chemically and physically the spring proved both constant and predictable, much more so than other lotic systems.Species diversity was low from November through February and increased in March, April, and May. Equitability followed the same trends as species diversity. Both indices were most affected by large fluctuations in the populations of aquatic insects.Significant changes in the numbers of amphipods, isopods, and total macroinvertebrates was evident over a seven month period. Males were present in the isopod population year-round, but only from November to January in the amphipod population. Breeding by the isopods occurred throughout the year and peaked during winter. Amphipods copulated only in the late fall and early winter.Significant diel peaks in the amphipod, isopod, and total invertebrate drift negatively correlated with light intensity levels. Amphipods and isopods did not exhibit any preferential upstream movement during either the day or night; however, total macroinvertebrate upstream movement was greater at night. The total number of invertebrates moving upstream were lower than values reported from other lotic environments.     

In situ exposure to low herbicide concentrations affects microbial population composition and catabolic gene frequency in an aerobic shallow aquifer

The aim of this study was to evaluate how the in situ exposure of a Danish subsurface aquifer to phenoxy acid herbicides at low concentrations (<40 micro g l(-1)) changes the microbial community composition. Sediment and groundwater samples were collected inside and outside the herbicide-exposed area and were analyzed for the presence of general microbial populations, Pseudomonas bacteria, and specific phenoxy acid degraders. Both culture-dependent and culture-independent methods were applied. The abundance of microbial phenoxy acid degraders (10(0) to 10(4) g(-1) sediment) was determined by most probable number assays, and their presence was only detected in herbicide-exposed sediments. Similarly, PCR analysis showed that the 2,4-dichlorophenoxyacetic acid degradation pathway genes tfdA and tfdB (10(2) to 10(3) gene copies g(-1) sediment) were only detected in sediments from contaminated areas of the aquifer. PCR-restriction fragment length polymorphism measurements demonstrated the presence of different populations of tfd genes, suggesting that the in situ herbicide degradation was caused by the activity of a heterogeneous population of phenoxy acid degraders. The number of Pseudomonas bacteria measured by either PCR or plating on selective agar media was higher in sediments subjected to high levels of phenoxy acid. Furthermore, high numbers of CFU compared to direct counting of 4',6-diamidino-2-phenylindole-stained cells in the microscope suggested an increased culturability of the indigenous microbial communities from acclimated sediments. The findings of this study demonstrate that continuous exposure to low herbicide concentrations can markedly change the bacterial community composition of a subsurface aquifer.     

Meiofauna assemblages discharged by springs from a phreatic aquifer system in the Netherlands

Groundwater meiofauna washed out of springs was studied by means of spring water filtration. The principal interest was a quantitative analysis of the number and diversity of outwash fauna in relationship to hydrological conditions. In addition, a comparative analysis was made of outwash fauna of different outlets of the same hydrological system. The spring complex studied is part of the Centraal Plateau in the southern-most part of The Netherlands. This area is characterized by, for The Netherlands, relatively high hydraulic heads and large permeability of aquifers. Influence of human activities (agriculture) in the recharge area was demonstrated by elevated concentrations of nitrate, chloride, and sulphate in the spring waters. No traces of groundwater pollution by heavy metals or pesticides were found. Temporal variation of meiofauna outwash was studied in a single rheocrene spring during one year. This rheocrene had a relatively large and constant discharge (1300±8 ¦h^–1). Meiofauna numbers fluctuated between 110–240 ind. m^–3. Considerable differences in meiofauna numbers in a one-off analysis of different springs (rheocrenes and helocrenes) were found. Numbers ranged from 140 to 5800 org. m^–3 and an inverse relationship with the amount of water discharged was shown. Organisms in the water filtrates were of multiple origin: aquifer(e.g. Niphargus, Parastenocaris), benthic spring head habitats (e.g. Gammarus, Chironomidae), and (saturated) soils near the outlets (e.g. Criconomatidae). Meiofauna abundance and composition is discussed in relation to hydrology and spring head morphology. A semi-quantitative analysis of outwash aquifer fauna was masked by the presence of epigean elements or elements of unclear origin. The outwash fauna of rheocrenes of large discharge supposedly is the most representative for the aquifer. Adaptations of hypogean populations to oligotrophic porous environments and the consequences for drift of juvenile stages is discussed.     

Spatial and temporal dynamics of the microbial community in the Hanford unconfined aquifer

Pyrosequencing analysis of 16S rRNA genes was used to study temporal dynamics of groundwater bacteria and archaea over 10 months within three well clusters separated by ∼30 m and located 250 m from the Columbia River on the Hanford Site, WA. Each cluster contained three wells screened at different depths ranging from 10 to 17 m that differed in hydraulic conductivities. Representative samples were selected for analyses of prokaryotic 16S and eukaryotic 18S rRNA gene copy numbers. Temporal changes in community composition occurred in all nine wells over the 10-month sampling period. However, there were particularly strong effects near the top of the water table when the seasonal rise in the Columbia River caused river water intrusion at the top of the aquifer. The occurrence and disappearance of some microbial assemblages (such as Actinobacteria ACK-M1) were correlated with river water intrusion. This seasonal impact on microbial community structure was greater in the shallow saturated zone than deeper zone in the aquifer. Spatial and temporal patterns for several 16S rRNA gene operational taxonomic units associated with particular physiological functions (for example, methane oxidizers and metal reducers) suggests dynamic changes in fluxes of electron donors and acceptors over an annual cycle. In addition, temporal dynamics in eukaryotic 18S rRNA gene copies and the dominance of protozoa in 18S clone libraries suggest that bacterial community dynamics could be affected not only by the physical and chemical environment but also by top-down biological control.     

Seasonal dynamics of cell numbers and biodiversity of marine heterotrophic bacteria inhabiting invertebrates and water ecosystems of the Peter the Great Bay,Sea of Japan

The annual changes in bacterial numbers and diversity of the heterotrophic microflora in invertebrates and ambient water were studied. During the whole period of observation, bacterial cell numbers were higher in invertebrate specimens than in the ambient water. The highest number of bacterial cells was detected in trepangs and sea urchins, while the lowest number of cells was detected in starfish. Based on the results of phenotypic analysis and analysis of fatty acid composition of bacterial cell lipids, 487 strains (out of the total of 502 isolates) of heterotrophic bacteria were identified to the genus level. Morphological differences between the winter and summer isolates of vibrios and halomonads were analyzed. The seasonal dynamics of the cell numbers of vibrios and halomonads was revealed. The gram-positive microflora was most often present in animals during the winter, fall, and spring periods. The diversity of heterotrophic bacteria was greater in the water column than in animal tissues.     

海洋细菌抗菌和细胞毒活性的初步研究

从不同海域的生物、海水和海泥中分离海洋细菌,利用琼脂扩散法和MTT法对细菌培养液的乙酸乙酯提取物进行了抗菌和细胞毒活性筛选。比较了活性菌株与来源的相关性．结果表明,在分离的341株海洋细菌中。42株细菌的代谢产物具有抗菌活性,7株具有细胞毒活性,其中来源于海洋无脊椎动物和海藻的活性菌株比例(22％和11％)大于来源于海水和海泥的细菌(7％和5％)．细菌分类鉴定结果显示,具有活性的细菌大部分属于假单胞菌属、发光杆菌属、梭状芽孢杆菌属、交替单胞菌属和黄杆菌属．     

Sulfur bacteria in sediments of two coastal ecosystems: the Bassin d'Arcachon and the Etang du Prévost, France

Enumeration of the functional groups of sulfur bacteria was performed in the sediments in the Bassin d'Arcachon, a mesotidal lagoon with strong tidal currents and dominant populations of seagrass (Zostera noltii), and in the Etang du Prévost, a shallow lagoon with moderate tidal fluctuations and dominant populations of floating seaweed (Ulva sp.). In addition, data were collected on the distribution of oxygen and sulfide at the water-sediment interface during diel cycles. Bacterial enumeration studies revealed highest numbers in the top two cm of the sediments for three functional groups of sulfur bacteria, these being the sulfate-reducing bacteria (SRB), the colorless sulfur bacteria (CSB), and the phototrophic sulfur bacteria (PSB). In both systems high numbers of SRB were encountered, suggesting ample availability of organic matter. A comparison between different sites in each ecosystem showed that sediments overlain by more stagnant water were dominated by PSB, whereas those overlain by more oxygenated water were dominated by CSB. Important factors are the physical forces induced by tidal currents and the degree of daily exchange of water between the lagoons and the sea. These factors may explain the differences observed between the two systems with regard to the development of anoxic conditions, more so than the level of eutrophication. It appears that rooted plants play an important role in the introduction of oxygen into the sediments, thus enhancing the competitive position of CSB compared to PSB. Mini-electrodes studies revealed high concentrations of free sulfide at the inner site of the Etang du Prévost but very low concentrations at the inner station of the Bassin d'Arcachon, which may be explained by the high iron input of the latter, rather than by differences in the rate of sulfide production.     

Chemoautotrophic Carbon Fixation Rates and Active Bacterial Communities in Intertidal Marine Sediments

Chemoautotrophy has been little studied in typical coastal marine sediments, but may be an important component of carbon recycling as intense anaerobic mineralization processes in these sediments lead to accumulation of high amounts of reduced compounds, such as sulfides and ammonium. We studied chemoautotrophy by measuring dark-fixation of ¹³C-bicarbonate into phospholipid derived fatty acid (PLFA) biomarkers at two coastal sediment sites with contrasting sulfur chemistry in the Eastern Scheldt estuary, the Netherlands. At one site where free sulfide accumulated in the pore water right to the top of the sediment, PLFA labeling was restricted to compounds typically found in sulfur and ammonium oxidizing bacteria. At the other site, with no detectable free sulfide in the pore water, a very different PLFA labeling pattern was found with high amounts of label in branched i- and a-PLFA besides the typical compounds for sulfur and ammonium oxidizing bacteria. This suggests that other types of chemoautotrophic bacteria were also active, most likely Deltaproteobacteria related to sulfate reducers. Maximum rates of chemoautotrophy were detected in first 1 to 2 centimeters of both sediments and chemosynthetic biomass production was high ranging from 3 to 36 mmol C m⁻² d⁻¹. Average dark carbon fixation to sediment oxygen uptake ratios were 0.22±0.07 mol C (mol O₂)⁻¹, which is in the range of the maximum growth yields reported for sulfur oxidizing bacteria indicating highly efficient growth. Chemoautotrophic biomass production was similar to carbon mineralization rates in the top of the free sulfide site, suggesting that chemoautotrophic bacteria could play a crucial role in the microbial food web and labeling in eukaryotic poly-unsaturated PLFA was indeed detectable. Our study shows that dark carbon fixation by chemoautotrophic bacteria is a major process in the carbon cycle of coastal sediments, and should therefore receive more attention in future studies on sediment biogeochemistry and microbial ecology.