Impact of polychaetes (Nereis spp. and Arenicola marina) on carbon biogeochemistry in coastal marine sediments†
© The Royal Society of Chemistry and the Division of Geochemistry of the American Chemical Society 2001
Received: 07 September 2001
Accepted: 16 October 2001
Published: 30 October 2001
Known effects of bioturbation by common polychaetes (Nereis spp. and Arenicola marina) in Northern European coastal waters on sediment carbon diagenesis is summarized and assessed. The physical impact of irrigation and reworking activity of the involved polychaete species is evaluated and related to their basic biology. Based on past and present experimental work, it is concluded that effects of bioturbation on carbon diagenesis from manipulated laboratory experiments cannot be directly extrapolated to in situ conditions. The 45–260% flux (e.g., CO2 release) enhancement found in the laboratory is much higher than usually observed in the field (10–25%). Thus, the faunal induced enhancement of microbial carbon oxidation in natural sediments instead causes a reduction of the organic matter inventory rather than an increased release of CO2 across the sediment/water interface. The relative decrease in organic inventory (Gb/Gu) is inversely related to the relative increase in microbial capacity for organic matter decay (kb/ku). The equilibrium is controlled by the balance between organic input (deposition of organic matter at the sediment surface) and the intensity of bioturbation. Introduction of oxygen to subsurface sediment and removal of metabolites are considered the two most important underlying mechanisms for the stimulation of carbon oxidation by burrowing fauna. Introduction of oxygen to deep sediment layers of low microbial activity, either by downward irrigation transport of overlying oxic water or by upward reworking transport of sediment to the oxic water column will increase carbon oxidation of anaerobically refractory organic matter. It appears that the irrigation effect is larger than and to a higher degree dependent on animal density than the reworking effect. Enhancement of anaerobic carbon oxidation by removal of metabolites (reduced diffusion scale) may cause a significant increase in total sediment metabolism. This is caused by three possible mechanisms: (i) combined mineralization and biological uptake; (ii) combined mineralization and abiogenic precipitation; and (iii) alleviation of metabolite inhibition. Finally, some suggestions for future work on bioturbation effects are presented, including: (i) experimental verification of metabolite inhibition in bioturbated sediments; (ii) mapping and quantification of the role of metals as electron acceptors in bioturbated sediments; and (iii) identification of microbial community composition by the use of new molecular biological techniques. These three topics are not intended to cover all unresolved aspects of bioturbation, but should rather be considered a list of obvious gaps in our knowledge and present new and appealing approaches.
It has long been recognized that activities of macrobenthic organisms have significant effects on sediment–water solute exchange and diagenetic reactions within sediments. Much experimental and modelling work done over the years on the effects of reworking and irrigation by burrowing animals such as polychaetes, crustaceans and bivalves [1–5] have not only enhanced our understanding of bioturbation effects, but also provided important knowledge on general mechanisms controlling diagenetic processes in sediments. It is now well established that the distribution of a porewater solute is determined by the balance between transport and reaction processes, and that irrigating infauna may change this balance dramatically by enhancing transport conditions. This has been modelled by a variety of excellent models such as the cylinder model of Aller and the non-local exchange model of Emerson. Although these two models basically are equivalent, the construction of the cylinder model allows the incorporation of additional realistic complexity (e.g. semipermeable burrow linings and periodic irrigation).
Nevertheless, the real complexity created from a multitude of behavioural activities by the wide variety of burrow-dwelling animals found in marine sediments is extremely difficult to describe mathematically. Activities, such as filter-or suspension-feeding and specific locations of faecal pellet deposition, may have serious impact on diagenetic reactions and solute distributions within sediments. While models may improve our general knowledge on bioturbation effects, the particular activity of any specific assembly of infauna can only be determined by examining both the impact of each individual species as well as of the entire community.
This paper attempts to review the current knowledge on the impact of common intertidal polychaetes, Nereis spp. and Arenicola marina, on fluxes and reaction rates involving carbon in coastal sediments. The behaviour and life habits of these animals are linked to measured fluxes and porewater profiles of CO2 as well as carbon reaction rates within sediments. The results are also used for the construction of simple (conceptual) models describing the impact of these species on carbon diagenesis in sediments. Finally, some guidelines are given for future work still needed to provide a more complete understanding of sediment carbon biogeochemistry and microbiology associated with Nereis spp. and A. marina.
Biology of Nereis spp. and Arenicola marina
The two Nereis species have been described as omnivores and detritivores feeding by swallowing surface sediment as well as plant and animal remains around the burrow opening. However, N. diversicolor also has the ability to live as a suspension-feeder. The ventilatory water current is then driven through a mucus net spun by the worm near the entrance. Suspended food particles are retained in the net, which is subsequently eaten. No such suspension feeding has been observed in N. virens.
Burrow ventilation by A. marina is driven from tail to head by peristaltic movements of the body. Water enters the burrow through the tail opening to the surface and exits by percolation into the sediment in front of the head and up through the feeding funnel.
The activities of benthic fauna have profound impact on physical, chemical and biological conditions in aquatic sediments. Sediment reworking and water irrigation (or ventilation) displaces both particles and porewater within the sediment. By volume, the amount of water moved is much larger than that of particles, and burrows act as channels for the direct communication between subsurface porewaters and overlying water. The enhanced solute exchange, therefore, redistributes dissolved reactants and products of microbial reactions within sediments.[23, 24]
Most infaunal animals actively irrigate their burrows with oxygen-rich overlying water. The renewal of burrow water serves important transport functions for the animals, such as supply of oxygen, removal of toxic metabolites and providing suspended food items. However, burrow irrigation may also affect the distribution of meiofauna and microorganisms, as well as the associated biogeochemical processes within the sediment.[4, 5, 25]
Examples of irrigation rate of Nereis virens, N. diversicolor and Arenicola marina. Results are presented both as individual rates [(g worm) h-1] and as population rates (m2 d-1). The abbreviations (ns) and (s) indicate non-suspension-feeding and suspension-feeding N. diversicolor.[9,28]
Individual/ml g-1 h-1
Population/1 m-2 d-1
N. diversicolor (ns)
N. diversicolor (s)
Macrofaunal reworking affects the stability and composition of coastal marine sediments. Thus, organic matter deposited in the sediment is usually redistributed, i.e. from surface to subsurface layers and vice versa. However, various burrow-dwelling animals disturb the sediment structure differently depending on their specific life habit and feeding type.[22, 30]
Examples of annual particle reworking rates by Nereis diversicolor and Arenicola marina. Results are presented both as volume of sediment m-2 and as depth of sediment reworked[19,52]
Volume/1 m-2 y-1
Free-living polychaetes, like Nereis spp., are actively moving around at the sediment surface or within their burrow systems in search for food. Occasionally they abandon burrows, either willingly or forced by inter- and intraspecific fights, and move some distance before digging a new 'home'. The amount of particles moved by these feeding and digging activities of Nereis spp. is quantitatively small (i.e. 2%) compared with A. marina (Table 2). However, the selective feeding of Nereis spp. on fresh plants, animals and microorganisms at the sediment surface associated with suspension feeding, subsurface defecation and mucus secretions along burrow walls may redistribute significant amounts of reactive organic matter within the sediment.
Organic matter is degraded (mineralized) in sediments by an array of aerobic and anaerobic microbial processes with a concurrent release of inorganic nutrients. The rates of decay depend on a variety of factors such as, organic matter quality (i.e., the content of protein, cellulose, lignin, etc.), age (decomposition stage) and temperature (season). The chemical composition of organic matter can be generalized according to: (CH2O) x (NH3) y (H3PO4) z , where x, y and z vary depending on the origin and age of the material. For marine organic matter (e.g. phytoplankton) having the Redfield composition the stoichiometry is as follows: x = 106, y = 16, and z = 1.
Almost all heterotrophic organisms with aerobic metabolism have the enzymatic capacity to perform a total oxidation of organic carbon. Organic matter may therefore be completely metabolized by a single organism to H2O, CO2 and inorganic nutrients using oxygen as electron acceptor according to eqn. (1):
(CH2O) x (NH3) y (H3PO4) z + xO2 → xCO2 + yNH3 + zH3PO4 + xH2O (1)
However, due to an efficient energy metabolism, a large fraction of the metabolized organic matter ends up as cell material. Aerobic decomposition is unique in the sense that oxygen-containing radicals such as superoxide anion (O2 -), hydrogen peroxide (H2O2) and hydroxyl radicals (.OH) are readily formed and consumed. These are capable of breaking strong chemical bonds and thus depolymerize relatively refractory organic compounds rich in aromatic structures like lignin.
Anaerobic decomposition occurs stepwise, involving several different functional types of bacteria. First, large and normally complex polymeric organic molecules are stepwise split into water-soluble monomers (amino acids, monosaccharides and fatty acids) by hydrolysis and fermentation under the production of energy and release of inorganic nutrients, e.g. mixed propionate and acetate formation (eqn. (2)):
6(CH2O) x (NH3) y (H3PO4) z → xCH3CH2COOH + xCH3COOH + xCO2 + xH2 + 6yNH3 + 6zH3PO4 (2)
These small organic acids are then oxidized completely to H2O and CO2 by a number of respiring microorganisms using a variety of oxidized inorganic compounds as electron acceptors.
The individual anaerobic respiration processes generally occur in a sequence with depth in the sediment according to the availability of electron acceptors: Mn4+, NO3-, Fe3+, SO42-and CO2 respiration. The actual sequence is determined by the ability of each electron acceptor to receive electrons, and thus the energy output per degraded organic carbon atom, e.g. manganese respiration is energetically more favourable than sulfate reduction. The suboxic zone contains the most potent anaerobic electron acceptors, Mn4+, NO3-, and Fe3+. The transition from one electron acceptor to the other downwards in the sediment occurs when the most favourable is exhausted, although some vertical overlap may occur. Two representative examples of anaerobic degradation stoichiometries are manganese and sulfate reduction (eqn. (3) and (4)):
CH3COOH + 4MnO2 + 8H+ → 4Mn2+ + 2CO2 + 6H2O (3)
CH3COOH + SO42- → 2CO2 + S2- + 2H2O (4)
A significant portion of sediment oxygen uptake is not caused by aerobic respiration, but is rather due to reoxidation of reduced inorganic metabolites (e.g., NH4+, Mn2+, Fe2+ and H2S) close to the oxic/anoxic interface. Thus, up to 85% of the sulfide produced by sulfate reduction is not trapped permanently by reactions with iron and other metals, but is continuously diffusing upwards to be reoxidized in the near-surface sediment. About 50% or more of the total sediment oxygen uptake is usually consumed directly or indirectly by oxidation of sulfide. Reoxidation can be a pure chemical process, but it is usually mediated by chemoautotrophic microorganisms.
The strict vertical distribution of electron acceptors, as mentioned above, is an over-simplification of the true spatial distribution. The influence of sediment inhomogeneities, such as worm burrows, on porewater profiles and vertical distribution of microbial processes has been clearly documented. Furthermore, patches associated with, e.g., faecal pellets may create anaerobic microniches, where anaerobic processes, such as denitrification and sulfate reduction, occur in otherwise oxic surface sediments.[39, 40]
Nevertheless, the usually observed decreasing degradation rate with depth in sediments is not solely caused by the less efficient electron acceptors in the deeper layers, but rather by decreasing degradability of organic matter.
Carbon oxidation in sediment inhabited by Nereis spp. and Arenicola marina
Impact on microbial reaction rates
The first studies examining the importance of Nereis spp. and Arenicola marina on sediment processes focused on the role of these animals for sediment–water fluxes and vertical porewater profiles of various solutes. [41–43] These studies were supplemented with measurements of e.g. burrow architecture and defecation rates.[12, 19] More recent biogeochemical studies employing a variety of experimental and modelling techniques have gained important new knowledge on the mechanisms controlling reaction rates in bioturbated sediments in general[23, 44] and more specifically in nearshore sediments affected by Nereis spp. and Arenicola marina.[9, 45, 46]
Irrigation is particularly important in enhancing solute exchange between overlying water and pore fluids. However, it is not an easy task to measure the impact of infauna on solute fluxes in nature due to the obvious lack of fauna-free sediment patches comparable to bioturbated locations. Sieving and homogenizing sediment to remove fauna and other inhomogeneities have partly solved this problem. Measurements are then done in laboratory microcosms by reintroducing known densities of animals, while defaunated controls are kept as a reference. Despite its inherent limitations (see later), this technique has been and still is widely used for certain purposes.[9, 45] Other studies use intact sediment microcosms, which are deoxygenated by N2 flushing in order to force the existing fauna out of the sediment followed by introduction of known densities of selected species when aeration is resumed. Unfortunately, only few studies have made comparable in situ measurements without manipulations, and few or none have yet been published for Nereis spp. and Arenicola marina.
Enhancement of benthic metabolism (O2 uptake or CO2 release) in sandy sediment inhabited by Nereis spp. and Arenicola marina. Values are given as percent difference between faunated and defaunated sediment (flux enhancement). The experimental conditions are indicated as: homog. = homogenized sediment in laboratory, intact = intact, but defaunated sediment in laboratory, in situ = in situ measurement a
Flux enhancement (%)
Only few attempts have been made to determine the flux enhancement by A. marina under manipulated laboratory conditions. The few data available indicate that this species have a higher capacity to increase fluxes than Nereis spp. Upward percolation of irrigated water through the feeding funnel forces porewater rich in solutes out of the sediment, which is more efficient than the combined action of radial diffusion into the burrow and advective transport out of the burrow as in the Nereis case.
Organic matter replenishment is another problem with the manipulated laboratory experiments. The microcosms are rarely continuously supplied with new organic substrates to balance the removal by decomposition. Even the supply from benthic diatoms may be impaired as the experiments usually are performed in darkness. As a consequence, true steady state will never be reached. The stable flux conditions usually observed after the initial porewater flushing (Fig. 9) can therefore only be considered a short-term pseudo steady state. In due course (months), fluxes must decrease to very low levels, and eventually be lower in bioturbated than defaunated sediment due to exhaustion of reactive organic matter.
The flux enhancement reached in manipulated systems, when the pseudo steady state is approached immediately after the initial porewater flushing, should be considered a measure of the enhanced metabolic capacity of all heterotrophic organisms in the sediment. However, this proposition is difficult to test experimentally under natural conditions because defaunated sediments underlying oxic water columns are rare. An alternative approach is to apply a speculative scenario based on the current knowledge on effects of Nereis spp. and Arenicola marina.
About 30% of the deposited carbon in the N. diversicolor sediment was oxidized and lost rapidly as excess CO2 flux to the water column. Incorporation into N. diversicolor tissues accounted for 45% of the carbon, while the remaining accumulated within the sediment. In the N. virens sediment, where no excess phytoplankton deposition occurred, decomposition resulted in a net loss of sedimentary carbon due to the enhancement of microbial decomposition by N. virens. Thus, two almost identical species may either enrich or impoverish the organic inventory of sediments depending on their life habit.
Causes for stimulated microbial decomposition
A number of macrofaunal activities are known or have been inferred to cause the enhancement of microbial metabolism and capacity for organic matter degradation in bioturbated sediments: particle manipulation, grazing, excretion/secretion, burrow/tube construction, irrigation and particle reworking. The two latter activities, which are the focus of this paper, are considered particularly important controlling factors for carbon diagenesis in Nereis spp. and Arenicola marina inhabited sediments.[4, 55]
The current knowledge on the underlying mechanisms for the impact of irrigation (defined here as downward transport of oxidants and upward transport of metabolites in sediments) and particle reworking (defined here as transfer of organic matter and redox sensitive minerals between redox zones) by species like Nereis spp. and A. marina on carbon diagenesis in coastal sediments will be summarized in the following. While acknowledging the simultaneous and inseparable nature of these activities, they will be treated individually and compared within and between species, when possible.
The role of oxygen
Quantitative estimates of the enhanced decomposition caused by injection of oxygen into actively irrigated burrows or by oxygen exposure due to particle reworking are rare. By the use of a modified version of the simple volumetric model presented by Kristensen and Holmer, rough estimates can be provided for the stimulated carbon reaction rate in sandy coastal sediment shortly after the introduction of macrobiotic activity in the form of either irrigated burrows of e.g. Nereis diversicolor or burrows reworked by e.g. Arenicola marina.
In both cases, the calculations are based on the following common assumptions: (i) all diagenetic processes occur in the upper L cm of the sediment column; (ii) the oxic surface zone is Lox cm thick; (iii) the rate of deposition (e.g. phytoplankton) or production (e.g. benthic diatoms) at the sediment surface of labile organic matter is similar irrespective of the species present; (iv) the fresh and labile detritus is mineralized A1 times faster (both in the presence and absence of oxygen) than the old and partly degraded detritus in the anoxic zone (R1ox = R1an = R1 = A1 R2an); (v) mineralization of old and partly degraded organic matter from anoxic zones is enhanced by a factor of A2 when exposed to oxygen at the surface during reworking or along irrigated burrow walls (R2ox = A2R2an); (vi) no depth dependent change in degradability of sediment detritus occurs in oxic and anoxic zones (dR/dx = 0), and the anoxic mineralization rate is independent of electron acceptors.
Ciox = R2an (V - (Vox + Vban + Vwox)) + R1 (Vox - Vbox) + R2ox Vwox (5)
Cdox = R2an (V - Vox)+ R1Vox (6)
where: V = L × 104 is total sediment volume (cm3 m-2) to depth L; Vox = Lox × 104 is volume oxic surface sediment without burrows; Vban = π r2(Lb - 2Lox) d is subsurface burrow lumen volume; Vbox = π r2 2Lox d is surface burrow lumen volume; Vwox = π (Box 2 + 2r Box) (Lb - 2Lox) d is oxic subsurface burrow wall volume. The initial enhancement of carbon oxidation caused by the presence of irrigated burrows with oxic walls is then, Ei = Ciox/Cdox.
In the reworking case with Arenicola marina (Fig. 17(b)) the following specific conditions are assumed: (i) reworking is continuous and subsurface sediment is distributed in an even layer on top of the sediment with a steady state thickness at least similar to the depth of the oxic surface layer (Lox); (ii) labile surface sediment with a thickness similar to that in the defaunated situation (L1 = Lox) is continuously pushed downward into the anoxic zone; (iii) the A. marina abundance is d individuals m-2; (iv) since burrows are assumed vertical and the presence of oxic subsurface sediment (e.g. burrow walls) is ignored, the burrow lumen can be excluded from calculations (these assumptions are clearly false, but necessary, when only reworking activities are considered). From sediment volumes and reaction rates in the various zones, total sediment carbon oxidation (m-2) in the presence and absence of sediment reworking can be estimated according to eqn. (7) (reworked) and (8) (defaunated), where the volume of reduced subsurface sediment deposited at the oxic surface (Vrox) is similar to the volume of buried sediment containing labile detritus (V1).
Crox = R2an (V- (V1 + Vrox)) + R1V1 + R2ox Vrox (7)
Cdox = R2an (V - Vox)+ R1 Vox (8)
where: V = L × 104 is total sediment volume (cm3 m-2) to depth L; Vox = Lox × 104 is volume oxic surface sediment without burrows; V1 = L1 × 104 cm3 m-2 is volume of buried sediment containing labile detritus; Vrox = Lox × 104 cm3 m-2 is volume of reduced subsurface sediment deposited at the oxic surface. However, due to the continuous input of labile detritus and deposition by reworking a dilution of the labile material into a larger sediment volume obviously occurs. Nevertheless, based on the assumption above, the volumetric reaction rate should remain unaffected. The initial enhancement of carbon oxidation caused by sediment reworking are then, Er = Crox/Cdox = (L + (A1+ A2 - 2)Lox)/(L + ((A1 - 1) Lox).
The reworking model (eqn. (7) and (8)) predicts that carbon oxidation in the presence of A. marina is enhanced by Er = 1.11, when the variables, L, Lox, A1 and A2 are similar to those used in the irrigation model, and that the enhancement is independent of worm abundance, d (Fig. 18). This prediction is only valid when reworked subsurface sediment containing reactive organic material covers the sediment surface to a depth similar to the oxic zone and that reaction rates are not so fast as to eliminate entirely the organic material exposed in a single transit. This means that below a certain threshold the enhancement must be proportional to abundance. As one average sized individual deposits 1.4 × 10-3cm d-1 sediment at the surface, a population size of 7 m-2 is needed to deposit a 0.3 cm layer (equivalent to Lox) every month. The actual threshold of abundance is probably lower than this limit, since there were no signs of changes in reactivity of aged materials exposed to oxygen for one month.
It is important to note that these model calculations are only valid for the initial conditions after introduction of macro-benthic activity. For a fixed influx of material, the steady state mass of reactive organic matter in a sediment deposit where decomposition rates are enhanced by faunal activities must be lower than for a defaunated situation (see the "Decomposition rates" sub – section above). Nevertheless, these model examples clearly illustrates that decomposition capacity of partly degraded organic matter along oxic walls of irrigated infaunal burrows is enhanced progressively more than by exposure of subsurface sediment to oxygen during reworking.
In the reworking case, where A. marina is used as model organism, oxygen effects due to irrigation of this worm are ignored. As mentioned earlier, the burrow structure and irrigation type of A. marina may result in comparable or larger impact on sedimentary reaction rates than that of N. diversicolor, and should be recognized and included in sedimentary budget predictions. As a consequence, the overall impact (including both reworking and irrigation) of A. marina may very well be higher than of N. diversicolor. Accordingly, Banta et al. reported up to 3-fold higher stimulation of carbon oxidation by A. marina than N. diversicolor.
Removal of metabolites
Irrigation not only promotes metabolic processes by supplying electron acceptors, like oxygen, to bioturbated subsurface sediments, but also net reactions in anoxic regions of inhabited sediments are stimulated. This implies that net rates of anaerobic microbial processes are controlled by the exchange of other solutes than oxygen during irrigation. Metabolites, such as CO2, ammonium and sulfide ions (Fig. 10) in particular, are efficiently removed from sediment porewaters by infaunal irrigation.
The proposed effect of infauna on metabolite dependent processes has not yet been confirmed from direct measurements in bioturbated sediments. However, relaxation of metabolite effects is a very plausible contributing explanation for the observed irrigation effect of Nereis spp. and Arenicola marina on benthic carbon mineralization.
Although the understanding of bioturbation effects in general and by Nereis spp. and Arenicola marina in particular has advanced considerably within the last couple of decades, the interactions with transport regime (diffusion scale), decomposition pathways (electron acceptor availability), microbial communities (bacterial populations) and associated processes are some of the important areas that remain largely unexplored. Furthermore, there is, as mentioned earlier, an urgent demand for conducting measurements by in situ approaches to avoid elucidating artefacts caused by the frequently used manipulative laboratory experiments.
The three possible cases suggested by Aller and Aller as explanations for the reaction rate response to changing diffusion scale (1, biological uptake; 2, abiogenic precipitation; and 3, inhibition) must be experimentally verified. The first step must be to determine the response of anaerobic sediment processes to the addition and removal of a variety of potentially inhibiting metabolites, either individually or in chosen combinations. This should be done in anoxic sediment incubations, where solute concentrations in the pore-water as well as reaction rates can be controlled and measured simultaneously. In the second step, reaction rates and the concentration of inhibitory metabolites should be measured in sediment microcosms with different densities of selected infaunal species. The third and most critical step will be to confirm that the interactions between reaction rates and inhibitory metabolites, which were revealed in steps one and two, are also active under in situ conditions. This task is complicated by the unpredictable seasonal, diurnal and spatial variations in the field and demands a competent small-scale approach conducted over long time series.
Electron acceptor availability
It has been shown, beyond any doubt, that infaunal irrigation increases the availability of oxygen as an electron acceptor in deep sediment strata. As a consequence, the availability of nitrate as electron acceptor for anaerobic respiration (denitrification) also increases due to downward irrigation transport and nitrification occurring in oxic burrow walls.[51, 65, 66] However, direct evidence for increased availability of Mn4+ and Fe3+ as electron acceptors in bioturbated sediments is lacking. Based on strong evidence, it has been argued, though, that the large contribution of Mn4+ and Fe3+ respiration to total benthic metabolism in manganese- and iron-rich deposits is primarily caused by bioturbation (particularly reworking).[67, 68] Although visual observations of brownish oxidized zones associated with deep burrow structures (Fig. 7) clearly indicate the presence of oxidized iron (and manganese), no studies have yet directly quantified the amount of Fe3+ present around burrow structures and the role of benthic animals for iron and manganese respiration in sediments. Another intriguing question arises: is mineralization of refractory organic substrates with Mn4+ and Fe3+ as electron acceptors potentially faster than sulfate reduction as shown for aerobic respiration? Thus, Kristensen and Holmer showed that the rate of mineralization with nitrate as electron acceptor is indistinguishable from the rate with sulfate reduction. These questions may be solved by mapping Mn4+ and Fe3+ distribution from vertical and radial burrow wall dissections combined with sediment incubations using a variety of aerobic and anaerobic techniques.
The microbial communities and associated diagenetic processes dominating in the oxic and oxidized zones around semipermanent burrows of infaunal species like Nereis spp. and Arenicola marina are usually considered identical to those in the equivalent zones at the sediment surface. However, the environmental conditions prevailing in burrows are basically different from those at the sediment surface. Burrows can be considered physically stable on a time-scale of days to weeks (lifetime of burrow structures) and chemically unstable on a scale of minutes (oxic–anoxic oscillations due to intermittent irrigation), whereas the sediment surface is physically unstable on a short-term scale (advective forces such as waves and currents) and chemically stable on a long-term scale (continuously oxic conditions in overlying water). Consequently, the specific environmental conditions may support the growth of different microbial communities in burrows and at the sediment surface. Thus, despite an apparent similarity in the suite of microbial processes (oxic respiration, nitrification and denitrification) occurring in both environments, the volume specific activities may vary considerably. This may be explained by different population sizes of the same microbial communities, or by the presence of specific microbial communities adapted to the physical and chemical conditions prevailing in the two environments. At present, there are no answers to these paradigms, but as studies have shown basic differences in the species composition of meiofaunal communities in the two environments,[71, 72] a similar situation may occur for microorganisms as well. Recent advances in molecular biology have created a new array of methodologies for examining the population structure of microbial communities in natural environments. Techniques such as gene probing and polymerase chain reaction (PCR) can provide a very specific and sensitive evaluation of similarities and differences in microbial communities. Microbiologists are now able to use small samples of microbial nucleic acids to identify unculturable bacteria, track genes, and evaluate genetic diversity in environmental samples. Although no studies have yet applied these techniques to burrow samples, they should, with adequate modifications, be reliable tools for describing variations of the microbial community structure in different sediment compartments.
Presented during the ACS Division of Geochemistry symposium 'Biogeochemical Consequences of Dynamic Interactions Between Benthic Fauna, Microbes and Aquatic Sediments', San Diego, April 2001.
This work was funded by grants from the Danish Environmental Research Program (1992–1996), Centre for Strategic Environmental Research in Marine Areas, and the Danish National Research Foundation No. 9601423 and 9901749.
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