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.
Diffusion scale
The three possible cases suggested by Aller and Aller[23] 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[59] 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.
Bacterial populations
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.[69] 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.[70] 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.[73] 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.
Note
Presented during the ACS Division of Geochemistry symposium 'Biogeochemical Consequences of Dynamic Interactions Between Benthic Fauna, Microbes and Aquatic Sediments', San Diego, April 2001.
Acknowledgements
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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