F.W.O contract G.0118.02
(2002-2005)
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Trophic significance and ecological fate of
microbial production and leaf litter in mangrove ecosystems F.W.O contract G.0118.02 (2002-2005) |
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This project wishes to contribute to understanding the role of several components -more specifically imported phytodetritus, local heterotophic microorganisms and local production by microphytobenthos- in the carbon cycle in mangrove ecosystems. The aim is (1) to gain insight into the carbon sources sustaining heterotrophic micro-organisms -bacteria and fungi- under natural conditions, (2) to assess how production (both primary and secondary) by micro-organisms is distributed in the ecosystem, and to compare this with the fate of mangrove litter inputs. The latter aim includes microbial degradation, transport to deeper sediment layers, and assimilation by higher organisms, with a particular focus on the carbon sources sustaining meiofaunal communities.
Both bulk and compound-specific stable isotope analysis will be used to assess the carbon sources used by natural populations of bacteria and fungi in mangrove sediments. Characterisation of available carbon sources will be made by analysis of POC, PN, C/N and d13C of sediment organic matter and possible carbon sources (i.e. mangrove litter, marine/estuarine suspended matter, microphytobenthos). These will be compared to d13C data on specific bacterial biomarkers (fatty acids such as i14:0, i15:0, a15:0, i16:0, 18:1w7c) and a fungal biomarker (18:2w6 and, if possible, ergosterol) to identify their main carbon sources. The extraction of these compounds is done on sediment samples, which also provides a fatty acid profile, in turn providing additional information on the different organic matter sources in the sediment and an estimate of bacterial biomass. The combination of these two approaches (relative importance of C-sources to the sediment, and relative importance of C-sources for bacteria and fungi) will indicate whether any selectivity for certain substrates exists and to which factors this might be related. In order to generate such data, sampling should be done in sites differing substantially in terms of origin of sediment organic matter – this can be achieved either by organising sampling in different regions, or by choosing sites within a region which display a comparable variability due to differences in subjection to tidal action or vegetation.
The major aim of this project is to gain insight into the ecological fate of microbial carbon production (both primary and secondary) in mangrove sediments, with a focus on the transfer to heterotrophic micro-organisms and invertebrates. The adopted strategy consists of doing 13C-enrichment experiments in the field, and following the distribution of the applied label in different components of the experimental plots. This approach was successfully applied recently to study the fate of microphytobenthos production in other marine ecosystem types. In different treatments (i.e. different small experimental fieldplots), 13C-labeling will be applied to local microphytobenthos (by addition of a NaH13CO3 solution), heterotrophic micro-organisms (e.g. through 13C-enriched glucose or acetate), or pre-labeled phytodetritus (using NaH13CO3) will be applied to the sediment surface. At different time steps (several hours to several days) different compartments are subsequently sampled, whereby both biomass and stable isotopic composition (bulk analysis for sediment, meio- and macrofauna; compound-specific analysis for micro-organisms) are measured in order to qualitatively survey the distribution of the label. By comparing these data with background stable isotope ratios (measured before the application of the label), the amount of the added label can be calculated for different compartments, allowing for quantitative evaluation of the fate of the 13C-label. As a comparative experiment, mangrove leaf material pre-labelled with 13C will be applied to other experimental plots. Such labelling can be achieved by growing mangrove seedlings in a closed environment with a 13CO2 atmosphere, or by regularly spraying them with 13C (and 15N)-labeled urea.
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