Decomposition of organic matter in the littoral sediments of a lake

This thesis deals with the microbial decomposition of organic matter in littoral sediments of lakes. Special attention was given to the initial step in the decomposition of polysaccharides that form a major component of macrophyte litter produced in these systems. This initial step, an extracellular enzymatic hydrolysis, is generally regarded as the rate limiting step in the decomposition of biopolymers in natural systems. The study site selected was an almost monospecific stand of common reed, Phragmites australis, that covers the upper littoral zone of Lake Gooimeer, The Netherlands.

The first two experimental chapters of this thesis deal with several general aspects of sedimentary carbon cycling, which are necessary for an implementation of a detailed study on the initial decomposition of biopolymers. In Lake Gooimeer, we could show that deep inside the reed bed, organic matter cycling was dominated by one source, namely macrophyte litter (Chapter 2). Plant litter is mainly made of lignocellulose, which has a rather simple and distinct polymeric composition. This makes the reed bed a good test system to study initial decomposition of biopolymers under natural conditions. Carbon sources gradually changed to an algal dominance going from inside the bed towards the lake (Chapter 2). This transition already begins inside the reed bed and no macrophyte derived material was found outside the bed, despite the high annual production of common reed.

Results of the budget study (Chapter 3) suggest that there are two major processes involved in the removal of sedimentary organic matter. Transport caused by erosion of the sediment during storms explains about 60% of the total carbon and only 30% was removed by mineralization. Mineralization rates were mainly determined by temperature, changes in oxic and anoxic conditions, and by amounts of organic matter present. Less than 5% of the annual production remained in the sediment over a period of 25 years. No short term accumulation could be detected during both years studied and results from 1992 even suggested a net removal of organic matter. This shows that sediment organic matter accumulation is variable and is governed by a delicate balance between input and outputs. Predicting sediment organic matter dynamics will be difficult in these type of ecosystems, since it depends to a large extent on episodic and variable sediment erosion during storms.

The remainder of the thesis deals with the initial decomposition of the two main reedlitter polysaccharides cellulose and arabino-xylan in sediments and the enzymes involved in this process. A method was developed to measure extracellular enzyme activities involved in cellulose decomposition by using artificial, fluorochrome labelled substrates (Chapter 4). Inhibition experiments with known substrates and inhibitors of cellulolytic enzymes were used to characterise the enzyme activities as measure in the assays. Results of the inhibition experiment suggested that the measured activity was of bacterial origin in the sediment used. This method was further extended to other enzymes and was used to study temporal and spatial variability of enzyme activities during two years in the littoral sediments (Chapter 5). Absolute activities were mostly determined by organic matter input or content. β-Glucosidase activities were among the highest ever detected in sediments, which is probably the result of the high litter cellulose input to the sediment. Patterns of enzyme activities showed a distinct change in a gradient through the reed bed, which was consistent with the polymeric composition of dominant sedimentary carbon sources (Chapter 2). This suggests that patterns of enzyme activities may be used to study biologically available carbon sources in natural systems.

Although the enzyme assays are easy to use and allow information from a large number of samples to be gathered and compared, they give little information on the actual in situ hydrolysis rates of natural occurring polysaccharides in sediments. This is mainly caused by the use of an artificial substrate that may not be representative of the form and availability of the natural substrates. Therefore, a new method was developed to measure hydrolysis rates of naturally occurring polysaccharides in sediments. This new method is based on the selective inhibition of microbial uptake of hydrolysis products by 3% toluene without affecting the extracellular decomposition of polysaccharides. This approach was thoroughly tested (Chapter 6) and subsequently used to study initial decomposition of individual polysaccharides in reed litter and sediments (Chapter 7). The accumulation of hydrolysis products was followed over time by high-performance liquid chromatography, which resulted in a sensitive method with a high degree of resolution for the products formed.

A similar pattern of accumulating carbohydrates was found for litter and sediments (Chapter 7). Ratios between glucose and xylose accumulation rates suggested that arabino- xylan was degraded more slowly that cellulose, which was not in agreement with apparent rates of glucose and xylose removal from litter. Accumulation of heterogenic oligomeric compounds besides xylose during the decomposition of arabino-xylan may explain this discrepancy. If direct uptake of these hetero-oligomers can be shown, it would have implications for carbon flow and the associated microbial populations. The turnover time of particulate glucose was estimated at 85 ± 14 d in the top centimetre of the sediment, and showed a three to four fold increase with depth. Comparison between glucose accumulation rates as a measure of cellulose decomposition and total carbon mineralization rates suggested that cellulose decomposition was a major process in the mineralization of organic matter in littoral sediments.

Finally, implications of these studies are discussed in view of general functioning of littoral zones and the mechanisms of polymer decomposition in natural systems (Chapter 8).