Nitrogen fixation (Acetylene reduction) in the sediments of the pluss-see : with special attention to the role of sedimentation

Sediments of productive lakes are usually rich in organic matter and, except for a thin surficial layer, anaerobic. These conditions seem to be favourable for heterotrophic nitrogen fixation. However, these sediments also contain relatively high ammonium concentrations. Ammonium represses the synthesis of the enzyme nitrogenase. Moreover, ammonium inhibits the activity of the enzyme in aerobic nitrogen fixers. These effects of ammonium seem to be functional. Nitrogen fixation is a highly endergonic process. Therefore it is more economic to use combined nitrogen (e.g. ammonium) than atmospheric nitrogen as a nitrogen source. Nevertheless, a number of workers have detected nitrogen fixing activity in ammonium rich sediments.

In the present investigation the significance of heterotrophic nitrogen fixation in the sediments or the nitrogen economy of the Pluss-See has been studied. Special attention has been paid to the role of organic matter supply and ammonium.

The surface area of the lake is 14 ha, the maximum depth is 29 m. Every year a stable thermal stratification develops in the lake, usually with an anaerobic hypolimnion.

The problem of nitrogen fixation in lake sediments has been approached in three ways:
1. Nitrogen fixation in the sediments was studied under controlled conditions in the laboratory;
2. The relation between nitrogen fixation and some properties of the sediments was studied under natural conditions;
3. The relation between nitrogen fixation in the sediments and processes within the lake was studied under natural conditions.

Laboratory studies (chapters 3, 4 and 5).

Nitrogen fixating activity was measured with the acetylene reduction assay. One of the requirements for this assay, the saturation of nitrogenase with acetylene, was not met (figure 5), because nitrogen fixation apparently occured in protected microsites with poor accessibility for acetylene (3.2.2). This protection may have also consequences for the measurement of nitrogen fixation with ¹⁵N ₂ (3.2.3).

Nitrogenase activity of the sediments was stimulated by the addition of organic substrates, as mannitol, glucose, fructose etc., suggesting that the activity of nitrogenase in situ in these sediments was limited by the availability of organic substrate (4-2). This suggestion is affirmed by the absence of a discontinuity in the Arrhenius plot (figure 17b).

Upon addition of organic substrate to the sediments two phases could be observed (figure 9). In the first phase acetylene reducing activity is constant, but higher than in control sediments. In this phase the activity of the already present nitrogenase is stimulated (increase of the actual activity). In the second phase, after the interstitial ammonium concentration has dropped below a certain threshold-value, the synthesis of nitrogenase is derepressed and an exponential increase of nitrogenase activity can be observed (increase of the potential activity). Because nitrogenase synthesis in situ had to be assumed above the derepression threshold, the conclusion was drawn that the dissolved ammonium concentration within the protected microsites was lower than in the bulk of the sediments (4.3). Apparently nitrogen fixation occures in ammonium rich sediments, because nitrogen fixers are not in contact with these high concentrations.

Field observations; relation between the nitrogenase activity and some other properties of the sediments (chapters 6 and 7).

During a year nitrogenase activity and some other characteristics of the sediments were measured at three stations in the lake: in the littoral sediments at 5 m water depth and in the profundal sediments at 15 and 29 m water depth. Highest nitrogenase activity was measured at the sediment surface at the deepest part of the lake (figure 27). Especially in the winter period very high rates were observed (figure 28). In the sediments at the deepest part of the lake the yearly fixed amount of nitrogen was estimated to be 0.24-1.10 g.m ^-2, depending on the conversion factor used. In the shallower regions this amount was estimated to be 0.15-0.77 g N.m ^-2.

Acetylene reduction in the littoral sediments was correlated with temperature (table II). In the profundal sediments no significant temperature variation could be observed. Acetylene reducing activity in the profundal sediments was correlated with the C/N ratio (figure 33), which could be shown to be an index for substrate availability in these sediments. In both littoral and profundal sediments acetylene reducing activity was highly significantly correlated with the maximum glucose uptake velocity (V _m ) of the heterotrophic population in the sediments (figure 41). The repression-derepression threshold of the interstitial ammonium concentration could be observed under natural conditions (figure 33). Acetylene reducing rates were higher at ammonium concentrations below this threshold. The ammonium adsorption coefficient (Ke) of the sediments seemed to be more important for the acetylene reducing activity than the ammonium concentration it self (table II). This finding suggests that the dissolved ammonium concentration in the protected microsites is lowered by adsorption.

Field observations; the relation between nitrogen fixation in the sediments and the sedimentation of suspended matter (chapters 8, 9 and 10).

Sedimentation of particulate organic matter was measured at the three stations. The measured rates were corrected for resuspension using the differences in carbon content between the settling particulate material and the carbon content of the surficial sediments (8.4; figure 47). Eight percent of the primary production reached the bottom at the deepest part of the lake (table VI). Redistribution of sediments resulting in sediment focusing is important in the lake. Both intermittent complete mixing and sliding of sediments on slopes are important for the focusing process. A correlation between sedimentation and acetylene reducing activity could be observed in the littoral sediments and at the deepest part of the lake (figure 63). No correlation was found at 15 m water depth. Only at the deepest part of the lake a correlation was found between the sedimentation and both the C/N ratio and Ke of the sediments (figure 61 and 62). These correlations and non- correlations could be explained by the transport of sediments within the lake, described by a simple focusing model (8.7). Using this model the efficiency of nitrogen fixation under natural conditions could be estimated to be high compared to the efficiency measured in pure and enrichment cultures (8.10).

Also using this model it could be shown (10) that nitrogen fixation may be important to the nitrogen economy of the sediments but not for the nitrogen economy of the whole lake. Nitrogen fixation is expected to be more important in lakes with a larger proportion of the primary production reaching the bottom.