Résumé
The degradation of an
Ulva lactuca mat (0.2
kg
dw
m
−2) was studied in a controlled flow-through mesocosm for 31
d. Sediment chambers without
U. lactuca served as controls. Fluxes of ∑CO
2, O
2, inorganic nitrogen, and urea were determined during the incubation period in addition to sulfate reduction rates, POC and PON content, enumeration of specific bacterial populations and evaluation of the physiological state of the added
U. lactuca thalli. After
U. lactuca addition to the chambers, there was an immediate increase in the efflux of ∑CO
2 from 11 to 27
mmol-C
m
−2
d
−1 and a concomitant increase in O
2 uptake from 11 to 23
mmol
m
−2
d
−1. These effluxes remained elevated throughout the incubation period. In contrast, the NH
4
+ efflux increased from 0.1 to 1.8
mmol
NH
4
+
m
−2
d
−1 during the first 3
d of incubation, followed by 6
d with a constant efflux rate, after which time it decreased gradually to 0.3
mmol
NH
4
+
m
−2
d
−1 by the end of the experiment. In total, NH
4
+accounted for 83% of the total nitrogen efflux after addition of
U. lactuca. During the 31
d incubation period there was a continuous colonization of the thalli by bacteria. Sulfate reducers associated with the thalli accounted for 3% of the carbon oxidation on day 31. The molar C:N ratio in mineralization products (the ratio between the efflux of ∑CO
2 and NH
4
+
+
NO
2
−
+
NO
3
−) increased from 15
mol
mol
−1 at day 11 after
U. lactuca addition to >80
mol
mol
−1 by the end of the incubation. Since the C:N ratio in the mineralization products was much higher than the original thallus material (8.9
mol
mol
−1) it is probable that a preferential incorporation of NH
4
+ into the increasing bacterial biomass occurred. The nitrogen for bacterial growth was most likely obtained from degradation of
U. lactuca thalli as there was no stimulation of urea-N turnover in the sediment during incubation. The net increase in bacteria cell number in the 18-mm thick thallus layer was estimated to be 7.6
×
10
9 to 2.4
×
10
10
bacterial
cells
cm
−3. In contrast, the bacterial cell number remained constant in the −Ulva incubations.