AME 27:241-248 (2002)  -  doi:10.3354/ame027241

Microbial communities and respiration on aggregates in the Elbe Estuary, Germany

Helle Ploug1,*, Heike Zimmermann-Timm2,**, Bernd Schweitzer3

1Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany
2Institute for Hydrobiology and Fisheries, University of Hamburg, Zeiseweg 9, 22765 Hamburg, Germany
3Institute for Limnology, University of Konstanz, Postbox 5560, 78434 Konstanz, Germany
*E-mail: **Present address: Institute for Ecology, Department for Limnology, Friedrich-Schiller-University of Jena, Winzerlaerstraße 10, 07743 Jena, Germany

ABSTRACT: Estuaries are characterized by a very high abundance of small aggregates enriched with bacteria, protozoa and sometimes metazoa compared to their abundance in the surrounding water. In this study, the microbial community and respiration rates on estuarine aggregates were analyzed by the combination of microscopy (bacteria, protozoa and metazoa enumeration), in situ hybridization and microsensor techniques. Aggregates were isolated and kept in darkness to prevent scavenging or production of new organic carbon. Bacterial abundance and respiration rates were stable during the first 2 d after collection. Bacterial numbers ranged from 0.84 x 106 to 23.7 x 106 cells aggregate (agg)-1. Protozoa were dominated by nanoflagellates, which varied between 24 and 1497 cells agg-1. Total protozoan biovolume accounted on average for 4 to 13% of the total estimated microbial biomass on aggregates, and the total grazing rate on attached bacteria was apparently low. Size-specific respiration rates were described by R = 5.86 x d1.82, where R is measured in ngC agg-1 h-1 and the diameter (d) is measured in mm. The average respiration rate was 0.150 μgC agg-1 d-1, and the estimated average bacterial respiration rate was 22 fgC cell-1 d-1. Total respiration and cell-specific respiration decreased to 0.039 μgC agg-1 d-1 and 2.6 fgC cell-1 d-1, respectively, after 6 d. The bacterial community was dominated by members of b-proteobacteria and the Cytophaga-Flavobacterium cluster, which together accounted for 54% of all 4',6-diamidino-2-phenylindole (DAPI) stained bacteria at the end of the study. The respiration rate on aggregates larger than 400 μm in the water column of the Elbe Estuary was 11.8 μgC l-1 h-1 as determined from the size-specific respiration rates, the aggregate abundance and size distribution. It accounted for 84 to 94% of the estimated total respiration in the upper water column. The critical ambient O2 concentration, at which the center of the aggregate interior turns anoxic, was found at ~16 μM.

KEY WORDS: Estuarine aggregates · Oxygen microelectrodes · Respiration · Protozoa · Bacteria · In situ hybridization

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