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Aquatic Microbial Ecology

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AME 25:55-63 (2001)  -  doi:10.3354/ame025055

Precision of bacterioplankton biomass determination: a comparison of two fluorescent dyes, and of allometric and linear volume-to-carbon conversion factors

Thomas Posch1,*, Maria Loferer-Krößbacher1, Guang Gao2, Albin Alfreider3, Jakob Pernthaler4, Roland Psenner1

1Institute of Zoology and Limnology, University of Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
2Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
3UFZ Center for Environmental Research, Department of Environmental Microbiology, Permoserstr. 15, 04318 Leipzig, Germany
4Max-Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany

Abstract: We studied the influence of methodology on the variability of bacterial direct counts and biomass estimates. Two frequently used fluorochromes, 4,6-diamidino-2-phenylindole (DAPI) and 3,6-bis(dimethylamino)acridine (acridine orange [AO]), were applied to determine bacterial abundances and sizes along a vertical profile in a oligo-mesotrophic freshwater lake (Piburger See, Austria). Only 90 ± 11% of AO stained bacteria were detected with the fluorochrome DAPI. On average, DAPI stained cells were half as large (48 ± 11% of mean cell volume) as cells stained with AO. The observed discrepancies in abundance and cell volumes were significantly related to community DNA synthesis rates, as suggested by lower differences at higher uptake rates of [3H]thymidine. In addition, a decrease in the relative DNA content cell-1 with increasing cell size was found in the bacterioplankton assemblage. Considering the staining properties of the 2 dyes, this may partially account for the observed differences in mean cell sizes. We summarized and evaluated most linear and allometric volume-to-carbon conversion factors published during the past 2 decades. Total bacterial biomass was estimated by applying several of these conversion factors to data sets determined from DAPI and AO stained preparations. Depending on the dye and conversion factor, bacterial biomass, averaged over the total water column, ranged between 5 μg C l-1 and 165 μg C l-1. As a result of this comparison we recommend the use of allometric conversion formulae specifically elaborated for a particular dye, i.e., CC = 218 x V0.86 (Loferer-Krößbacher et al. 1998) for DAPI stained bacteria and CC = 120 × V0.72 (Simon & Azam 1989, recalculated by Norland 1993) for AO stained cells (where CC is cellular carbon content [fg C], and V is bacterial volume [μm3]). In addition, these 2 formulae produced biomass estimates closest to the median values of estimates by all the investigated conversion factors.

KEY WORDS: Bacterial size · Epifluorescence direct counting · Volume-to-carbon conversion

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