Lizhu Wang & John C. Priscu*. Department of Biology, Montana State University, Bozeman, Montana 59717, USA (*authorfor correspondence); Present address: ...
Hydrobiologia 277: 145-158, 1994.
145
© 1994 Kluwer Academic Publishers. Printedin Belgium.
Stimulation of aquatic bacterial activity by cyanobacteria Lizhu Wang & John C. Priscu* Department of Biology, Montana State University, Bozeman, Montana 59717, USA (*authorfor correspondence); Present address: DNR Area Fisheries Office, 7372 State Highway 25 SW, Montrose, MN 55313, USA Key words: bacterioplankton, cyanobacteria, extracellular products, bacterial stimulation
Abstract The time-course response of natural bacterial populations and isolates from lake water to various densities of the filamentous cyanobacteria Aphanizomenon flos-aquae and Lyngbya birgei collected from the same lake is reported. The cyanobacteria were separated from the bacteria by dialysis membranes that allowed only dissolved cyanobacterial products to pass. Bacterial 3 H-thymidine incorporation and cell number were significantly (p300 cells total) in each sample were counted with a Nikon Labophot epifluorescence microscope using the acridine orange direct count technique (Hobbie etal., 1977). Bacterial cell volumes were calculated after measurement of appropriate geometric dimensions using a calibrated ocular micrometer. Our microscope set-up allowed us to resolve dimensions as small as 0.2 pm. Cell dimensions were measured on days 1 and 5 in experiments 1 and 2 and on day 5 of experiment 3. The average of at least 40 cell measurements from each sample were used to compute cell volume. Cell volume was converted to cell carbon using the coefficient of 560 fg C m 3 (Bratbak, 1985). The amount of DOC utilized by bacteria during the 5 day incubation of experiments 1 and 2 was estimated by subtracting bacterial carbon on day 5 from that on day 1. This rate (expressed as a daily increase)
Statistical analysis To determine the effect of cyanobacterial density on bacterial growth, simple linear regressions between cyanobacterial chlorophyll and bacterial thymidine incorporation or cell number were made. Such regressions were done at each timepoint for both cyanobacterial species treatment. Simple linear regressions were also applied to test the association between DOC changes and estimated DOC utilized by bacteria. To test the effect of cyanobacteria on bacterial activity, timecourse bacterial thymidine incorporation and cell
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Fig. 1. Bacterial thymidine incorporation (A, B) and cell number (C, D) at different densities of A.flos-aquae (A, C) and L. birgei (B, D) over the incubation in experiment 1. o6 h, day 1, A day 3, A day 5. The lines represent least-squares linear fits through the data.
150 Table 1. Correlation coefficients (r) for least-squares linear fits between cyanobacterial chlorophyll a, and bacterial thymidine incorporation and cell number in the 3 experiments. Treatment
Experiment 1
Experiment 2
Experiment 3
Day 1
Day 3
Day 5
6h
Day 1
Day 3
Day 5
Water
Surface
Thymidine uptake 0.55** A.flos-aquae L. birgei 0.53**
0.82*** 0.96***
0.92*** 0.84***
0.94*** 0.86***
0.93*** 0.84***
0.94*** 0.86***
0.90*** 0.79**
0.99*** 0.79**
NA NA
NA NA
Cell number A.flos-aquae L. birgei
0.92*** 0.96***
0.93*** 0.75**
0.94*** 0.82***
0.84*** 0.87***
0.96*** 0.92***
0.84*** 0.81**
0.92*** 0.81**
0.90*** 0.85***
0.96*** 0.91***
6h
0.71** 0.48
** Significant atp
