AME 14:19-28 (1998)  -  doi:10.3354/ame014019

Dynamics of heterotrophic bacteria attached to Microcystis spp. (Cyanobacteria)

Jakob Worm1,*, Morten Søndergaard2

1Section of Genetics and Microbiology, Department of Ecology and Molecular Biology, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
2Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød, Denmark

We studied the ecology of heterotrophic bacteria attached to the mucilaginous colonies of Microcystis spp. (Cyanobacteria) in the eutrophic lake Frederiksborg Slotssø, Denmark. The succession in bacterial abundance, production and potential aminopeptidase activity in 20 μm fractionated samples was followed during periods in which Microcystis dominated the phytoplankton. We operationally defined that nets of 20 μm mesh-width segregated bacteria associated with Microcystis (Microcystis-associated bacteria, MB; >20 μm size fraction) from the mainly free-living bacteria in the filtrate (FB, <20 μm size fraction). According to this definition, the contribution of MB during summer 1995 and autumn 1994, respectively, averaged 10 ± 4 (± standard deviation) and 37 ± 12% of total bacterial biomass and 25 ± 13 and 43 ± 16% of total bacterial production, as estimated from thymidine (TdR) incorporation. During summer, MB further contributed 55 ± 18% of total leucine incorporation measured at 600 nM leucine and 53 ± 12% of total potential aminopeptidase activity. Although 20 μm mesh-width nets also retained particles other than Microcystis, our results indicate that Microcystis was a 'hotspot' for bacterial activity, comparable to larger aggregates known as marine or lake snow. During summer, growth rate and specific aminopeptidase activity of MB generally exceeded those of FB, which points to diversified microenvironments or species compositions. In order to balance gain and loss rates within the community of MB, we hypothesize that a large fraction of MB produced were exported from Microcystis to the surrounding water, only modified by the loss due to viral lysis. This idea arose from reported low loss rates of Microcystis and continuous measures indicating that a surplus of more than 70% of MB production (TdR) was not reflected as biomass increases within the community of MB. According to this hypothesis Microcystis may be considered as a bacterial 'incubator' for the surrounding water.

Attached bacteria · Microcystis · Eutrophic lake · Microhabitat

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