Abstract
Many freshwater phytoplankton species have the potential to form transient nuisance blooms that affect water quality and other aquatic biota. Heterotrophic bacteria can influence such blooms via nutrient regeneration but also via antagonism and other biotic interactions. We studied the composition of bacterial communities associated with three bloom-forming freshwater phytoplankton species, the diatom Aulacoseira granulata and the cyanobacteria Microcystis aeruginosa and Cylindrospermopsis raciborskii. Experimental cultures incubated with and without lake bacteria were sampled in three different growth phases and bacterial community composition was assessed by 454-Pyrosequencing of 16S rRNA gene amplicons. Betaproteobacteria were dominant in all cultures inoculated with lake bacteria, but decreased during the experiment. In contrast, Alphaproteobacteria, which made up the second most abundant class of bacteria, increased overall during the course of the experiment. Other bacterial classes responded in contrasting ways to the experimental incubations causing significantly different bacterial communities to develop in response to host phytoplankton species, growth phase and between attached and free-living fractions. Differences in bacterial community composition between cyanobacteria and diatom cultures were greater than between the two cyanobacteria. Despite the significance, major differences between phytoplankton cultures were in the proportion of the OTUs rather than in the absence or presence of specific taxa. Different phytoplankton species favoring different bacterial communities may have important consequences for the fate of organic matter in systems where these bloom forming species occur. The dynamics and development of transient blooms may also be affected as bacterial communities seem to influence phytoplankton species growth in contrasting ways.
Highlights
The diatom Aulacoseira granulata and the toxic cyanobacteria Microcystis aeruginosa and Cylindrospermopsis raciborskii are common phytoplankton species in eutrophic freshwater lakes and reservoirs around the world [1,2,3]
C. raciborskii cultures inoculated with lake bacteria exhibited significantly higher growth yield compared to the control (ANCOVA, p,0.001, F = 16.34) (Figure 1)
Bacteria were not detected by scanning electron microscopy (SEM) in M. aeruginosa and A. granulata control cultures (Figure 2a, c)
Summary
The diatom Aulacoseira granulata and the toxic cyanobacteria Microcystis aeruginosa and Cylindrospermopsis raciborskii are common phytoplankton species in eutrophic freshwater lakes and reservoirs around the world [1,2,3]. These phytoplankton can form transient blooms that greatly impact the energy and nutrient flux in aquatic environments because of the locally high amount of organic matter being produced. Cellular interactions between microorganisms are important for the dynamics of microbial populations while influencing microbially mediated biogeochemical cycles [5,9]. Despite the suggested importance of interactions between freshwater phytoplankton and heterotrophic bacteria, our understanding of such interactions is still very limited [10]
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