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.

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The phylogeny of the family Selenastraceae was investigated by light microscopy, 18S rDNA, rbcL and ITS-2 analyses. Various morphological features traditionally used for species and genera identification were investigated. All selenastracean strains studied have naked pyrenoids within the chloroplast, except the genus Chlorolobion, which presented starch envelope. The molecular analyses showed that no morphological criterion considered so far is significant for the systematics of the Selenastraceae, but a set of features may be suitable to identify the genera Ankistrodesmus and Chlorolobion. Phylogenetic analyses showed the genera Monoraphidium, Kirchneriella and Selenastrum were not monophyletic and not distinguishable as separate genera. The polyphyly of the genus Selenastrum led to the description of two new genera, Curvastrum gen. nov and Messastrum gen. nov.

Mixotrophy is important to ecosystems functioning. Assuming that limiting resources induce phagotrophy in mixotrophs, we used a factorial experimental design to evaluate how nutrient and light affects phagotrophy in two mixotrophic phytoflagellates belonging to different lineages. We estimated cell-specific grazing rates (CSGR) by analyzing prey ingestion using microscopy and flow cytometry (FC). Furthermore, we tested if the acidotropic probe LysoTracker green (LyTG) can be used to differentiate autotrophs from mixotrophs. Cryptomonas marssonii (cryptophyte) had higher CSGR in high-nutrient treatments. Although it seems counterintuitive, phytoflagellates likely uses phagotrophy to obtain organic growth factors instead of inorganic nutrients when photosynthesis is more favorable. In contrast, CSGR in Ochromonas tuberculata (chrysophyte) increased when light decreased, suggesting that it uses phagotrophy to supplement carbon when autotrophic growth conditions are suboptimal. Measurements of CSGR obtained by FC and microscopy were significantly correlated and displayed the same trend among treatments, although FC rates tended to be higher. Fluorescence with LyTG did not differ from the control in the non-phagotrophic chlorophyte. Contrarily, addition of LyTG significantly increased the fluorescence in chrysophytes and cryptophytes, although no differences were observed among treatments. This approach allowed for differentiation between phagotrophic and non-phagotrophic flagellates but failed to quantify mixotrophy.