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.

Extracellular and intracellular dissolved organic matter (E‐DOM and I‐DOM, respectively) produced in axenic cultures by the cosmopolitan cyanobacteria Microcystis aeruginosa were characterized, and photoreactivity, bioreactivity, and the effects of phototransformations on bioavailability of E‐DOM and I‐DOM were assessed in laboratory experiments. Chromophoric (CDOM) and fluorescent (FDOM) properties were analyzed by spectrophotometry, and chemical signatures were obtained by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR‐MS). E‐DOM was more heterogeneous regarding optical characteristics as indicated by the presence of multiple CDOM peaks and the more even distribution of FDOM components, and had less absolute and relative numbers for molecular formulae containing nitrogen (N) than I‐DOM. E‐DOM was more photolabile, with greater photochemical removal of dissolved organic carbon (DOC), CDOM and FDOM, and exhibited more pronounced photochemical alterations in molecular formulae composition than I‐DOM. E‐DOM was less biolabile, with less biological removal of DOC, CDOM, and protein‐like fluorescent material, and supported lower rates of bacterial productivity than I‐DOM. Phototransformations reduced the bioavailability of E‐DOM and I‐DOM, as shown by lower bacterial productivity and longer DOC and CDOM half‐lives in bioassays with photoirradiated DOM in comparison to those with nonirradiated DOM. Analyses of CDOM, FDOM, and FTICR‐MS revealed that the decrease in bioavailability was caused primarily by photolysis of the biolabile DOM, indicating competition between photochemical and biological removal of cyanobacterial DOM. These results provided insights into mechanistic effects of independent and combined photochemical and biological degradation on M. aeruginosa‐derived DOM.

The aim of this study was to characterize the extracellular polysaccharides (EPS) released by a freshwater Thalassiosira sp. (Bacillariophyceae) and evaluate their degradation by heterotrophic microbial populations from the same habitat of Thalassiosira sp., a tropical eutrophic reservoir. The EPS were purified by anion exchange column chromatography, the monosaccharide composition was determined by GC, and the linkages of the monosaccharides by GC-MS. The EPS is a mannose-rich heteropolysaccharide composed of two different acidic fractions. Both of these fractions are composed of mannose, rhamnose, fucose, xylose, galactose, glucose, glucuronic acid, and N-acetyl glucosamine but with different proportions. N-acetyl galactosamine occurs only in fraction 1 and galacturonic acid only in fraction 2. We monitored the concentrations of the monosaccharides in the EPS during its degradation using pulse amperometric detection in an HPLC. The decay patterns of the monosaccharides were varied and the deoxy sugars, fucose and rhamnose, were degraded at a slower rate than the other components, increasing their relative concentrations and the hydrophobic feature of the EPS. The possibility of a selective degradation, which enhances the stickiness of the EPS, promoting transparent exopolymeric particles and aggregate formation, is discussed based on the literature data.

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Several studies have focused on the release of carbohydrates by phytoplankton because of the ecological significance of such compounds. This process increases the supply of carbon to the heterotrophic community, enhancing the phytoplankton/bacteria associations. In this article, we report investigations on the carbohydrate release, both polymeric and free sugars, in axenic batch cultures of three tropical freshwater phytoplanktonic species from different taxonomic positions: Cryptomonas tetrapyrenoidosa (Cryptophyceae), Staurastrum orbiculare (Zygnematophyceae) and Thalassiosira sp. (Bacillariophyceae). The total carbohydrate release rate was increased in the stationary growth phase in all the species under study. Most of the carbohydrates released by the three species were present in the polymeric form, although both polymeric and free carbohydrates could supply carbon enough to support bacterioplanktonic populations, according to rates of consumption found on literature. The composition of the carbohydrates differed significantly from one species to another, indicating that carbohydrate release might be a speciesspecific process. We also observed that the contributions of some components from polymeric sugars, such as fucose, rhamnose and arabinose increased with the advancing age of the cultures.

1. The dissolved organic matter, especially carbohydrates, released by phytoplanktonic organisms may be ecologically important, through its influence on carbon cycling and microbial diversity. Here axenic cultures of three phytoplanktonic species, Cryptomonas tetrapyrenoidosa (Cryptophyceae), Staurastrum orbiculare (Zygnematophyceae) and Thalassiosira duostra (Bacillariophyceae), were inoculated with a microbial community from the same habitat in which the algae had been isolated (a tropical reservoir). Replicate cultures were not inoculated. 2. In both axenic and co-inoculated cultures, phytoplanktonic density and extracellular carbohydrate production were monitored microscopically and by high performance liquid chromatography with a pulse amperometric detector, respectively. Bacterial population density was also monitored by epifluorescence microscope in the microbial co-inoculated cultures. 3. Both bacterial and phytoplanktonic densities increased for 11 days in all cases. The use of extracellular carbohydrates by bacteria was also showed for all phytoplanktonic species. Of the three species of phytoplankton, only T. duostra had a faster population growth in the presence of bacteria, and reached a higher biomass than in axenic culture.