2008, Bahulikar, Rahul A., Kroth, Peter G.

Diatoms are dominant organisms in phototrophic biofilms in aquatic habitats. They produce copious amounts of extracellular polymeric substances (EPS), which mainly consist of carbohydrates and traces of proteins and glycoproteins. This study focuses on the characterization of EPS from a total of 14 diatoms belonging to the six genera Achnanthes, Cymbella, Fragilaria, Punctastriata, Staurosira, and Pseudostaurosira, all of which were isolated from epilithic biofilms of the littoral zone of Lake Constance. EPS from all isolates were extracted by a sequential extraction procedure resulting in five different fractions. The monosaccharide composition of each fraction was analyzed by HPLC equipped with a pulse amperiometric detector, yielding results similar to those obtained by probing the EPS structures with monomer-specific fluorophore-linked lectins. Significant differences in carbohydrate composition occurred in the different fractions of single isolates. Most of the diatom isolates in our study form chain-like colonies in which the cells are attached to each other by intercellular pads. Here we demonstrate that these pads can be dissolved in hot bicarbonate and that they show a heterogeneous composition of monosaccharides in contrast to other fractions, which mostly were dominated by one or two monosaccharides. Principal component analysis indicates a correlation between carbohydrate composition of EPS fractions and the phylogenetic relationship of the respective species, indicating that EPS analyses under defined culture conditions may support taxonomic analyses.

2008, Bruckner, Christian G., Bahulikar, Rahul A., Rahalkar, Monali, Schink, Bernhard, Kroth, Peter G.

The composition of diatom-associated bacterial communities was studied with 14 different unialgal xenic diatom cultures isolated from freshwater epilithic biofilms of Lake Constance, Germany. A clear dominance of Alphaproteobacteria was observed, followed by Betaproteobacteria, Gammaproteobacteria, Bacteroidetes, and Verrucomicrobia. Pure cultures of the diatom Cymbella microcephala, which was found to be dominant in epilithic biofilms in Lake Constance, were cocultivated with six associated bacterial strains. All these bacterial strains were able to grow in C. microcephala cultures in the absence of organic cosubstrates. Diatom growth was generally enhanced in the presence of bacteria, and polysaccharide secretion was generally increased in the presence of Proteobacteria. The monomer composition of extracellular polysaccharides of C. microcephala changed in relation to the presence of different bacteria, but the dominant monomers were less affected. Our results indicate that these changes were caused by the diatom itself rather than by specific bacterial degradation. One Bacteroidetes strain strongly influenced carbohydrate secretion by the alga via extracellular soluble compounds. Biofilms were formed only in the presence of bacteria. Phylogenetic analysis and coculture studies indicate an adaptation of Proteobacteria and Bacteroidetes to the microenvironment created by the diatom biofilm.

2008, Medlin, Linda, Jung, Ines, Bahulikar, Rahul A., Mendgen, Kurt, Kroth, Peter G., Kooistra, Wiebe H. C. F.

The separation of Fragilaria and Synedra has been the subject of a long standing debate amongst diatomists. For those advocating keeping the two genera separate, several additional genera have been separated from both Fragilaria and Synedra based on morphological grounds alone. A molecular analysis has now been applied to evaluate the status of several of these new genera as well as selected taxa that can be unequivocally assigned to either Fragilaria or Synedra. Fragilaria and Synedra sensu stricto are good monophyletic genera and they are sister clades. The new genera separated from Fragilaria are not monophyletic and fall into several clades that are well separated from Fragilaria sensu stricto and thus, deserve some taxonomic ranking but not as individual genera. In contrast, the taxa separated from Synedra appear with one exception to be monophyletic and thus valid from a molecular standpoint.