To investigate the changes in the morphology and polysaccharide content of Microcystis aeruginosa (Kütz.) Kütz. during flagellate grazing, cultures of M. aeruginosa were exposed to grazing Ochromonas sp. for a period of 9 d under controlled laboratory conditions. M. aeruginosa responded actively to flagellate grazing and formed colonies, most of which were made up of several or dozens of cells, suggesting that flagellate grazing may be one of the biotic factors responsible for colony formation in M. aeruginosa. When colonies were formed, the cell surface ultrastructure changed, and the polysaccharide layer on the surface of the cell wall became thicker. This change indicated that synthesis and secretion of extracellular polysaccharide (EPS) of M. aeruginosa cells increased under flagellate grazing pressure. The contents of soluble extracellular polysaccharide (sEPS), bound extracellular polysaccharide (bEPS), and total polysaccharide (TPS) in colonial cells of M. aeruginosa increased significantly compared with those in single cells. This finding suggested that the increased amount of EPS on the cell surface may play a role in keeping M. aeruginosa cells together to form colonies.

In the experiment we investigated the effect of grazing by different sorts of zooplankton on the induction of defensive morphology in the cyanobacterium Microcystis aeruginosa. The results showed that protozoan flagellate Ochromonas sp. grazing could induce colony formation in M. aeruginosa, whereas M. aeruginosa populations in the control and the grazing treatments of copepod Eudiaptomus graciloides, cladoceran Daphnia magna, and rotifer Brachionus calyciflorus were still strongly dominated by unicells and paired cells and no colony forma occurred. In the protozoan grazing treatment, the proportion of unicells reduced from 83.2% to 15.7%, while the proportion of cells in colonial form increased from 0% to 68.7% of the population at the end of the experiment. The occurrence of a majority of colonial M. aeruginosa being in the treatment with flagellates, indicated that flagellate grazing on solitary cells could induce colony formation in M. aeruginosa. The colonies could effectively deter flagellate from further grazing and thus increase the survival of M. aeruginosa. The colony formation in M. aeruginosa may be considered as an inducible defense against flagellate grazing under the conditions that toxin cannot deter flagellate from grazing effectively.

BackgroundEnzymes of the cellulose synthase (CesA) family and CesA-like (Csl) families are responsible for the synthesis of celluloses and hemicelluloses, and thus are of great interest to bioenergy research. We studied the occurrences and phylogenies of CesA/Csl families in diverse plants and algae by comprehensive data mining of 82 genomes and transcriptomes.ResultsWe found that 1) charophytic green algae (CGA) have orthologous genes in CesA, CslC and CslD families; 2) liverwort genes are found in the CesA, CslA, CslC and CslD families; 3) The fern Pteridium aquilinum not only has orthologs in these conserved families but also in the CslB, CslH and CslE families; 4) basal angiosperms, e.g. Aristolochia fimbriata, have orthologs in these families too; 5) gymnosperms have genes forming clusters ancestral to CslB/H and to CslE/J/G respectively; 6) CslG is found in switchgrass and basal angiosperms; 7) CslJ is widely present in dicots and monocots; 8) CesA subfamilies have already diversified in ferns.ConclusionsWe speculate that: (i) ferns and horsetails might both have CslH enzymes, responsible for the synthesis of mixed-linkage glucans and (ii) CslD and similar genes might be responsible for the synthesis of mannans in CGA. Our findings led to a more detailed model of cell wall evolution and suggested that gene loss played an important role in the evolution of Csl families. We also demonstrated the usefulness of transcriptome data in the study of plant cell wall evolution and diversity.