MALDI-TOF MS based discrimination of coccoid green microalgae (Selenastraceae, Chlorophyta)

The morphological simplicity of the cells in the family Selenastraceae has been a barrier to a good establishment of well-defined taxonomic groups, and consequently impairing the application of these organisms in the biotechnological field. Biomolecules are phenotypic expressions that can be useful in the taxonomy of microorganisms. In this scenario, the present study used the spectra of organic compounds (mainly proteins) of whole cells obtained by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique as a tool to differentiate freshwater coccoid green microalgae of the family Selenastraceae at the species and strains levels. Firstly, a test to choose the matrix and sample preparation method that could result in higher signal-to-noise ratio was performed with the species Ankistrodesmus fusiformis and Coelastrum sphaericum. The best spectra were obtained using α-cyano-4-hydroxycinnamic acid (HCCA) and matrix suspension method, which were used for chemotaxonomic analysis of 18 microalgal strains belonging to 12 different species. Spectra of microalgae cells were collected in two different mass ranges, from 0.4 to 2.0 kDa and from 2.0 to 20.0 kDa. Each strain yielded unique spectra with a good resolution of peaks and reproducibility, which was used for the cluster analyses. The dissimilarity analysis showed that it was possible to distinguish microalgal strains and species based on the obtained spectra. Geographic differences for the genus Selenastrum were evidenced in both mass ranges analyzed. Furthermore, our data corroborated some previous studies with Selenastraceae, using molecular markers, regarding to the polyphyly of the genus Monoraphidium. The spectra collected at higher masses contained more significant information for the taxonomy of green coccoid microalgae, allowing a better distinction between the genus Ankistrodesmus and Monoraphidium. The good classification results allied with practicability and quickness for data acquisition using MALDI-TOF MS favors its application in taxonomic studies.