Conclusion
The discovery of new microbial sources of cellulases is a crucial strategy to reduce costs of various industrial processes using such enzymes. Cellulases are produced by various microorganisms including bacteria, fungi and actinomycetes. Recently was reported that they are also produced by some animals like termites and crayfish without certainties about his role in vivo 45. The search, isolation and identification of new cellulose degrading microorganisms from different environments are of crucial importance to get new cellulases with unique and distinctive characteristics.
Microalgae are considered a valuable source of new enzymes with biotechnological potential. However, the presence of cellulolytic enzymes is meagre studied form this photosynthetic microorganisms.
Different works published during the last decade report cellulolytic activity (either by experimental evidence or by bioinformatic analysis) in C. reinhardtii , V. carteri , G. pectorale andA. protothecoides but the genus Scenedesmus had not been analysed3,5,46.
This is the first bioinformatic analysis of Scenedesmaceae cellulases reported. It comprises GH5 and GH9 β-1,4-endoglucanases, GH1 β-glucosidases and GH10 exoglucanases. Our results shows that GH9 endoglucanases analyzed are phylogenetically closer to invertebrates, termites and bivalve rather than higher plant, bacteria or fungi. On the other hand, most of GH1 β-glucosidases analyzed are evolutionarily closer to enzymes of other microalgae, however, four of them are grouped in a branch close to the bacteria enzymes, result that suggests the probable gaining of their genes by horizontal transfer. In contrast, GH5 and GH10 studied enzymes are evolutionarily closer to enzymes of other microalgae and higher plants.
Most of the analyzed enzymes present signal peptides for membrane anchoring or extracellular secretion. This result suggests the presence of extracellular cellulolytic machinery in Scenedesmaceae. Only some of the analyzed enzymes were found to have additional modules and linkers besides its GH domains, and particularly a few endoglucanases have CBM modules, from CBM1 and CBM2 families.
The combination of GH catalytic domains together with CBMs and, in some cases linkers, propose that these cellulases would present an enhanced cellulolytic activity.
The presence of this battery of enzymes in the photoheterotrophic algaeScenedesmus suggest that these organisms are perfectly prepared for use of cellulose as carbon source. This strategy would represent an advantage that would have allowed Scenedesmaceae to occupy many environments in nature.
The findings reported in this work explores just one family within the Chlorophyta taxon, but it increases the evidence in favor of the presence of conserved cellulolytic machinery in photoheterotrophic organisms and encourages to continue with the search for cellulases in other species of microalgae.