**Gea Guerriero 1,\*, Kjell Sergeant 1, Sylvain Legay 1, Jean-Francois Hausman 1, Henry-Michel Cauchie 1, Irshad Ahmad <sup>2</sup> and Khawar Sohail Siddiqui 2,\***


Received: 10 May 2018; Accepted: 8 June 2018; Published: 15 June 2018

**Abstract:** The assumption that cellulose degradation and assimilation can only be carried out by heterotrophic organisms was shattered in 2012 when it was discovered that the unicellular green alga, *Chlamydomonas reinhardtii* (Cr), can utilize cellulose for growth under CO2-limiting conditions. Publications of genomes/transcriptomes of the colonial microalgae, *Gonium pectorale* (Gp) and *Volvox carteri* (Vc), between 2010–2016 prompted us to look for cellulase genes in these algae and to compare them to cellulases from bacteria, fungi, lower/higher plants, and invertebrate metazoans. Interestingly, algal catalytic domains (CDs), belonging to the family GH9, clustered separately and showed the highest (33–42%) and lowest (17–36%) sequence identity with respect to cellulases from invertebrate metazoans and bacteria, respectively, whereas the identity with cellulases from plants was only 27–33%. Based on comparative multiple alignments and homology models, the domain arrangement and active-site architecture of algal cellulases are described in detail. It was found that all algal cellulases are modular, consisting of putative novel cysteine-rich carbohydrate-binding modules (CBMs) and proline/serine-(PS) rich linkers. Two genes were found to encode a protein with a putative Ig-like domain and a cellulase with an unknown domain, respectively. A feature observed in one cellulase homolog from Gp and shared by a spinach cellulase is the existence of two CDs separated by linkers and with a C-terminal CBM. Dockerin and Fn-3-like domains, typically found in bacterial cellulases, are absent in algal enzymes. The targeted gene expression analysis shows that two Gp cellulases consisting, respectively, of a single and two CDs were upregulated upon filter paper addition to the medium.

**Keywords:** cellulase; glycosyl hydrolase family 9; carbohydrate binding module (CBM); bioinformatics; RT-qPCR
