Background ( Pseudo ) Bacteroides cellulosolvens is a cellulolytic bacterium that produces the most extensive and intricate cellulosomal system known in nature. Recently, the elaborate architecture of the B. cellulosolvens cellulosomal system was revealed from analysis of its genome sequence, and the first evidence regarding the interactions between its structural and enzymatic components were detected in vitro. Yet, the understanding of the cellulolytic potential of the bacterium in carbohydrate deconstruction is inextricably linked to its high-molecular-weight protein complexes, which are secreted from the bacterium. Results The current proteome-wide work reveals patterns of protein expression of the various cellulosomal components, and explores the signature of differential expression upon growth of the bacterium on two major carbon sources—cellobiose and microcrystalline cellulose. Mass spectrometry analysis of the bacterial secretome revealed the expression of 24 scaffoldin structural units and 166 dockerin-bearing components (mainly enzymes), in addition to free enzymatic subunits. The dockerin-bearing components comprise cell-free and cell-bound cellulosomes for more efficient carbohydrate degradation. Various glycoside hydrolase (GH) family members were represented among 102 carbohydrate-degrading enzymes, including the omnipresent, most abundant GH48 exoglucanase. Specific cellulosomal components were found in different molecular-weight fractions associated with cell growth on different carbon sources. Overall, microcrystalline cellulose-derived cellulosomes showed markedly higher expression levels of the structural and enzymatic components, and exhibited the highest degradation activity on five different cellulosic and/or hemicellulosic carbohydrates. The cellulosomal activity of B. cellulosolvens showed high degradation rates that are very promising in biotechnological terms and were compatible with the activity levels exhibited by Clostridium thermocellum purified cellulosomes. Conclusions The current research demonstrates the involvement of key cellulosomal factors that participate in the mechanism of carbohydrate degradation by B. cellulosolvens. The powerful ability of the bacterium to exhibit different degradation strategies on various carbon sources was revealed. The novel reservoir of cellulolytic components of the cellulosomal degradation machineries may serve as a pool for designing new cellulolytic cocktails for biotechnological purposes. Electronic supplementary material The online version of this article (10.1186/s13068-019-1447-2) contains supplementary material, which is available to authorized users.