The extracellular polymeric substances of desmids (Conjugatophyceae, Streptophyta): chemistry, structural analyses and implications in wetland biofilms

Abstract

S.N. Kiemle, D.S. Domozych and M.R. Gretz. 2007. The extracellular polymeric substances of desmids (Conjugatophyceae, Streptophyta): chemistry, structural analyses and implications in wetland biofilms. Phycologia 46: 617–627. DOI: 10.2216/06–97.1

Desmids represent a group of advanced green algae that are commonly found in biofilm communities of freshwater wetlands. Desmids secrete significant amounts of extracellular polymeric substances (EPS) that form an extensive mucilaginous sheath external to the cell wall and function in adhesion, gliding-based movements and ultimate ensheathment within the biofilm complex. We have initiated biochemical and structural analyses of the EPS of desmids isolated from biofilms from the southeastern Adirondack region of New York including Penium cylindris, Penium spirostriolatum, Cosmarium sp. 1, Cosmarium sp. 2, Pleurotaenium trabecula, Tetmemorus brebissonii, Netrium digitus, Netrium oblongum, Netrium interruptum and Netrium interruptum 2509. Cosmarium sp. 1 EPS appeared as a homogeneous sheath that encapsulated the cells, whereas P. trabecula EPS occurred as dispersed patches, and N. oblongum EPS was reticulated and striated. Polysaccharides were the major component of the EPS (52–76%), and lectin labeling revealed differences in polymer organization between saccoderm (labeled with Con-A, WGA and HPA) and placoderm (labeled with BS-I, PSA, UEA and HPA) desmids. Desmid EPS had significant uronic acid (3–29%) and protein (2–10%) content, and the polysaccharides were sulfated to varying degrees. Xylose and Fucose were the predominant monosaccharides with the major glycosyl linkages t-Xylp, 3,4-Fuc and t-Fuc. The unique EPS from N. oblongum was rich in galactose and uronic acid (29.3% w/w), extracellular polymers of P. spirostriolatum were composed predominantly of arabinose, and T. brebissonii EPS was enriched in glucose and galactose. EPS of P. trabecula and T. brebissonii was highly sulfated (10.2% and 14.7%, respectively). The EPS from New York and UTEX strains of N. interruptum were unique from each other. Overall, desmid EPS exhibits a conserved motif with the predominant component an anionic polysaccharide. The presence of deoxy-sugar subunits indicates the potential for hydrophobic interaction where anionic components may play important role in ionic cross-linking. The establishment of this baseline biochemistry provides the foundation for future dissection of core EPS molecules and associated functional groups and studies of EPS involvement in specific stages of biofilm development (e.g. adhesion).

Key Words::

• Biofilms
• Carbohydrate chemistry
• Conjugatophyceae
• Cosmarium
• Desmid
• Extracellular polymeric substances
• EPS
• lectin labeling
• Netrium
• Penium
• Pleurotaenium
• Streptophyta
• Tetmemorus
• Wetlands

ACKNOWLEDGMENTS

Part of this study was supported by NSF grant DBI-04141 (D.S.D.).