Red microalgal cell-wall polysaccharides: biotechnological aspects

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The area of sugars and glycosylation is not as well developed as other fields in cell biology owing to biotechnological constraints. However, the biotechnological potential of sugars, including polysaccharides, is the driving force pushing research efforts to meet the challenge. Algae produce cell-wall sulfated polysaccharides, with those of the red unicells, which dissolve into the medium, having unique characteristics—structure, composition, fluid dynamics, and extreme stability. These characteristics, combined with polysaccharide bioactivities, offer a vast range of potential applications. Research has thus been directed toward an in-depth understanding of the molecular structure, biosynthesis, and characteristics of the red microalgal sulfated polysaccharides and to the development of molecular-genetic tools, aiming at large-scale production for applications that can benefit humanity.

Introduction

The biotechnological potential of natural polysaccharides is currently gaining increased recognition as a result of two parallel, seemingly unrelated, processes—the tendency of global markets to switch from synthetic to natural products and a growing understanding of the functions of sulfated sugars and of the importance of glycosylation in the post-genomic era. In this context, the polysaccharides of seaweeds have been under investigation for many years, but those of the microalgae remain almost unstudied and elucidating the role of these complex sugars in cell metabolism poses an exciting challenge. This review is thus dedicated to the information that is available on red microalgal cell-wall polysaccharides and their potential biotechnological applications.

Polysaccharides play significant roles in a variety of functions in the cells of different organisms. In algae, polysaccharides serve mainly as storage and structural molecules. In seaweeds, the structural cell-wall polysaccharides usually consist of an outer amorphous mucilage matrix, commonly made up of linear sulfated galactan polymers (carrageenans, agarans, and alginates) and an inner rigid component, cellulose fibrils. In the red microalgae, the cell walls lack this cellulose microfibrillar component; rather they are encapsulated within a sulfated polysaccharide in the form of a gel [1]. During growth in a liquid medium, the external part of the polysaccharide undergoes dissolution from the cell surface into the medium (soluble fraction) [2, 3], whereas most of the polysaccharide (∼50–70%) remains attached to the cell (bound fraction). A variety of functions have been suggested for the red algal cell wall: mechanical support [1], biorecognition [4], and ion-exchange capacity [5]; In the red microalgae, the polysaccharide supplies the cells with environmental protection: the gel structure protects against desiccation; its stability to temperature, pH, and salinity [3, 6] protects against environmental extremes, and its antioxidant activity is probably used as a free radical scavenger [7, 8].

The Arad laboratory has taken a multidisciplinary approach to developing the biotechnology of red microalgae. This effort requires the coordination of various disciplines: chemistry (composition and structure), physiology (effect of environmental conditions), biochemistry (biosynthesis and sulfation pathways), biotechnology (applications and bioactivities) [9, 10••, 11••], and bioengineering a commercial/large-scale cultivation system [9, 10••, 12]. Since modern biotechnology demands molecular-genetic studies, we have made significant progress in the field of red microalgal genomics by establishing expressed sequence tag (EST) databases of two species of red microalgae, Porphyridium sp. and Dixoniella grisea [13], and in the development of molecular-biology tools [11••, 13, 14•].

Section snippets

Chemical characterization

The chemical characteristics of red algal cell walls have been investigated mainly in seaweeds, with knowledge about the chemical structure and characteristics of red microalgae being more limited. This lag is mainly due to the complexity of the red microalgal polysaccharides and to the lack of specific enzymes that degrade them [6, 9, 15]. The studies that have been performed on the red unicells have been devoted mainly to four species from different habitats: Porphyridium sp. (seawater), P.

Physicochemical characterization

One of the main characteristics of the red microalgal polysaccharides that makes them suitable for industrial applications is their fluid-dynamic behavior [9, 10••, 13, 17, 29, 30, 31, 32••, 33, 34, 35, 36, 37]—highly viscous aqueous solutions at relatively low polymer concentrations, yielding rheological properties comparable with industrial polysaccharides, for example, xanthan [17, 29, 30, 31]. Aqueous solutions of Porphyridium sp. polysaccharide were found to be stable (as reflected by

Cell-wall polysaccharide formation

In contrast to the cell-wall polysaccharides of seaweeds, which are characterized by an organized structure, composed of repeating disaccharide blocks, the polysaccharide structure of the red microalgae is more complex. Various approaches have been taken to elucidate cell-wall formation in red microalgae; these include the use of synchronized cultures, cell-wall-modified mutants, inhibitors of polysaccharide formation, carbon partitioning and elucidation of sulfation pathways, and glycoprotein

Biotechnological aspects

The increasing market demand for natural polysaccharides for the food, cosmetics, and pharmaceutical industries cannot be met by currently available conventional sources—red and brown macroalgae. These traditional sources of polysaccharides, which are usually harvested from their natural habitats [50, 51], are being depleted by intensive harvesting and detrimental environmental conditions. An attractive alternative may be found in the sulfated polysaccharides of the red microalgae, which offer

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

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