Algae are a large and very diverse group of organisms ranging from simple unicel­lular microalgae to giant macroalgae. The morphology of macroalgae or seaweeds resembles the terrestrial plants but the biochemical composition is significantly dif­ferent. The major carbon storage products in terrestrial plants are starch and fructo — san [13-15] that can be easily converted to biogas. However, the main components of terrestrial plant cell walls are cellulose/hemicellulose fibers embedded into a pec­tin matrix and cemented together by lignin [16-18]. This lignocellulosic complex is recalcitrant to biological degradation and requires intensive chemical (acid hydroly­sis, alkaline wet oxidation, ammonia fiber expansion) or thermal pretreatment (steam explosion, hot water) before biological conversion [19-22].

The major components of macroalgae are polysaccharides, algal cell wall lack lignin. The main components of cell envelopes are ulvan and xylan in green algae; carrageen, agar, and xylose in red algae; alginate and fucoidan in brown algae. Cellulose is a structural component of the cell wall in many genera, but only in some green algae is the ratio on a level comparable to terrestrial plants. The main storage polysaccharides in macroalgae are floridean starch in red algae; chlorophy — cean in green macroalgae; laminarin; and mannitol in brown macroalgae.

The biochemical composition of microalgae and cyanobacteria are significantly different from macroalgae. Often carbohydrates are a minor component of cell dry weight, whereas proteins and lipids account for the bulk of microalgal dry weight.

One of the challenges in AD of algae is significant variation in biochemical com­position not only among different phylum or genera, but also among similar species. Biochemical composition depends on many environmental factors, such as tem­perature, salinity, light intensity, and nutrient availability [23-27] .