The specific organic constituents of brown algae are alginic acid, laminarin, mannitol, and fucoidan (Table 11). Among these compounds, mannitol lacks polymeric struc­ture; it is soluble, and can be easily transferred into the cell [139]. Mannitol can be utilized by anaerobic microorganisms with the formation of acetate and hydrogen as the major products, and minor production of ethanol, formate, lactate, and succinate [140]. The degradation of laminarin by anaerobic organisms was studied by inocula­tion of an anaerobic reactor with bacteria from the human gut [141]. The authors reported almost complete (>90%) usage of laminarin during 24 h with the formation of butyrate and other VFA. Alginate has a more complex molecular structure and usually forms a gel in algae. The alginate lyases are enzymes found to be responsible for alginate depolymerization [142, 143]. Biological degradation of soluble

Na-alginate gel is 6-8 times faster compared to Ca-alginate gel due to calcium cross bridging in the polysaccharides [144]. The products of alginate depolymerization are a mixture of oligosaccharides with different length [145], which are further degraded to 4-deoxy-L-erythro-5-hexoseulose uronic acid [146, 147]. The final products of alg­inate degradation are glyceraldehyde-3-phosphate and pyruvate [146, 147].

AD of fucoidan has not been studied in detail, but several fucoidan-degrading marine bacteria were isolated and characterized [148] . Several studies prove the possibility of AD of fucoidan containing waste sludge from alginate extraction [136, 149]. Some fucans are resistant to anaerobic fermentation possibly due to specificities of molecular structure in particular strains [148, 150, 151]. The AD of algal proteins and polyphenols and their impact on overall digestibility is an area that needs more attention. It is assumed that polyphenols associate with proteins and polysaccharides in the cell envelope that decreases their availability for biological degradation [150-152].

Brown algae are one of the most studied algal feedstocks for AD. Examined spe­cies include Macrocystis pyrifera, Ascophyllum nodosum, Durvillea antarctica, Sargassum spp., and Laminaria spp. According to the chemical composition, the theoretical methane yield of 0.52 L/g VS and 0.49 L/g VS were predicted for M. pyrifera and Laminaria sp. [79]. The authors reported an experimental methane yield for M. pyrifera of 0.43 L/g VS (82% VS reduction) but only 0.24-0.3 L/g VS (50-60% VS reduction) for Laminaria saccharina. The significant difference in VS reduction was explained by variability in chemical composition between these gen­era. Laminaria has a higher content of fucoidan, laminarin, and alginate but lower content of mannitol (Table 13) . The ratio between experimental and theoretical methane yield among M. pyrifera species is highly correlated with the mannitol content [153). Mannitol, in contrast to polysaccharides, can be easily and com­pletely degraded by anaerobic microorganisms (Fig. 8a) [79, 153]. The methane yield from L. saccharina samples harvested in spring (4.2% of mannitol and lami — naran, 23% of alginate from TS) provided only 50-65% of the methane yield in comparison to L. saccharina samples harvested in autumn (36% of mannitol and laminaran, 15% of alginate from TS) [154].

The methane yield from Sargassum spp. using a BMP test showed that two spe­cies from this genus Sargassum fluitans and Sargassumpteropleuron have small potential for biomethane production [121]. All tissues added at 0.12-0.20 L/g VS exhibited a methane yield that was only 33-46% of the theoretical methane yield (Table 18). Sargassum species appeared to be a poor feedstock for AD possibly due to low mannitol (3.5-4.5% from TS) and a higher content of fibers (36.5-40.6% from VS) (Table 13).

The ADP in continuously stirred-tank reactors proved that M. pyrifera is the best substrate for methane production among brown macroalgae tested. The digester fed by the M. pyrifera with higher mannitol content had large methane yield and stabil­ity at higher OLR (up to 9.6 gVS/L-day) in contrast to a digester fed with M. pyrifera with lower mannitol content or Sargassum species (Fig. 8b) . Using continuous reactors, the methane yield obtained for different species decreased in the following order: M. pyrifera (0.24-0.35 gVS/L-day), Laminaria sp. (0.2-0.28 gVS/L-day), D. antarctica (0.18 gVS/L-day), Sargassum sp. (0.08-0.15 gVS/L-day), A. nodosum (0.11 gVS/L-day).