The potential of algal biomass as a source of liquid and gaseous biofuels has been the subject of considerable research over the past few decades, with researchers strongly agreeing that algae have the potential of becoming a viable aquatic energy crop with a higher energy potential compared to that from either terrestrial biomass or municipal solid waste. However, neither microalgae nor seaweed are currently cultivated solely for energy purposes due to the high costs of harvesting, concentrating and drying. Anaerobic digestion of algal biomass could theoretically reduce costs associated with drying wet biomass before processing, but practical yields of biogas from digestion of many algae are substantially below the theoretical maximum. New processing methods are needed to reduce costs and increase the net energy balance. This review examines the biochemical and structural properties of seaweeds and of microalgal biomass that has been produced as part of the treatment of wastewater, and discusses some of the significant hurdles and recent initiatives for producing biogas from their anaerobic digestion.

Macroalgae represent a potential biomass source for the production of bioethanol or biogas. Their use, however, is limited by several factors including, but not restricted to, their continuous supply for processing, and low biofuel yields. This review examines recent pre-treatment processes that have been used to improve the yields of either biogas or bioethanol from macroalgae. Factors that can influence hydrolysis efficiency and, consequently, biofuel yields, are highly affected by macroalgal composition, including content of salts, heavy metals, and polyphenols, structural make-up, as well as polysaccharide composition and relative content of carbohydrates. Other factors that can influence biofuel yield include the method of storage and preservation.

The pretreatment of seaweed by washing in freshwater is often used in seaweed biofuel research studies. However, the effect of washing seaweed prior to anaerobic digestion (AD) does not appear to have been greatly studied. This study examines washing Sargassum muticum with freshwater and its effect on ultimate and proximate analyses, salt content, methane production from anaerobic digestion, and leachate loss from ensiling. Washing with freshwater significantly (p < 0.01) increased moisture content (unwashed 85.6%, washed 89.1%) but significantly (p < 0.05) reduced ash (unwashed 32.7% dry weight dw, washed 30.6% dw) and salt content (unwashed ash containing 51.5%, washed 42.5%). The dry biomass higher heating value was significantly (p < 0.05) increased by washing due to the lower ash content (11.5 to 12.6 kJ g⁻¹ dw). There was no significant change in the protein or lipid content, although washing increased the nitrogen content (3.85–4.77% dw). Washing significantly (p < 0.05) increased leachate losses during ensiling, with total leachate losses increasing after washing (12.7–25.2%). The methane yield from anaerobic digestion (28 days) was not statistically significantly different (p > 0.05) between unwashed (0.225 L CH₄ g⁻¹ VS) and washed samples (0.177 L CH₄ g⁻¹ VS). However, washing delayed biomethane production.

Microalgae have become an attractive natural source of a diverse range of biomolecules, including enzymatic and non-enzymatic antioxidants; nevertheless, economically sustainable production of such compounds from microalgae biomass is still challenging. The main hurdles are: (a) increasing microalgae yield; (b) achieving optimal cultivation conditions; (c) energy-efficient and cost-effective downstream processing (extraction and purification); (d) optimal storage of post-processed antioxidant molecules. This review provides a detailed overview of enzymatic and non-enzymatic antioxidants in the cellular metabolism of the commercially important microalgae Dunaliella, industrial applications of antioxidant enzymes, strategies to enhanced antioxidant accumulation in cells, and the opportunities and limitations of current technologies for antioxidant enzymes production from microalgae biomass as an alternative to common microbial sources.