A variety of algae are currently cultivated commercially, especially for applications in food and feed production, but also for other applications such as fertilizer and the production of materials. Also, there is limited harvesting of uncultivated algae (Critchley et al. 2006). There have been proposals to exploit aquatic biomass for the production of biofuels.

Marine Phytobiomass

Most of the surface of the Earth consists of seas, mainly oceans. A variety of pro­posals exist to exploit the seas for the production of biofuels. Macroalgae, macro­phytes and microalgae have been considered in this context. Microalgae include both prokaryote and eukaryote photosynthetic micro-organisms. In the context of exploiting macrophytes, floating man-made structures to cultivate the Macrocystis pyrifera (giant kelp) have been proposed (Wilcox 1982; Bungay 2004). Varieties of the brown seaweed Laminaria, which is currently harvested for food (Chopin et al. 2001), have been suggested as a convenient source of carbohydrates to be con­verted into ethanol (Hornet al. 2000). The highly salt-tolerant microalgaDunaliella, which, for instance, occurs in the Dead Sea, has also been proposed as a source of transport biofuel (Ben-Amotz et al. 1982).

However, there is a major snag regarding the proposal to use the sea for the production of algae, which may serve the supply of transport biofuels. Actual phy­tobiomass in the seas is in the order of 1-2% of total global plant carbon. Photosyn­thesis in the seas is much higher (in the order of 40-50% of total photosynthesis; Rosing et al. 2006), but most of the photosynthetic yield (approximately 80-88%) is quickly consumed. In the case of microalgae, consumption is mainly by zooplankton (‘grazers’), while 2-10% is subject to viral lysis (Wilhelm and Suttle 1999). Thus, substantial direct appropriation of the products of photosynthesis by humans in the seas in general would necessitate a major overhaul of the marine food web. For the successful growth of desired microalgae, probably dramatic changes in ocean com­position, such as a switch to much higher salinity, may be required (Sawayama et al. 1999; Joint et al. 2002; Ugwu et al. 2008). Large-scale exploitation of macroalgae is cumbersome. Proposals to exploit the giant kelp Macrocystis require pumping of deep seawater to the ocean surface, massive man-made structures to support kelp growth and regular replanting (Bungay 2004).