While aiming at transport biofuels, the growth of microalgae with high levels of oil (triacylglycerol) followed by lipid extraction has drawn most attention (Scragg et al. 2002; Wijffels 2008; www. oilgae. com; Dismukes et al. 2008; Liu et al. 2008). Such lipids can then subsequently be converted into replacements for fossil fuels, in ways similar to vegetable oils from terrestrial plants. There is currently some use of biodiesel based on algal oils, as pointed out above. There have also been proposals to convert algal biomass into methanol via synthesis gas or into bio-oil via pyrolysis (Hirano et al. 1998; Sawayama et al. 1999). Strains of the photosyn­thetic microalga Botryococcus braunii may contain and secrete substantial amounts of isoprenoid hydrocarbons: n-alkadienes and trienes, methylated squalenes and terpenoids (Guschina and Harwood 2006). When subjected to catalytic cracking, these hydrocarbons can be converted into transport biofuels (Banerjee et al. 2002). It has also been suggested that an intermediate in the synthesis of isoprenoids by Botryococcus braunii (isoprenylpyrophosphate) may be converted into isopentanol, which may be used as a gasoline additive (Fortman et al. 2008). The slow growth of Botryococcus braunii has not been conducive to its application.

Microalgae may be produced in open ponds converting solar irradiation into biomass which may be harvested and converted into biofuels. Open ponds used for growing microalgae are man-made structures (made from, for example, plas­tic or concrete) with 10-20 cm of water that are subjected to circulation and mix­ing (Chisti 2007). Closed systems (‘bioreactors’) have also been proposed for the purpose of growing photosynthetic micro-organisms to produce transport biofuels (Chisti 2007, Wijffels 2008). In closed systems, heterotrophs, organisms that graze on algae (zooplankton) and viruses can be excluded, and monocultures of desirable species can be maintained. In open ponds, sustained generation of a specific pho­tosynthetic micro-organism with relatively little contamination of other species and subject to low heterotrophic conversion would seem only possible under extreme circumstances, such as very high salinity and/or a high pH (Joint et al. 2002; Ugwu et al. 2008). Sustained open pond production has been successful for a limited num­ber of algae such as Spirulina, Chlorella and Dunaliella (grown at high pH and/or NaCl concentrations). For other organisms, most growth can take place in a closed bioreactor, which then may be eventually followed by a short period in an open pond (Huntley and Redalje 2007).

Freshwater Macrophytes

In fresh waters, there has been an emergence of invasive macrophytes with high pri­mary production per hectare. Increased levels of nutrients (‘eutrophication’) and the import of macrophytes from other continents have been conducive to this emergence (Gassmann et al. 2006; Gunnarsson and Petersen 2007). Among these macrophytes, the water hyacinth (Eichhornia crassipes) has been studied in the context of bio­fuel production (Gunnarsson and Petersen 2007; Malik 2007). The water hyacinth has emerged as a major invasive organism (‘pest’) in tropical freshwater systems especially outside its natural range (South America). Water hyacinth biomass forms floating mats which interfere with shipping, power generation, drinking water pro­duction and irrigation, are detrimental to fish stocks and may be conducive to a num­ber of infectious diseases (Odada and Olago 2006; Gunnarsson and Petersen 2007; Malik 2007). Due to these negative impacts, there are efforts to reduce the pres­ence of Eichhornia crassipes in tropical surface waters, which have met with at least some success (Odada and Olago 2006). The need to control the water hyacinth is evidently at variance with high yields, but when the water hyacinth generates substantial amounts of floating biomass, energetic use thereof may be considered (Gunnarsson and Petersen 2007; Malik 2007).