Algal organisms are photosynthetic macroalgae or microalgae growing in aquatic environments. Macroalgae, or “seaweeds,” are multicellular plants growing in salt or fresh water. They are often fast growing and can reach sizes of up to 60 m in length (McHugh 2003). They are classified into three broad groups based on their pigmen­tation: (1) brown seaweed (Phaeophyceae), (2) red seaweed (Rhodophyceae), and (3) green seaweed (Chlorophyceae).

Microalgae are microscopic photosynthetic organisms that are found in both ma­rine and freshwater environments. Biologists have categorized microalgae into a va­riety of classes, mainly distinguished by their pigmentation, life cycle, and basic cellular structure. The three most important classes of microalgae in terms of abun­dance are the diatoms (Bacillariophyceae), the green algae (Chlorophyceae), and the golden algae (Chrysophyceae). The cyanobacteria (blue-green algae) (Cyano — phyceae) are also referred to as microalgae. This applies, for example, to Spirulina (Arthrospira platensis and A. maxima). Diatoms are the dominant life form in phy­toplankton and probably represent the largest group of biomass producers on Earth. It is estimated that more than 100,000 species exist.

Microalgae are primitive organisms with a simple cellular structure and a large surface-to-volume-body ratio, which gives them the ability to uptake large amounts of nutrients (Sheehan et al. 1998). The photosynthetic mechanism of microalgae is similar to land-based plants, but, due to their simple cellular structure and to the fact they are submerged in an aqueous environment where they have efficient access to water, CO2, and other nutrients, they are generally more efficient in converting solar energy into biomass (Carlsson et al. 2007). The growth medium must contribute the inorganic elements that help make up the algal cell such as nitrogen, phosphorus, iron, and sometimes silicon (Grobbelaar 2004).

Microalgae can be used for bioenergy generation (biodiesel, biomethane, bio­hydrogen), or combined applications for biofuel production and CO2 mitigation. Microalgae are veritable miniature biochemical factories and appear more photo­synthetically efficient than terrestrial plants (Pirt 1986) and are efficient CO2 fixers (Brown and Zeiler 1993).

The existing large-scale natural sources of algae are bogs, marshes, and swamps — salt marshes and salt lakes. Microalgae contain lipids and fatty acids as membrane components, storage products, metabolites, and sources of energy. Algae contain anywhere from 2 to 40% of lipids/oils by weight. Essential elements include nitro­gen (N), phosphorus (P), iron, and, in some cases, silicon (Chisti 2007). Minimal nutritional requirements can be estimated using the approximate molecular formula of the microalgal biomass: CO0.48Hi.83N0.hP0.0i. This formula is based on data pre­sented by Grobbelaar (2004).

The production of microalgal biodiesel requires large quantities of algal biomass. Macro — and microalgae are currently mainly used for food, in animal feed, in feed for aquaculture, and as biofertilizer. Biomass from microalgae is dried and marketed in the human health food market in the form of powders or pressed in the form of tablets. Aquatic biomass could also be used as raw material for cofiring to produce electricity, for liquid fuel (bio-oil) production via pyrolysis, or for biomethane gen­eration through fermentation. Biomethane can be produced from marine biomass (Demirbas 2006).