Commercial fertilizers, used for long periods, have adverse effects on soil productivity and environmental quality, so interest in environmentally friendly, sustainable agricultural practices has been on the rise. In developing and implementing sustainable agriculture tech­niques, biofertilization is of great importance to alleviate deterioration of natural ecosystems and to reduce the impact of environmental pollution while integrating nutrient supply into agriculture. Biofertilizers include mainly nitrogen-fixing, phosphate-solubilizing, and plant growth-promoting microorganisms, as in the case of microalgae.

Marine algae and algae-derived products have been widely used as nutrient supplements and as biostimulants or biofertilizers to increase plant growth and yield. The regulatory sub­stances cytokinins, auxins, gibberellins, and betaines in algae can induce plant growth (Valente, Gouveia et al., 2006), but their roles as macro — and micronutrients also make them valuable components of biofertilizers. A few commercial products based on marine algae are ready available for use in agriculture, but ongoing research has featured several alga species in terms of ascertaining their effects on plant growth. For instance, recent work with Laminaria digitata indicated that this marine macroalga (traditionally used as soil amendment in many parts of the world) improves seed germination and rooting in terrestrial plants (Thorsen, Woodward et al., 2010).

Several pieces of evidence confirmed that microalgae are beneficial in plant cultivation by producing growth-promoting regulators, vitamins, amino acids, polypeptides, and antibacterial and antifungal substances that exert phytopathogen biocontrol as well as poly­mers, especially exopolysaccharides that improve both plant growth and productivity (de Mule, de Caire et al., 1999). Other indirect growth-promotion effects may be claimed, such as enhancing the water-holding capacity of soils or substrates, improving availability of plant nutrients, and producing antifungal and antibacterial compounds (Schwartz and Krienitz,

2005) . In hydroponic cultivation, microalgae present a few extra benefits: The oxygen pro­duced by photosynthesis avoids anaerobiosis in the root system while releasing such growth-hormones as auxins, cytokins, gibberelins, abscisic acid, and ethylene (Schwartz and Krienitz, 2005). Equally important and promising is the high N:P ratio exhibited by microalgae, which is an extra indicator of its potential as fertilizer.

10.5.1 Animal Feed

The moisture content of fresh marine algae is quite high and can account for up to 94% of their biomass. However, marine algae contain such nutritional elements as proteins, lipids, carbohydrates, vitamins, and minerals that are in high demand for animal feed (Zubia et al., 2008). In particular, the ash content is high compared to that of vegetables (Murata and Nakazoe, 2001) and includes both macrominerals and trace elements.

Fish feeding represents over 50% of the whole operating costs in intensive aquaculture, with protein being the most expensive dietary source (Lovell, 2003). Nowadays 24% of the fish harvested by fisheries worldwide is used to produce fish meal and fish oil, thus putting high pressure on fisheries that aquaculture has attempted to alleviate. This demand promoted extensive efforts to find alternative sources of protein sources for aquatic feed; unfortunately, plants are poor protein sources in fish diets owing to their deficiency in certain essential amino acids, their content of antinutritional compounds, and taste problems.

Conversely, microalgae have been traditionally used to enrich zooplankton, which will in turn be used to feed fish and other larvae. In addition to providing proteins contain essential amino acids, they carry such other key nutrients as vitamins, essential PUFAs, pigments, and sterols, which may then be transferred upward through the food chain (Guedes and Malcata, 2012).

On the other hand, contamination by bacteria that attack fish can potentially devastate aquaculture farms. Microalgal fatty acids longer than 10 carbon atoms can induce lysis of bacterial protoplasts; said ability depends on composition, concentration, and degree of unsaturation of free lipids (Guedes et al., 2011b). The contents of carotenoids are important in aquaculture as well. In fact, artificial diets that lack natural pigments preclude such organisms as salmon or trout to acquire their characteristic red color (muscle), which, in nature, is a result of ingesting microalgae containing red pigments; without such a color, a lower market value will result (Guedes and Malcata, 2012).