Humic Acid Stimulation of Growth and Optimization of Biochemical Profiles on Two Microalgal Species Proposed asLive Feeds in Aquaculture

摘要

Microalgae have an important role in aquaculture as a means of enriching zooplankton for on-feeding to fish and other larvae. In addition, microalgal provide energy which is transferred through the food chain to higher trophic levels via the zooplankton intermediates (Brown, 2002). Tney are characterized by high nutrient production of compounds such as proteins, lipids, carbohydrates (Renaud et al., 1999), sterols (Mohammady, 2004), polyunsaturated fatty acids (Sargent et al., 1997), and minerals (Fabregas and Herrero, 1986).The biochemical composition of microalgae can be manipulated readily by changing the growth conditions (Brown et al., 1997). Nutrient enrichment and optimization of these conditions is of practical use to aquaculturists who wantto optimize the level of specific nutrient(s) in the micraolgal that are needed by the fish. The enrichment of the microorganisms' nutrient medium by humic substances (HS) was studied (Muller-Wegener, 1988). This organic matter is commonly distributed in the natural habitats such as water, soil and sediments (Coates et al., 2002). It is easily available as compost (Canellas et al., 2002) for conducting to different crops (Mackowiak et al., 2001; Canellas et al., 2002) and has highly recognized benefitswhich are attributed to the numerous functional groups HS has(Coates et al., 2002). HS have been extensively studied for their possible contribution to phytoplankton growth as a source of nitrogen (Carlsson et al., 1995) and carbon (Doblin et al., 1999). Other studies (Doblin et al, 2000) attributed the benefits of humic substances to microalgae for the reason that they act as metal-bending ligands that modulate the availability of trace elements. Furthermore, a combination of metabolic (Doblin et al.,1999) and membrane permeability alterations (Vigneault et al., 2000) triggered by HS might also enhance algal growth.