58 JUNE 2022 • WORLD AQUACULTURE • WWW.WA S .ORG and growth media of several microalgal species (Tarakhovskaya et al. 2007). For instance, Stirk et al. (2013) identified the phytohormones auxin, gibberellin and cytokinin in axenic cultures of 24 species of microalgae belonging to four classes: Ulvophyceae, Trebouxiophyceae, Chlorophyceae and Charophyceae. Biofertilizers are substances containing living organisms that, when applied to seed, plant or soil, promote crop growth (Vessey 2003). Response of crops to microalgae as a biofertilizer was documented in the 1960s, when Gupta and Lata (1964) reported a positive influence of three species of microalgae (Fisherlla mucicolo, Scytonema hofmanni and Nostoc sp.) on a rice paddy field. The microalgae accelerated germination, promoted seedling growth, increased grain yield and protein content of the seed. Microalgae plays a role in the maintenance and build-up of soil fertility, consequently increasing rice growth and yield as a natural fertilizer (Song et al. 2005). Microalgae increases soil water holding capacity due to their “jelly” structure and prevents weed growth (Thajuddin and Subramanian 2005). Research on the agriculture applications of microalgae have focused on rice production for various reasons, primarily because most paddy soils contain a natural population of microalgae (Mishra amd Pabbi 2004). In a study that evaluated the effects of microalgae on a crop other than rice, Garcia-Gonzalez and Sommerfeld 2016) observed positive effects of aqueous extract and dry application of Acutodesmus dimorphus (microalgae) on tomato plant growth. A plant biostimulant is an organic substance that, when applied in small quantities, enhances nutrition efficiency, abiotic stress tolerance, growth and development such that the response cannot be attributed to application of traditional plant nutrients. Biostimulants can be “hormone-containing products,” “humic substances” and “amino Introduction Microalgae are a diverse group of 1-50 µm photoautotrophic, unicellular organisms (Cooper and Smith 2015) sometimes found in soils and on rock surfaces (Sahu et al. 2012) but mainly in water bodies. Microalgae are a rich source of carbohydrates, lipids, proteins, pigments, oils, plant biostimulants and other plant growth-promoting substances such as vitamins, amino acids and sugars (Borowitzka 2013, Mishra and Pabbi 2004, Stirk et al. 2002). In crop production, microalgae are capable of fixing atmospheric nitrogen into a bioavailable source of ammonium required for plants (Malik et al. 2001). Microalgae are also a potential source of bioactive compounds (auxin, gibberellin and cytokinin) (Rodríguez et al. 2006) and several companies have recently started to commercialize microalgae as plant biostimulants (van der Voort et al. 2015) and biofertilizers. In pond and outdoor tank aquaculture, phytoplankton growth needs to be managed to avoid water quality problems such as low dissolved oxygen, high ammonia concentration, daily pH change, etc. Phytoplankton grow using nutrients not assimilated by fish. The more feed added to a pond, the greater the algae growth. Farmers control algal growth by regular water exchange, which is a waste of water and nutrients. Lately some entrepreneurs have been filtering out algae mechanically and selling them as plant growth biostimulants. Numerous compounds active in improving crop yield are synthesized by microalgae (Tarakhovskaya et al. 2007. The elemental composition of various microalgae species includes macronutrients and trace elements such as nitrogen, phosphorus, potassium, zinc, selenium and manganese necessary for crop growth (Campanella et al. 1998, Tibbetts et al. 2015, Volkman and Brown 2006). Additionally, growth promoting substances and various bioactive compounds have been identified in cellular extracts Aquaculture Effluent is a Good Fertilizer But Not a Biostimulant of Plant Growth and Flowering Ghaith Amro, Hadi Jaafar and Imad P. Saoud FIGURE 1. Tilapia aquaculture tanks used for algae extraction and irrigation. FIGURE 2. Flowering French Marigold irrigated with water from tilapia aquaculture tanks.
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