Plant Physiology - Musienko M.M. 2001

Plant Physiology and Biotechnology: Achievements and Development Prospects
Microalgae and their cultivation. Biologically active substances of algae and water quality

The Significance of algal culture in PHYSIOLOGICAL AND BIOCHEMICAL research, coupled with the fact that Algae serve as the primary photosynthetic link in ecosystems and play a crucial role in shaping the Chemical Composition and organic matter reserves of Water bodies, has intensified interest in their study over recent decades.

Algal metabolites have been found to exhibit allergenic, mutagenic, and carcinogenic effects, as well as anti-hormonal, growth-promoting, bactericidal, insecticidal, fungicidal, and algicidal activity (Sirenko, Kozytska, 1988). Consequently, the study and preparative isolation of these compounds are of great theoretical and practical importance in biotechnology.

Over the past thirty years, various species of microalgae have been cultivated on chemically standard nutrient media to obtain their metabolites—most frequently secondary ones of practical value: Spirulina as a source of valuable Proteins, Vitamins, CARBOHYDRATES, and γ-linolenic acid; Dunaliella as a source of β-carotene and glycerol; Porphyridium as a source of Polysaccharides and arachidonic acid; Chlamydomonas as a source of polysaccharides and soil conditioners; and various species of blue-green algae as nitrogen fixers and a source of ammonia.

In recent years, the microalga Spirulina has gained immense popularity due to its Cells containing a significant amount of high-quality protein. Its amino acid profile closely approaches the optimal protein standards established by the FAO. It has been found to contain 9 Essential Amino Acids alongside an equal number of others, making Spirulina an exceptionally valuable food product. Its productivity is quite high; for instance, up to 20 g of dry matter can be harvested daily from 1 m² of a water body, which, when converted to an annual yield, exceeds wheat productivity by nearly 10-fold.

In many countries, algae are used as vitamin supplements in animal feed and as fertilizers. Algal biomass enriches the soil with phosphorus, potassium, iodine, and Trace Elements, while also improving its microbiological properties by introducing nitrogen-fixing microflora, among others.

Microalgae are cultivated for biomass production using both batch cultures and continuous culture Methods. The growth and biomass accumulation of a batch culture begin the moment a portion of it is transferred to a culture medium.

A variety of nutrient media exist, but when developing their composition, several critical factors must be taken into consideration. First and foremost, the total salt concentration is determined by the ecological conditions of the species' natural habitat and includes Macronutrients such as potassium, magnesium, sodium, calcium, sulfate, and phosphate. Algal growth depends primarily on the availability of nitrogen, which may be supplied as nitrates, ammonium nitrogen, or urea, with the choice depending on the algal species and the optimal pH value. Since most microalgae species contain 7–9% nitrogen relative to dry biomass, producing 1 g of cells per 1 L of culture requires 500–600 mg of KNO₃ L^-1. Inorganic carbon is introduced in the form of CO₂ or bicarbonate. To prevent the precipitation of calcium, magnesium, and certain trace elements, media with an acidic pH are utilized. Vitamins—specifically thiamine and cobalamin (vitamin B₁₂)—are also required for the growth of many algal species.

Biomass accumulation in a batch culture begins as soon as a portion of it is transferred to a fresh culture medium. Biomass concentration is determined by Cell counts, dry biomass weight, or the quantification of a specific biochemical component. The rate of biomass concentration increase is calculated using the formula:

where μ is the specific growth rate, or the index of biomass accumulation per unit of time; x is the biomass concentration,

Culture growth is divided into the following phases: lag phase, acceleration phase, logarithmic phase (balanced growth), deceleration phase, stationary phase, and death phase.

During the lag phase, a freshly inoculated culture adapts to the culture medium, whereas the subsequent acceleration phase is marked by the sequential increase of specific biomass parameters. First, RNA content rises, followed by protein, and finally, dry biomass.

In the logarithmic phase, the growth rate remains constant while the biomass concentration increases.

Throughout the logarithmic growth phase, the ratio between various intracellular biochemical components remains constant; therefore, this state is referred to as balanced or exponential growth.

During the deceleration (or retardation) phase, the Biochemical Composition of the cells Changes in the reverse sequence to that observed in the acceleration phase. In the stationary phase, the biomass remains constant, though its other parameters may vary.

The final biomass concentration is naturally closely related to nutrient depletion, pH changes, and the accumulation of metabolites that inhibit culture growth.

In the final phase, The ratio of Respiration to Photosynthesis exceeds unity, potentially leading to cell death resulting from lysis.

Algae cultures isolated from natural habitats must be continuously maintained and protected from contamination. The illumination intensity for propagation under cool-white fluorescent lamps should be 2150–4300 lx, whereas a lower intensity of 540–1100 lx is sufficient to slow down growth and preserve the culture. A Temperature of 15–20 °C is adequate to maintain viability. Subculturing intervals depend on the algal species and cultivation conditions. Unicellular non-motile algae should be subcultured every 6–12 months, while flagellated species require more frequent transfer. Cryopreservation in liquid nitrogen is successfully used for the preservation of certain species.

It should be noted that installations for algal biomass production can be utilized for such diverse purposes as wastewater Treatment, feed additive production, industrial recovery of various Organic compounds (glycerol, mannitol, Lipids), as well as the synthesis of biochemical compounds, pigments, and natural colorants.

Among algal metabolites, simple and terpenoid alcohols, terpene esters, aldehydes, Hydrocarbons, sulfur-containing compounds, Sugars and Polysaccharides, organic acids, lipids, and their derivatives have been identified. The volume of exogenous metabolites accounts for a substantial proportion of the generated organic matter. For instance, in certain blue-green algae, they comprise 6–13%, while in some diatoms and the majority of blue-green algae, the figure reaches 40–60%.

The Biological Significance of exudative metabolite release and its underlying mechanisms remain insufficiently understood. This process is often associated with the need to build up nutrient reserves in the water to support the early Developmental Stages of daughter cell spores and to sustain Selection/30.html">The population as a whole. It is possible that these metabolites play a role in adaptive processes, expanding geographical ranges, suppressing competing species, and supporting the functioning of symbiotic organisms.

Upon cell death, 70–80% of phosphorus compounds and 20–30% of nitrogen compounds are released into the water, along with proteins, carbohydrates, lipids, and other intracellular Biopolymers. These compounds not only interact with one another but also undergo phytochemical and biological transformation. As a result of such processes, even more physiologically active substances appear in the water. Proteins break down into various reactive Aromatic Compounds, such as indoles, skatoles, and amines. Amines, particularly in the presence of nitrates in the water and a low pH, promote The formation of carcinogenic nitrosamines.

Lipids and their derivatives are also transformed in the water. Furthermore, allergens of algal origin have been detected in aquatic environments. All of these factors affect water quality. In addition, water chlorination leads to the formation of organochlorine compounds characterized by heightened biological activity, notably carcinogenicity. Consequently, accelerated algal growth in eutrophic water bodies must be regarded as a factor that significantly impacts not only the functioning of hydrobiocenoses but also the formation of natural water quality.