Nutrition in Algae

Occurrence and Distribution
Structure of Thallus
  Unicells and Unicell Colonial Algae
  Filamentous Algae
  Siphonous Algae
  Parenchymatous and Pseudoparenchymatous Algae
Following our definition of the term algae, most algal groups are considered photoautotrophs, that is, depending entirely upon their photosynthetic apparatus for their metabolic necessities, using sunlight as the source of energy, and CO2 as the carbon source to produce carbohydrates and ATP. Most algal divisions contain colorless heterotropic species that can obtain organic carbon from the external environment either by taking up dissolved substances (osmotrophy) or by engulfing bacteria and other cells as particulate prey (phagotrophy). Algae that cannot synthesize essential components such as the vitamins of the B12 complex or fatty acids also exist, and have to import them; these algae are defined auxotrophic.

However, it is widely accepted that algae use a complex spectrum of nutritional strategies, combining photoautotrophy and heterotrophy, which is referred to as mixotrophy. The relative contribution of autotrophy and heterotrophy to growth within a mixotrophic species varies along a gradient from algae whose dominant mode of nutrition is phototrophy, through those for which phototrophy or heterotrophy provides essential nutritional supplements, to those for which heterotrophy is the dominant strategy. Some mixotrophs are mainly photosynthetic and only occasionally use an organic energy source. Other mixotrophs meet most of their nutritional demand by phagotrophy, but may use some of the products of photosynthesis from sequestered prey chloroplasts. Photosynthetic fixation of carbon and use of particulate food as a source of major nutrients (nitrogen, phosphorus, and iron) and growth factors (e.g., vitamins, essential amino acids, and essential fatty acids) can enhance growth, especially in extreme environments where resources are limited. Heterotrophy is important for the acquisition of carbon when light is limiting and, conversely, autotrophy maintains a cell during periods when particulate food is scarce.

On the basis of their nutritional strategies, algae are into classified four groups:

  • Obligate heterotrophic algae. They are primarily heterotrophic, but are capable of sustaining themselves by phototrophy when prey concentrations limit heterotrophic growth (e.g., Gymnodium gracilentum, Dinophyta).
  • Obligate phototrophic algae. Their primary mode of nutrition is phototrophy, but they can supplement growth by phagotrophy and/or osmotrophy when light is limiting (e.g., Dinobryon divergens, Heterokontophyta).
  • Facultative mixotrophic algae. They can grow equally well as phototrophs and as heterotrophs (e.g., Fragilidium subglobosum, Dinophyta).
  • Obligate mixotrophic algae. Their primary mode of nutrition is phototrophy, but phagotrophy and/or osmotrophy provides substances essential for growth (photoauxotrophic algae can be included in this group) (e.g., Euglena gracilis, Euglenophyta).