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).