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:
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