Algae may be heterotrophic!

Basic features of  algae - unicellular to multicellular organisms, typically autotrophic (phototrophic). Yes they are typically autotrophic.

But in the recent issue of Nature Communications Vol. 3, Article number: 1214; doi:10.1038/ncomms2210 it has been reoprted that the green alga Chlamydomonas reinhardtii not only engages in photosynthesis, it also has an alternative source of energy. A group of scientists from Department of Biology, Algae Biotechnology and Bioenergy–Center for Biotechnology (CeBiTec), Bielefeld University, 33615 Bielefeld, Germany has presented another paradigm of adaptation for C. reinhardtii, highlighting its photoheterotrophic ability to utilize cellulose for growth in the absence of other carbon sources. They claim C. reinhardtii may thus serve as biocatalysts for cellulosic biofuel production.

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Schematic  diagram of Chlamydomonas

Chlamydomonas reinhardtii is an unicellular algae and is the most widely used laboratory species. Naturally it occurs in  several habitats. Cells of this species are haploid, and can grow on a simple medium of inorganic salts, using photosynthesis to provide energy. They can also grow in total darkness if acetate is provided as an alternative carbon source. When deprived of nitrogen, haploid cells of opposite mating types can fuse to become a diploid zygospore which forms a hard outer wall that protects it from adverse environmental conditions. When conditions improve (or when the scientist restores nitrogen to the culture medium and provides light and water), the diploid zygote undergoes meiosis and releases four haploid cells that resume the vegetative life cycle. Chlamydomonas is used as a model system for research on many very fundamental questions in cell and molecular biology.
(source : http://www.chlamy.org/info.html )

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