Red algae
Over a billion years ago, a heterotrophic eukaryote engulfed a cyanobacterium, and started its evolutionary descent into the supergroup archaeplastida, in which red algae branched apart from the line of green algae that would go on to produce plants. For photosynthesis, red algae contains a handfull of pigments that help absorb light at different wavelengths, giving them a variety of colors possibilities. Phycoerythrin is a red pigment that can mask the green chlorophyll (they have chlorophyll a[4]), making red algae red[4]. The color tends to get darker the deeper the alga lives, since it needs more absorptive pigment to make more out of the lesser light available. They also have phycocyanin, which appears bluish. Some red algae even lack pigmentation and function as parasites on other red algae[1, p590].
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Mature gametophyte
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References
[1]Campbell & Reece, Biology, 8th Edition
[2]http://www.mbari.org/staff/conn/botany/flora/reds.htm
[3]http://www.seaweed.ie/aquaculture/noricultivation.php
[4]http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Protists.html
[2]http://www.mbari.org/staff/conn/botany/flora/reds.htm
[3]http://www.seaweed.ie/aquaculture/noricultivation.php
[4]http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Protists.html