Glaucophyte algae (Phylum Glaucophyta) are unicellular freshwater algae, and are thought to resemble a very ancient ancestor of plants. Study of these organisms gives us clues to the origin of photosynthesis within Archaeplastida. The current hypothesis for the origin of the chloroplast within this group is known as secondary endosymbiosis. According to this hypothesis, a free-living photosynthetic bacterium (a cyano-bacterium) was engulfed by a eukaryotic ancestor but not digested, leaving a chloroplast with 2 membranes. This bacterium continued to live inside its new host, providing it with a constant supply of sugar. While plants photosynthesize using the molecules chlorophyll a and chlorophyll b, glaucophytes photosynthesize via chlorophyll a but do not have chlorophyll b. However, glaucophytes have additional photosynthetic molecules, known as phycobiliproteins. While the engulfed cyanobaterium provided the eukaryote with sugar, this eukaryotic cell provided the photosynthetic bacterium (which is considered a chloroplast once engulfed) the ability to break down those sugars to produce much more ATP via cellular respiration. Where a cyanobacterium can only harvest a net 2 ATP from a glucose molecule, a eukaryote with a mitochondrion can harvest a net 36 ATP!

Glaucophyta.   Glaucocystis sp.  Photo: NEON_ja 2007. Source: Wikimedia Commons. 

Glaucophyta.  Glaucocystis sp. Photo: NEON_ja 2007. Source: Wikimedia Commons.