Because of their likely common ancestry, red and green algae have sometimes been included with the land plants in the formal plant kingdom, a grouping more recently called the Archaeplastida (“ancient [chloro] plastids”). Land plants cer- tainly arose from green algae, but the relationship between red and green algae, is still controversial.
The first verified algal fossils, estimated at 1.2 billion years of age, were of a red alga, Bangiomorpha pubescens (Butterfield 2000) (Fig. 2.11). Identification of these fossils is based more on cell wall structure than any remnants of pigments in them, and they strongly resemble some modern forms of red algae. The green
Figure 2.8 Diatom cells are enclosed in fitted glass shells resembling shoe boxes, and come in an astounding array of intricately ornamented shapes. Some stick together in
algae appear to be somewhat younger, first appearing as fossils about 1 billion years ago, and brown forms of algae are probably only half as old as them. There are various estimates that both red algae and green algae are older than the fossil record indicates, and that the first eukaryotes may have appeared as early as 2.7 billion years ago (Knoll 2014).
The chloroplasts of modern red algae contain accessory photosynthetic pig- ments called phycobilins, which include the reddish phycoerythrin mentioned above. Phycobilins are abundant in cyanobacteria, but not present at all in green and brown chloroplasts. This suggests that the first chloroplasts were like those of red algae, and that the novel pigments of green and brown algae came later.
There have been several prominent theories for how the green chloroplasts, and the green algae that possess them, came about. Among modern free-living photo- synthetic bacteria, there are some, called prochlorophytes (or Chloroxybacteria, in the terminology of Margulis), that have the same general mix of accessory
Figure 2.9 A. Euglena is an animallike protist without a cell wall, whose ancestors
acquired chloroplasts through secondary endosymbiosis with a green alga; B. Ceratium
is one of the diverse group known as dinoflagellates, which have acquired chloroplasts through both secondary and tertiary endosymbiosis involving mostly red and brown algae. It is the occasional population explosions of a dinoflagellate that cause red tide, which results in large-scale death of fish and other larger animals, due to a toxic byproduct of dinoflagellate metabolism. Drawings from Oltmanns 1905.
Diplomonad s Tr ichomonad s Eu glenoid s Kine to plasti ds Dinofla ge llates Ci liates Wa te r mold s Ancestral Eukaryote Archaeplastids
endosymbiosis endosymbiosis kleptoplasty
Di at om s Golden Alg ae Brow n Al ga e Re d Alg ae Gr een Al ga e Te rr estr ial Plant s Sl ime mold s Fu ng i Animal s
Figure 2.10 Within the broad tree of eukaryotic life, based on host cell characteristics, acquisition of photosynthesis through endosymbiosis or kleptoplasty has occurred in nearly every major clade, including the animals.
100
µm
A
C B
Figure 2.11 Fossils of Bangiomorpha pubescens are the oldest verified evidence of
eukaryotic algae. They strongly resemble modern red algae of the genus Bangia.
pigments as the green algae: they lack the phycobilins of other cyanobacteria and have chlorophyll b and carotenoids like green algae. A few years back, this caused a lot of excitement. The prochlorophytes were thought to be quite distinct from cyanobacteria, and potentially the ancestors of green algal chloroplasts. It was proposed that the red algae originated through endosymbiosis with a phycobilin-bearing cyanobacterium, and the green algae through a completely separate endosymbiosis with a prochlorophyte.
Alas, the story is not that simple. With conflicting evidence from modern DNA analysis, the relationship between red and green algae has re-emerged as a controversial issue (see Graham et al. 2000 for a good summary). For example, a DNA-based analysis (Chen et al. 2005) showed that the several kinds of prochlo- rophytes may not be closely related to each other, nor to the chloroplasts of green algae. It seems also that chlorophyll b can easily appear as a mutant form of chlo- rophyll a. Alternatively, “green genes” may have evolved once and moved to other organisms through horizontal gene transfer. In any case, it seems likely that green photosynthetic pigments evolved independently several times as an adaptation to the wavelengths of light found in shallow surface waters.
In 2007, Adrian Reyes-Prieto and colleagues summarized the available compara- tive DNA studies for both chloroplast and nuclear genomes, and concluded that red, green, and brown chloroplasts were more closely related to one another than to any known cyanobacterium, and therefore probably had a common ancestor. They also concluded that there was most likely a single ancestral host cell that captured the cya- nobacterium that became the grandfather of all chloroplasts. Those red chloroplasts evolved new color forms as their host cells adapted to different light environments.
Soon after, however, reports by Kim and Graham (2008) revealed that the rela- tionships of red, green, and other algae are more complex than earlier thought. The red and green algae may have descended from a single host cell undergoing endosymbiosis with a single cyanobacterium or with several related cyanobac- teria, or different host cells may have undergone endosymbiosis with different cyanobacteria. Nozaki et al. (2009) also concluded that red and green algae had a common origin but that the ancestors of brown algae were more closely related to green algae, but had lost their original chloroplasts before acquiring new ones through kleptoplasty. In other words, the issues of relationship among algae are far from resolved.