Red Algae is a strange group more or less confined to cool waters having a temperature near 50C. They have a very poor number of genes required for a multicellular organism and this is explained as the reason why they never colonized land. The genome of red algae like Cyanidioschyzon merolae has 4775 genes; Porphyridium purpureum has 8355 genes and Chondrus crispus has 9,606 genes (Collen et al, 2013) and while a single-celled green alga Chlamydomonas reinhardtii has 14,516 genes and Arabidopsis thaliana has 27,416 genes. An unusually high number of genes in these algae are unique in having no known counterparts in other organisms. This indicates that these plants retained only the genes for bare essentials by stripping down its genome and eliminating genes that perform redundant functions in other organisms. Chondrus has 82 genes for making ribosomes(compared to 349 in the green plant Arabidopsis), only one light-sensing protein: a cryptochrome while other plants has several. C. crispus also has very few small introns – sections of RNA inside genes that get edited out during the production of proteins. The few it has are small and probably serve vital regulatory functions, increasing or decreasing protein production as conditions warrant while the rest of the eukaryotes – all Earthly life except for bacteria and archaea -- have introns galore (about 139,418 in the human genome).
Reasons for the few genes
The hot acidic water in which earlier members like Cyanidioschyzon merolae and Galdieria suphuraria grew, might be the reason for this poor genetic diversity and their restriction to Ocean. It is proposed that a genome squeeze induced by such an extreme environment resulting in shrinking the genome drastically may also explain why Chondrus has an unusually high number of genes with no known counterparts in other organisms. It is also proposed that sometime soon after red algae evolved they adapted to an environment that exerted strong selective pressure for small body size, the ability to get by on very little food, or perhaps both. The consequence was the drastic reduction in genome size, pruning introns, non-coding DNA, and superfluous genes from the genome. Without a large and redundant genome from which evolution could play and easily create new genes, they lacked the genetic potential necessary to leave the ocean for the brave new world of land. An example of epigenetics!!!The lesser number of genes in the group may be responsible for restrictive distribution of this group. The known members of this group are 5000–6000.
The great morphological and ultrastructural diversity
This group shows great morphological and ultrastructural diversity in having are at least three patterns of Golgi association, three methods by which cells achieve multinuclearity in development, and several methods of establishing intercellular connections through cellular fusions and pit-plug formation, five distinct patterns of mitosis, differing in details of microtubule number and mode and location of formation, number and disposition of gaps in the nuclear envelope, shape and size of the nuclear-associated organelles, and three patterns of cytokinesis, three distinct patterns by which the various reproductive structures and an alternation of generations that may have three generations rather than two.
Similarity to Cyanobacteria
They are very similar to Cyanobacteria in lacking flagella in reproductive cells, endoplasmic reticulum, chloroplast lamellae separated from each other (not stacked in stroma) and presence of pigments like phycocyanin and Phycoerythrin.; photosynthetic reserve stored in the cytoplasm; plastids with non-aggregated thylakoids; accessory pigments including phycoerythrin and phycocyanin in phycobilisomes on the thylakoid surfaces.
The unique characters of this group include red algal chloroplast (containing phycobilisomes, plastoglobuli, genophores, ribosomes, and pyrenoids in some species), floridean starch ( a glucose polymer containing branched polysaccharides like amylopectin as the main storage food which is stored in granules, which are localized in the cell cytoplasm – outside of the chloroplast) and a variety of photosynthetic pigments, predominantly chlorophyll a, with accessory pigments found in the phycobiliproteins (allophycocyanin, phycocyanin and phycoerythrin), as well as different carotenoids (violaxanthin, antheraxanthin, lutein, zeaxanthin, β-cryptoxanthin, α-carotene and β-carotene). Red algae also lack centrioles, the cellular microstructures that help orchestrate cell division
Why they migrated to deep oceans
As the surface of water bodies got filled with photosynthetic Eukaryotes, there occurred a struggle to get sunlight. Therefore the red algae dived deep in oceans and survived because of the pigment phycoerythrin which absorbs the blue light filtering through the upper layers of water. Though the primitive members thrived in hot acidic waters, the later evolved forms thrived in deep waters. Red algae originated along with green algae and Glaucophytes with the primary endosymbioses.. The earliest fossils date back to 2 billion years before present and are superficially similar to the extant taxa of the Porphyridiales, Bangiophyceae. Red algae diverged from other eukaryotes at an early stage and a secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other microbial eukaryotes.
Reference
1. Collén, J et al (2013) "Genome structure and metabolic features in the red seaweed Chondrus crispus shed light on evolution of the Archaeplastida." Proceedings of the National Academy of Sciences 110, no. 13 (2013): 5247-5252.
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