Red Algal and Other Extremophiles
Figure 3: Examples of plastids across Archaeplastida lineages. Plastids in each example are denoted by stars. a. Surface view of the cortex of red alga Agardhiella sp. viewed at 1000× TM and then magnified further digitally after initial micrography. Here, the plastids are visible as elongated and amorphous light red spots along the outer borders of the cells. b. Micrographs showing plastids in other major Archaeplastida groups. From left to right: leaf of Elodea sp. (an aquatic tracheophyte) magnified to 400× TM showing many plastids within each cell; view of a single cell of Spirogyra sp., a filamentous green alga (charophyte) at 1000× TM showing a spiral arrangement of chloroplasts; a small colony of glaucophyte algae (ID unconfirmed but possibly a Glaucocystis sp.) at 400× TM with cells containing plastids (“cyanelles”).
few times in evolutionary history, endosymbionts have become bona fide energy-producing organelles like mitochondria and plastids [see 7,16,17]. Once these cells become fixtures within the host cell, they undergo massive genome reduction, and many of their genes are transferred to the host nucleus in a process called endosymbiotic gene transfer (EGT). In order to make sure that despite losing many genes, this cell-turned- organelle can still function in a useful way, the host nucleus must contain an adequate repertoire of genes to complement the organelle’s remaining functions [18,19] (Figure 6b(v)). Fur- thermore, because certain pathways must be conserved for the organelle to remain functional, this might not be possible between the organellar genome and the host genome alone fol- lowing such massive genome reduction and EGT. To remedy this problem, it has been shown that genes contemporaneously acquired horizontally from other sources can compensate for missing genes from the other two sources. It is hypothesized that plastids would not be what they are currently without gene complementation via HGTs from Chlamydial cells that were incorporated into the ancestral plastid-bearing organism’s nuclear DNA [20].
Cyanidiophyceae as Models for Studying HGT It is by HGT that Cyanidioschyzon spp. and Galdieria
spp. (Figure 1), the two genera of Cyanidiophyceae that have been sequenced and analyzed, have acquired new genes with novel functions,
likely from many different extremophile
Figure 4: General differences between prokaryotes and eukaryotes visualized by a representative organism from each domain of life. (a.) Prokaryote: filaments (each containing many cells) of the cyanobacterium Oscillatoria sp. (b.) Eukaryote: a desmid (green alga) Closterium sp. While superficially both of these organisms may look somewhat similar, they each only have the cellular components consistent with the domain that they are part of. For example, like all eukaryotes, the desmid cell contains a membrane-bound nucleus as well as organelles, a cell wall that does not contain peptidoglycan (not all eukaryotes have cell walls), linear DNA, and a cytoskeleton, among other features. Like all prokaryotes, the cyanobacterium has circular DNA concentrated in a nucleoid region rather than a nucleus and a cell wall that contains peptidoglycan. As shown in (c.), both eukaryotes and prokaryotes can share some cellular features such as ribosomes, flagella (neither pictured organ- ism has this), cytoplasm, and a plasma membrane, although their structures may deviate. Interestingly, both organisms in this figure share their primary metabolic strategy, that is, they are photosynthetic, however, as a eukaryote, the machinery for photosynthesis is located within the membrane-bound chloroplast, in contrast to cytoplasmic membranes in prokaryotes. Photosynthesis graphic used with permission from KEGG [41].
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