Red Algal and Other Extremophiles
Figure 1: Photomicrograph of Galdieria sulphuraria strain YNP5572. Pictured at 1000× total magnification (TM). Brightfield. Their color warrants an expla- nation: members of the “red” algae, the Cyanidiophyceae are actually green. Before more modern methods of sequencing and classification,
this made
researchers think that they were other types of algae, like green or glaucophyte. They actually got their green color because, as they moved to higher light envi- ronments, they evolved to no longer need the red pigment phycoerythrin, and thus they no longer reflect red light [40].
of the characteristics that make the cells of eukaryotes more complex than prokaryotes. Moreover, there are both asexually and sexually reproducing eukaryotes; both strategies are responsible for transferring genetic information to the next generation vertically (Figure 6a). Traditionally, eukaryotes were thought to rely heavily on
the processes of gene duplication and mutation that allow traits to be selected for by the forces of evolution. Novel adaptation of these traits allowed organisms to evolve [5]. Tese are slow processes and because of this, eukaryotes are not able to acquire new genes as quickly as prokaryotes. Based on this paradigm, it would be an extremely lengthy and unlikely process for an organism to acquire the genetic properties necessary to go
from a regular aquatic environment to one where it would need genes for temperature tolerance, salt tolerance, acid tolerance, light fluctuations, and xenobiotic/heavy metal detoxification, as in the case of the Cyanidiophyceae. However, as the field of genomics has progressed and more work studying the development of novel genes has been published, it is now known that gene and genome duplications are only two of many diverse pathways that lead to the development of novel genes [6]. One such pathway that is now the topic of many studies in the mainstream scientific literature is HGT. Because eukaryotic cells are more complex than prokaryotic cells, it was previously thought that they generally could not acquire new genes horizontally like bacteria, and when they did it was extremely uncommon and mostly in the form of endosymbiotic gene transfer (discussed below) [7]. However, over the past twenty years or so, genome sequencing projects have led to an accumulation of examples of HGT in eukaryotes [8], including work from our collaborators and in our lab that has suggested that these HGTs confer novel adaptive traits to their recipient genomes [9–12]. Just like in prokaryotes, HGT is proving to be an important fundamental mechanism of evolution, and especially in microbial eukaryotes like the Cyanidiophyceae that can more easily pass on HGTs due to their unicellular nature. Tat is, each cell is a germ cell and can propagate novel HGTs in the population via mitosis. Tere are a variety of mechanisms whereby a eukaryote
can acquire new genes horizontally (Figure 6b). One method is the uptake of exogenous genetic material from the environment (analogous to transformation in prokaryotes; Figure 6b(i)), whereby bits of nucleic acids with properties that allow them to insert themselves into the nuclear DNA of a eukaryotic cell, are taken up and incorporated. Another is via phagocytosis, the process by which one cell or unicellular organism engulfs a substance (which can be another cell/organism) and brings it into the cell where it is typically digested. It is possible for gene transfer to occur during this process where genetic material is transferred from the consumed cell/organism to the nucleus of the phagocytic cell [13] (Figure 6b(ii)). A third method is for a eukaryotic cell to be infected by a virus that injects DNA or RNA into the host cell in order to make more viruses, and during this process some of the viral genetic material gets incorporated into the host genome (Figure 6b(iii)). Another method, similar to exogenous uptake, is when some eukaryotic organisms (for example, some red algae; Porphyrid- ium purpureum [14,15]) actually have plasmids of their own and can host bacterial plasmids in their nucleus where they can replicate and likely incorporate plasmid DNA into other DNA-bearing entities
like organ-
Figure 2: Schematic bifurcating phylogenetic tree showing three Cyanidiophyceaen red algae species and some of the extremophilic traits they have acquired. Topology based on tree from Rossoni et al. 2019 [12]. Outgroup taxa include two other red algae (P. umbilicalis, P. purpureum) and two green algae (C. reinhardtii, O. tauri).
2020 November •
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elles, the red algal nuclear DNA, or genomes of other organisms (Figure 6b(iv)). Lastly, when a cell ingests another cell but does not digest it, it can become an endosymbiont and, a
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