Red Algal and Other Extremophiles
Figure 7: Composite of six red algae genera representing many major groups within the Rhodyphyta. (a.) Porphyridium sp. (b.) Batrachospermum sp. (c.) Polysi- phonia sp. (d.) Agardhiella sp. (outer cortex view) (e.) Rhodymenia sp. (f.) Galdieria sulphuraria.
Learning More about HGT in Eukaryotes Although the Cyanidiophyceae have HGTs as approxi-
mately 1% of their gene inventory and this makes them good models for studying this process, they are certainly not unique. Many other extremophilic eukaryotes have HGTs, ranging from protists like ciliates and dinoflagellates [29,30] to desiccation and radiation-tolerant animals like rotifers [31]. One example is ice-binding proteins in Arctic/Antarc- tic diatoms that were acquired from prokaryotes or viruses that survive cold temperatures, giving diatoms the ability to more easily adapt to polar climates [32]. When consider- ing extremophiles, traits related to extremophily are rather conspicuous, allowing us to more easily identify the impact of more noticeable HGT events. However, studies in recent years have shown that HGTs are in fact ubiquitous across the eukaryotic tree of life and that they also play equally impor- tant roles in lifestyle changes not related to extremophily. One example is mesophilic diatoms that have acquired genes for ferritin, a protein that allows them to sequester iron, which is typically a limiting nutrient in marine environ- ments [33]. HGTs have also been implicated in the transition of some organisms from free-living to parasitic lifestyles and in the colonization of land by plants [34,35,36]. Moreover, HGTs don’t always have to be from prokaryotes; there are many cases of eukaryote-to-prokaryote and eukaryote-to- eukaryote HGT as well. One example is the oomycetes, a group of protists that live as plant parasites and thus behave similarly to fungal parasites of plants. It turns out that across the oomycete species surveyed, all have acquired a variety of HGTs from fungi, and this has not only allowed them to become more efficient plant pathogens, but they are so
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convergently similar to their fungal gene donors that they were mistakenly classified as fungi when originally discov- ered [37,38]. While these examples provide exciting insights, they
are not comprehensive enough for us to come to robust uni- versal conclusions. A majority of eukaryotic diversity is still largely unexplored, especially in fungi and protists. As more genomes are sequenced and cryptic and rare species are iden- tified, the eukaryotic tree of life can be resolved with a higher degree of precision. Molecular methods of characterization are one important way to go about this work, but they come with biases that may hinder developing a more exhaustive survey of eukaryotes, which is why it is also important to complement these types of analyses with conventional light microscopy [39], a practice that has dwindled in this field due to time constraints and reliance on molecular methods. Microbial eukaryotes like protists are vastly understudied, under-classified, and overlooked in most textbooks and biol- ogy classes. One of the best ways to learn about them will always be to simply observe them. Using these skills to cata- logue these microorganisms is important for understanding broad patterns in eukaryotic ecology and genomics, as well as finding new models for interesting processes like HGT integration. Novel species can be identified and collected in the hot springs of Yellowstone National Park like the Cyanid- iophyceae once were, or they can be found in a local pond or soil sample. Maybe an unknown alga in a pond has over- come a unique set of desiccation challenges and the result- ing adaptations are evident in their genome architecture, or maybe a diatom lineage has begun to diverge aſter many gen- erations living amongst pollution and a unique assemblage of
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