Red Algal and Other Extremophiles


prokaryote donors that live in the same environment, or from viruses. Previous work in our lab (the Bhattacharya lab at Rutgers University) and with our collaborators has confirmed this bioinformatically through orthogroup analysis and phylogenomic methods [11,12], showing that these putative HGTs found across various species and strains of these algae are more similar to genes found in prokaryotes than in the algal web of life. My current work, funded by a NASA FINESST grant in Planetary Science, is to validate these findings functionally, by testing what these genes do in the extremophile Cyanidiophyceae and how their encoded functions have been incorporated into existing metabolic networks. Most


genes that are horizontally transferred to a


new genome are quickly lost. Tese are transient “failed experiments” of evolution that we will never know about. In order to become a universal feature of the species (fixed), they must outlast the other fundamental forces of evolution like genetic driſt and natural selection that would lead to the removal of the new gene from the population. One way for an HGT-derived protein to overcome these obstacles is for it to occupy a position in a pre-existing metabolic pathway that allows it to be functionally integrated into the new genome. Once there, if its presence increases fitness of the organism, the foreign gene may subsequently undergo duplications that allow it to enhance its functional benefit even further. Tere are examples of this in many eukaryotic organisms including the green alga Ulva mutabilis, the gastrointestinal parasite Blastocystis sp., and beetles [21,22,23]. Our group has already shown that such HGTs exist, making up approximately 1% of cyanidiophyceaen genomes, and that they appear to provide adaptive advantages by conferring extremophilic traits to these organisms, which act as drivers of evolution, and allow them to occupy such hostile niches. Te next step, which is currently underway, is to test this hypothesis by doing experiments to measure expression levels of genes (including HGTs) across a variety of physical and chemical conditions and then modeling the dynamics of the metabolic networks that the HGT-derived proteins have become a part of to show how they function in congruence with native proteins. Some of the HGTs acquired by members of this lineage


make these algae rather unusual. For example, algae are photosynthetic; that is, they have a plastid and obtain energy primarily from light. But interestingly, all strains of Galdieria that we grow in our lab can live in complete darkness, which doesn’t seem to make sense! Tat would be like putting a houseplant in a totally dark room indefinitely and expecting it to grow. Galdieria can do this because the ancestor of this lineage had acquired genes for heterotrophy (consuming food, not making its own) that allowed the organism to feed on carbon sources like glucose and glycerol when light is not available [12]. Sugar uptake must have become advantageous in the population that acquired the genes for it; the algal cells that could take up fixed carbon could also produce energy in both the presence and absence of light compared to other cells that could only generate energy when light was available. Tis increased the fitness of the heterotrophic cells, which would have led to selection of these individuals and, ultimately, for the HGTs that conferred this trait to reach fixation in the Galdieria population.


32


Algae on Earth and Algae in Space? Aside from their importance to our ongoing research, the


Cyanidiophyceae are fascinating study systems for the insights they provide to fields like ecology and astrobiology. Rhodophyta (the red algal clade) is among the earliest branching lineages (it diverged ca. 1.4 Ga) of organisms in Archaeplastida (the superphylum that contains all primary plastid-bearing organ- isms except for Paulinella sp., that is, algae and land plants), and the Cyanidiophyceae represent the first major split within Rho- dophyta (Figure 7). Studying such organisms provides insights into ancient traits and physiologies. As primary producers, algae (“algae” here refers to all “phytoplankton” which include organisms with secondary and tertiary plastids) are important ecologically, and along with Cyanobacteria, they produce nearly one-half of the oxygen present in the atmosphere (land plants produce the other ∼54%) [24] and serve as the food source for organisms at higher trophic levels. Atmospheric oxygen likely played a causative role in the evolution of multicellular tissue- bearing organisms like Metazoans (animals) whose radia- tion has contributed to the vast biodiversity on Earth today [25]. Furthermore, large blooms of algae can lead to anoxia in aquatic environments and result in cascading effects to food webs that kill many organisms and cause economic and some- times medical harm to humans [26]. Algae also have important applications in bioremediation and biofuel production, there- fore, understanding their evolution and physiology has broad impacts on many human endeavors that are oſten overlooked by non-phycologists [27]. Te ability to convert arsenic and mercury to less toxic


forms make Galdieria and Cyanidioschyzon good bioremedi- ators, and this allows them to survive in geothermal environ- ments with very high concentrations of these elements. Some well-studied examples of such locations are the hot springs of Yellowstone National Park and Soos National Park in the Czech Republic. Not only do these habitats provide the kinds of stresses that facilitated the transition to polyextremophily in these algae and provide a niche where these organisms can thrive without a lot of competition, they also resemble condi- tions of the early Earth as well as the volatile conditions that may exist on exoplanets elsewhere in the universe. Currently lacking the capability to explore such locales, it helps to be able to study organisms on Earth that are thriving in such conditions and to use these habitats as a proxy for how life has thrived in the past and may live elsewhere. Tis allows us, along with our collaborators at NASA, to gain a firsthand understanding of some of the biochemical and molecular processes associated with the biological limits of life as we know it. Although it would be astronomically unlikely for algae to be found somewhere else in the universe, it is less unlikely that some sort of nucleic acid-based lifeform would exist [28]. With nucleic acids like DNA and RNA come pro- teins, and proteins form the basis of metabolic networks. Understanding how genes are exchanged between organisms and how novel genes arise and trigger rapid adaptation adds to our fundamental knowledge of genomics and evolution, as well as the environmental limits of life that we can study first- hand on Earth. Tis kind of work will inform the search for habitable worlds elsewhere and the study of extraterrestrial life if it is found in the future.


www.microscopy-today.com • 2020 November


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