Red Algal Extremophiles: Novel Genes and Paradigms
Julia Van Etten Graduate Program in Ecology and Evolution, Rutgers University, New Brunswick, NJ 08901
julia.vanetten@
rutgers.edu
Abstract: The Cyanidiophyceae are a class of unicellular red algae that live in environments at the extremes of temperature, pH, salt, and heavy metal concentrations. These photosynthetic organisms have been able to occupy these rather unusual niches by acquiring a variety of genes from extremophilic prokaryotes and/or viruses via the process of horizontal gene transfer (HGT). Elucidating how the proteins encoded by these genes integrate into existing metabolic systems is crucial to understanding and evaluating the significance of HGT in eukaryotes and its role in conferring adaptive traits. This article addresses the fundamentals of HGT in model red algae, its function as a driver of evolution across the web of life with a focus on eukaryotes, and summarizes ongoing work and the future directions of this field.
Keywords: red algae, eukaryotes, horizontal gene transfer, adaptation, extremophiles
Introduction Successful adaptation to a new or changing environment
is the hallmark of evolutionary success for any organism. Whether that means corals ultimately adapting to prolonged heat stress due to climate change, plants occupying new urban niches, or changes to the human population following a global pandemic, stressful environmental conditions drive evolution by selecting for traits encoded by the genes that make up genomes. Most organisms cannot cope with every new stressor; aſter all, 99.9% of species that have ever lived are now extinct [1]. So how would a group of red algae that were previously occupying mesophilic habitats (moderate, such as oceans and lakes) evolve to thrive in some of the most extreme and inhospitable environments on Earth? Te Cyanidiophyceae (example pictured in Figure 1) are
unicellular red algae found in aquatic habitats worldwide. What makes them different from other algae and most other organisms on the planet is that some species are able to survive and reproduce under a myriad of extreme environmental conditions. Tese include high temperatures up to 56°C (∼133°F), high and low light, pH that approaches 0, and high concentrations of salt and toxic chemicals like mercury and arsenic [2]. Tis combination of factors allows these organisms to qualify as extremophiles, and because they live in many different kinds of extremes, they are actually termed “polyextremophiles.” It is important to note most other red algae (Rhodophytes) live in mesophilic conditions and that the common ancestor of this clade was likely a mesophile as well [3] (Figure 2). Tis means that the polyextremophilic members of the Cyanidiophyceae acquired the ability to thrive under extreme conditions at some point in their evolutionary history, that is, they were not ancestral traits of all red algae. Because this
lifestyle strategy was advantageous to their
survival, it became fixed within populations, allowing them to be the only photosynthetic organelle (plastid)-bearing organisms to live in such volatile conditions (Figure 3). But
28 doi:10.1017/S1551929520001534
how is this possible? And why were these algae able to adapt to extremes while others have not? Both genomic and microscopy techniques are important tools in analyzing genetic diversity and the adaptations organisms have made to living in extreme environments.
Horizontal Gene Transfer Te first
important concept to keep in mind is the
distinction between prokaryotes and eukaryotes. Prokaryotes are tiny, single-celled or filamentous organisms that lack a nucleus (Figure 4). Tey have rigid cell walls that protect them from the outside world, and they obtain their energy from light, by breaking down inorganic molecules in their environment, or by consuming organic molecules necessary for survival. Prokaryotes include all bacteria and archaea on Earth. Tese organisms reproduce asexually by splitting into two identical cells, a process called binary fission. Tey can also exchange genes between one another in a process called conjugation, where they swap small rings of DNA (called plasmids) with each other, or they can receive new bits of genetic information from viruses (transduction), or from the external environment (transformation). When they reproduce asexually, they pass their genes on to the next generation, from parent to offspring, and their offspring can then reproduce and continue this pattern. Tis process is vertical (Figure 5a). However, when they receive foreign genetic information during their lifetime, they can ultimately pass on these new genes not present in their ancestors. Tis process is called horizontal gene transfer (HGT) (Figure 5b) and is the most common way prokaryotes add genetic diversity and new, oſten adaptive traits to their populations [4]. In contrast, eukaryotic cells are quite different. Although
some are single-celled or filamentous like most protists (including algae), many, like most plants, fungi, and animals, are multicellular. What distinguishes eukaryotes from prokaryotes is that each of their cells contains a nucleus where most or all of their DNA is enclosed by a membrane (Figure 4). Tis, and the fact that DNA transcription occurs in the protected nucleus, whereas the decoding of mRNA to create proteins occurs in the cytoplasm, makes it more difficult for eukaryotes to receive genetic material from external sources. Tey also all (or nearly all) have membrane-bound organelles like mitochondria, which allow them to take in organic molecules and convert them to energy. Some eukaryotes, like plants and algae, also have plastids, which allow them to convert light energy into chemical energy via photosynthesis. Mitochondria and plastids were once free-living prokaryotes billions of years ago before they were taken in by a host eukaryote and permanently maintained via the processes of endosymbiosis and organellogenesis. Tese features are some
www.microscopy-today.com • 2020 November
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