Chapter: Protists and the Origin of Eukarya

Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy. Instead, it is "better regarded as a loose grouping of 30 or 40 disparate phyla with diverse combinations of trophic modes, mechanisms of motility, cell coverings and life cycles.The protists do not have much in common besides a relatively simple organization—either they are unicellular, or they are multicellular without specialized tissues. This simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants. Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protists are responsible for a range of serious human diseases, such as malaria and sleeping sickness. Eventhough they are extremely abundant they have relatively low species diversity, occupying only about 10% of all Eukaryotes.

Unlike Bacteria and Archaea, protists are not monophyletic. They are paraphyleticGroups that do include all the descendants of the most recent common ancestor are said to be monophyletic. A paraphyletic group is a monophyletic group from which one or more of the clades is excluded to form a separate group. In other words, a group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants.

Currently, the term protist is used to refer to unicellular eukaryotes that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues. These terms are not used in current taxonomy, and are retained only as convenient ways to refer to these organisms. However, they do not accurately represent phylogenetic relationships.The taxonomy of protists is still changing. Newer classifications attempt to present monophyletic groups based on structure, biochemistry, and genetics. Because the protists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. So, in other words, protists shouldn’t really be thought of as a single group, rather they are several different groups that share a specific physical trait (unicellularity).

Protists and Humans

Like bacteria, protists can have detrimental effects on human. In 1845, a famine struck Ireland with a vengeance. It was a period of mass starvation, disease and emigration between 1845 and 1852. During the famine approximately 1 million people died and a million more emigrated from Ireland, causing the island's population to fall by between 20% and 25%.The cause of the famine was a potato disease commonly known as potato blight.[5] Although blight ravaged potato crops throughout Europe during the 1840s, the impact and human cost in Ireland – where one-third of the population was entirely dependent on the potato for food – was exacerbated by a host of political, social and economic factors.

The potato blight was caused, not by a bacteria or a fungus but by a protist (Phytophthora infestans). Protists are also responsible for inflicting heat flashes followed by extreme chills (and even death) in the tropical jungles across the world. This protist is Malaria. Malaria has a two stage life cycle. Gametes get picked up by a mosquito and are fertilized. And when a mosquito inserts its mouthparts into another animal, the protist find its way to the liver cells and reproduce via meiosis. They explode out of the liver cell infecting the blood cells, which is picked up by a mosquito.

Algal Blooms

Algal blooms, also known as red tides, are also caused by ocean-dwelling protists. These protists are known as dinoflagellates. These protists reproduce like mad in the presence of natural fluctuations in chemicals in the ocean. As a by product of the life cycle, they produce chemicals that are toxins to fish and shellfish. When those shellfish (like oysters) are harvested, they retain those chemicals and can poison humans.

Aquatic Food Chains

Global warming is real. And everyone is concerned about the deforestation of the world’s tropical rain forest. And these are very important carbon fixators. However, the lungs of the Earth are microscopic and rarely thought of. They are phytoplankton, and they fix over half of the world’s carbon. And just like plants on land, they form the base of the food chain in the world’s ocean. Phytoplankton are protists that float on the top layer of the ocean and convert light energy from the sun into chemical energy via photosynthesis. This provides the energy for almost all of the oceans life forms: from unicellular herbivores to whales to the great white shark.

Protists and Climate Change

In the global carbon cycle, phytoplankton are the most important factor. They are the primary carbon sink, converting carbon dioxide into sugar molecules, via photosynthesis. In other words, the more carbon dioxide that exists in the atmosphere, the more carbon that is available for phytoplankton to convert into sugar. So in this way phytoplankton populations are likely increasing and helping to mediate global warming. However, these little organisms can only fix so much of the problem. They are not able to keep up with the skyrocketing carbon dioxide levels.

This is a global map of photosynthesis of the Earth. As you can see, the highest concentrations of photosynthesis in the oceans is actually in the Arctic oceans, followed by the a band around the equator. Also, there is relatively low abundance of phytoplankton in the middle of the oceans. These patterns are caused from global oceanic circulation patters. These circulations cause major upwellings of nutrients at the poles causing huge blooms of phytoplankton in these regions. This is why there is such a robust fishery in Alaska even though it is blood cold. It is all about the nutrients.

Diversification of Protistsa

Even though protists are paraphyletic, meaning that they don’t all have synapomorphic characteristics that bind them together, what they do have in common is that they are eukaryotic. This means were more than likely unicellular, they had a nucleus, mitochondria, and lacked a cell wall. Let’s see how these likely came to be.

Origin of the nucleus

The leading hypothesis to explain the origin of the nuclear envelope is based on the infoldings of the plasma membrane. It is thought that the cellular membrane began to fold into itself, and at some point separated from the outer cell membrane and came to envelope the chromosomes. Two lines of evidence support this hypothesis. Infoldings of the plasma membrane occur in some bacteria today. And second, the nuclear envelope and ER of contemporary eukaryotes are continuous. It is thought that the major advantage of the development of the nucleus was the separation of translation and transcription. In bacteria, translation and transcription can occur simultaneously. This allowed eukaryotes a specific way to differentiate gene expression.

 Origin of the Mitochondrion

It is currently thought that the mitochondrion was taken hostage by an early eukaryotic cell over 2 billion years ago. This theory is known as endosymbiosis, which is when an organism of one species lives inside an organism of another species.

The  current theory holds that the earliest eukaryotic cell performed glycolysis in order to generate ATP. This process is known as fermentation and only produces a net of 2 ATP. It is thought that a single unicellular eukaryotic cell that had a nucleus engulfed a bacterium (that has come to be known as the mitochondrion). And instead of digesting for food, the eukaryotic cell retained it. This proved to be one of the most important interactions for the evolution of life on Earth. Why? That eukaryotic cell could take one molecule of ATP and net 32 ATP. They could make 16 times more energy with the same molecule. As good as the Prius is, it will never be 32 times more efficient than my 1969 Camaro with a 454 Big Block with nitrous injection.

So what did the mitochondrion get from this relationship? The mitochondrion got 2 things. First, it got protection. Kind of like the mafia. It’s gotta pay its fair share to the boss; but you don’t have to worry about anybody bustin’ your balls. It also got free access to pyruvate (which is what it needs to produce the energy). No longer did the mitochondrion have to look for it in its environment. It is fed a constant supply of it. However, what is extraordinarily interesting is that this happened once….in a single cell. And it was so successful that all protists, plants, fungi and animals on Earth stemmed from this single cell. That is pretty awesome to think about.

So where is the evidence? There is actually quite a bit of evidence to support this idea. Mitochondria are TINY. In fact, they are exactly the same size as the average bacterium. The strongest evidence is that mitochondria have their own genome (their own DNA). And in fact, if you sequence the DNA of the mitochondrion and compare it with taxa from the entire tree of life, the DNA sequence is most closely related to a bacteria (not a eukaryote). With their genome, they can manufacture their own proteins. And they have double membranes. It is thought that when the original eukaryote engulfed the mitochondrion, the membrane of the original eukaryote wrapped up the mitochondrion (kind of like a bag inside of another bag. Only one other organelle does that. And it is thought that this organelle was also consumed by a descendent of this evolutionary powerhouse of a eukaryote. And that organelle was the chloroplast.

 Origin of the Chloroplast

Photosynthesis originated in bacteria. Then the spawn of that crafty eukaryote that engulfed the mitochondrion outdid itself by repeating the same process with the perfect complement to the mitochondrion, the chloroplast. However, there is one difference. If you look closely at a chloroplast, you will see that it has not two membranes, but four. The only logical explanation is that another eukaryotic cell engulfed a cyanobacterium, but didn’t consume it. In other words, it held the cyanobacterium hostage and made it make sugars for it. Then our original eukaryotic hero came across and engulfed that eukaryote (yet didn’t consume it).  This produced a super organism. A single cell that could produce carbon containing molecules (for example, sugar) and could break it down with extraordinary efficiency. And plants were set to take over the world.

How do Protists eat?

One way protists quote-unquote eat is known as phagocytosis. In this process, protists wrap their bodies around a prey, release digestive juices. And break down thier prey into the molecules with which they use to go through cellular respiration. Protists most commonly eat through absorption. This is a form of consumption where nutrients are taken directly from their environment. This way of feeding is ubiquitous in decomposers and parasites.