The kingdom Protista comprises organisms which are unicellular or unicellular-colonial and which form no tissues. ~ Robert Whittaker in 1969
The mishmash multiformity of life is well exemplified by protists, which represent a failure of classification by dint of diversity. Protists are a catchall category of eukaryotic microbes. Most are unicellular, typically defined by what they are not: fungus, plant, or animal; though their similarities to these other life forms are striking. Among the protists are algae, plankton, and protozoa.
Protists are a disparate grouping of 30–40 phyla, with little in common except relative simplicity, notably their lacking tissue differentiation. Owing to their oddities and variety, protist classification has been an ongoing controversy.
Trophically, various protists may be photosynthetic autotrophs or heterotrophic. Protists are either plant-like, as with algae, or animal-like, such as protozoa.
Euglena go both ways. They have chloroplasts for photosynthesis, but will hunt prey, typically smaller protists, when they can’t get enough light energy.
Individually, protists are microbial; such as diatoms, which are an algal form with 110,000 species. Numerous protists, including some diatoms, form visible colonies.
All protists can reproduce asexually. Unicellular protists do so by mitosis. Multicellular protists often produce spores.
Many protists, though not all, can also reproduce sexually. Sex affords faster adaptability by diversifying genotypes, increasing the probability that at least some of a population will survive the stress of environmental adversity. Reproductivity flexibility itself confers adaptability to live in different habitats.
Brown kelp, a common large seaweed, produces spores (a sporophyte). A spore develops into a microscopic life that attaches to submerged surfaces, often in deep, dark waters. These microscopic bodies produce male and female gametes (a gametophyte). Mating of male and female gametes results in a zygote that grows into a kelp that lives near the surface: a photosynthesizing brown alga. Hence brown kelp has a dual life cycle involving both asexual and sexual reproduction: alternation of generations.
One of the protozoan parasites that causes malaria, Plasmodium falciparum, has a complex life cycle involving multiple forms, some reproducing asexually, other sexually. The genetic implications of sexual versus asexual reproduction in these protozoa are not yet understood.
Algae (singular: alga) are a vastly diverse group of simple autotrophs. Undifferentiated tissue is what renders algae as protists rather than plants. Like plants, most algae are photosynthetic.
Algae are categorized by color: green, yellow-green, red, and brown. Algae range from unicellular microbes, such as diatoms, to giant kelp, which may grow to 65 meters. Seaweed – including kelp (a brown algae) – are a colonial form; the largest and most complex marine algae.
Chlamydomonas reinhardtii is a tiny (10 µm) single-cell photosynthetic green alga, found worldwide in soil and fresh water. When carbon dioxide is in short supply for photosynthesis, C. reinhardtii resorts to consuming cellulose. It is the only known vegetative organism capable of herbivory.
Even unicellular organisms evaluate their individual needs. Diatoms adapt their behavior flexibly, moving towards potential mates or food sources depending on how hungry they are for sex or nutrients. ~ German biologist Georg Pohnert
Diatoms live within a unique silica cell wall that makes for a bivalve shell. Because of it, diatoms are an alga with a prosperous afterlife.
140 million years makes for a lot of dead diatoms. While other planktonic biomass produces petroleum in the not-so-great beyond, diatom shells that drop to water’s bottom make for harder stuff: a mildly abrasive, porous, and absorptive diatomaceous earth, which is used in paint and toothpaste, among other applications.
Volvox is a genus of freshwater green algae. Free-swimming cells join together – up to 50,000 strong – to create colonies that are gelatinous hollow spheres formed by collective secretions.
Each cell sends out 6 fine fibrils. The fibrils connect with different neighboring cells, creating a communication network.
Volvox cells coordinate to swim in unison. While single cells have some individuality, they work together for the good of the colony.
The colonial sphere develops cellular specialization, including distinct anterior and posterior poles. Eyespots become more developed in the front (anterior), better allowing a colony to swim toward the light. Encountered encumbrances are sensed and swum around.
Plankton typify the protist’ classification dilemma, in being characterized only as teeny critters that can’t swim against a current.
Bacterioplankton are waterborne bacteria and archaea, rendering the term bacterioplankton an incomplete misnomer. A more inclusive and descriptive term would be saproplankton, because these plankton remineralize organic material.
Phytoplankton practice photosynthesis. Among them are prokaryotes (cyanobacteria) and eukaryotes (diatoms, coccolithophores, and dinoflagellates).
Phytoplankton alter their biochemical composition according to nutrient availability. ~ American biochemist Patrick Martin
Phosphorus is essential to all organisms. It can be extremely scarce in open-ocean surface waters. Phytoplankton compensate by tucking away a reserve of phosphorus when it is available.
Conversely, phytoplankton take various measures to access phosphorus when supply is scant. They reduce their inventory of phosphorus-containing biochemicals, increase their affinity and uptake rate of phosphorus, and produce enzymes to hydrolyze extracellular dissolved organic phosphorus molecules.
Dinoflagellate have 1 to 3 whiptails (flagellates), hence the name. That is about the only commonality among the 2,000 species.
Most dinoflagellates have an odd nucleus, called a dinokaryon, with chromosomes attached to the nuclear membrane. These chromosomes lack histones, and so stay condensed throughout interphase, which is strange.
Once considered a primitive prokaryotic holdover owing to their peculiar practice of genetic management, the chromosomal doings of dinokaryons are instead simply distinctive to other eukaryotic life.
Overall, dinoflagellates cover the food chain range: as autotrophs, heterotrophs (phagotrophs), symbionts (e.g., coral), and parasites.
A popular pastime for social dinoflagellates is to bloom into concentrations of more than a million per milliliter of water. Some produce neurotoxins and kill hapless fish. Others are innocuous. A few are flashy: bioluminescent blooms that blink when disturbed.
Hard times can make a dinoflagellate congregate for protection. 2 cells fuse, entering hibernation after tucking in extra fat, and forming a hard shell, sometimes even spikes.
When prospects return to promising, dormant dinoflagellates break out of their shells, separate, and start life anew. Ah, to be young again, with a new tail at the start of life’s tale.
Cyanobacteria were long considered algae, but these prokaryotes do not fit the profile. Algae are eukaryotes, performing photosynthesis via chloroplast organelles. Cyano-bacteria lack these organelles, conducting their photosynthetic conversions in specialized infolded cytoplasmic membranes (thylakoid membranes). Yet cyanobacterial photosynthesis is like plant photosynthesis. This is unsurprising, in that plant chloroplasts arose from endosymbiotic cyanobacteria living inside early eukaryotic cells.
All plants practice photosynthesis 1 way, with incredible efficiency. In contrast, there are 5 different systems of photosynthesis for prokaryotes: 2 are like plants, albeit somewhat simpler; the other 3 are considerably different, and not very efficient at all. 1 bacterial photosynthetic process produces sulfur as its byproduct, not oxygen.
Zooplankton are protozoa or metazoa that feed on other plankton and telonemia. While protozoa are protists for life, metazoic meroplankton are protist posers: planktonic for only part of their lives, usually the larval stage.
Meroplankton graduate to a nektonic (free-swimming) or benthic (bottom-dwelling) adulthood. Marine worms, crustaceans, sea stars, sea urchins, and most fish get their start as meroplankton.
Plankton derives from the Greek for “drifter.” But meroplankton are not casual drifters. When threatened by a predator, they try to leap out of the way: springing into the air for a jump of up 40 times their length (17 cm), traveling 0.66 meters per second in midair, while somersaulting at up to 125 revolutions per second. Their aerial bulleting is often successful: lowering the odds of becoming a meal for a small predator to just 1.1%.
Protozoa are a diverse group of 50,000 species; one that has grown big for its britches. Once considered protists, some new classification schemes grant protozoa their own kingdom.
Protozoa are among the most ancient eukaryotes; presumably originating through parasitic, then mutualistic, associations of prokaryotes. This follows along the evolutionary lines that led to the eukaryotes of the plant and animal kingdoms.
Trichomonads are single-celled, anaerobic protozoans with multiple flagella (typically 4–6). Trichomonas vaginalis (Tv) is a parasitic trichomonad which causes trichomoniasis (aka trich). Tv is the most common of the 3 trichomonads that infects humans.
Sexually transmitted, Tv resides in the urogenital system, with the vagina its preferred domicile: a warm, moist, inviting place with plentiful nutrition. Plus, men are a convenient carrier to the next vagina.
Once inside, the pear-shaped parasite paddles to an ideal spot on the lining of the vagina or cervix, where it sprawls out, amoeba-like. Tv then helps itself to a banquet of host cells. (Surprisingly, ~70% of women and men do not have symptoms when infected. Symptoms typically begin 5–28 days after exposure. Symptoms include itching, stinky vaginal discharge, burning when urinating, and pain during sex.)
Tv doesn’t work alone. Other microbes living in the vagina, and some inside the parasite itself, get into the fray.
Host immune response can be problematic. Tv kills T and B cells by poisoning them.
Neutrophils cope better. Neutrophils are immune system first-responders that attempt to engulf invaders. The problem in attacking Tv is that the protozoan is larger than a neutrophil. So, 3–6 neutrophils mob Tv and nibble it to death: a process called trogocytosis. Tv typically succumbs with 3–8 bites.
Tv’s preemptive countermove is to nestle into the vagina so that it cannot be surrounded. To do so, Tv peppers vaginal cells with tiny bubbles of proteins and RNA which prime host cells for parasite attachment by altering their surface membrane.
The vagina is home to a bacterial microbiome which is dominated by Lactobacillus in healthy women. The vagina feeds its lactobacilli. In return, the bacteria excrete an acidic substance that prevents many disease-causing microbes from taking hold of host cells.
Tv treats Lactobacillus as a threat: eating them and other protective bacteria while cultivating microbes amenable to mischief, which includes cooperating with Tv.
Tv also hosts its own helpful microbes: species of Mycoplasma bacteria and an assortment of viruses called TVVs (Tv-virus). TVVs assist Tv in sticking to host cells. TVVs and Mycoplasma also magnify inflammation and infection symptoms, thereby sowing confusion in the immune system as to the source of the infection.
Tv thrives when it has friends: showing more energy and growing faster. The mutualism improves nutrient collection, most notably arginine. Immune cells need arginine to make the nitric oxide which kills infectious microbes. By eating the arginine, Tv’s bacterial helpers thwart the immune system by removing the pesticide supply.
Ciliates are well-known protozoa, characterized by a peach fuzz lining of hair-like cilia that let them swim for it, whatever it may be. Ciliates swim wherever water is found: in soils, ponds, lakes, rivers, and oceans.
Other protozoa are not so ambitious. Once settled, sporozoans don’t move. Instead, they lodge inside hosts as parasites.
Cilia are for more than just swimming about. A ciliate sweeps food into its mouth using its cilia. Food is packed into a vacuole which travels the digestive tract, where the contents are broken down via lysosomes until small enough to diffuse through the food vacuole into the cell. Anything left in the vacuole when it reaches the cytoproct (anus) is discharged via exocytosis: directing a secretory vesicle into extracellular space.
Amoeba are a primitive protozoan that can reproduce both sexually and asexually.
While unicellular, amoebae extend their cytoplasm to form a pseudopodium (false foot), thus affording movement.
Amoebas gobble other protists: diatoms and plankton. Amoebae eat with their feet. Chemical stimulation from the amoeba’s food induces psuedopodia to form, enveloping the meal-to-be, while forming a cavity, or vacuole, that acts as a digestive area.
A digestive enzyme secreted into the cavity breaks down the morsel into soluble chemicals that then diffuse into the cytoplasm: phagocytosis. Many heterotrophic protists – phagotrophs – eat by phagocytosis.
Despite looking like blobs, amoebas are careful to maintain their figure. They know how much they have eaten. Though food may be abundant, amoebae don’t overeat.
While the term microbe entwines tiny, not all unicellular organisms are microscopic. Deep in the ocean trenches, on abyssal plains, up to 10.6 km down, live xenophyophores: single-celled protozoans, some 42 known species, that grow to 20 cm in diameter. These are ancient creatures, dating to the Ediacaran (635 – 542 mya).
Xenophyophores are delicate; basically, lumps of viscous fluid – cytoplasm – with numerous nuclei evenly distributed throughout. Their bodily sprawl is contained in a ramose (branched) system of tubes called a granellare, made of an organic cement-like substance.
Xenophyophores are tireless bottom feeders; rooting through sea floor mud for unknown nutrition. But what is known is that what goes in comes out as slime.
This fecal matter, termed stercomes, mixes with their secreted cement to form stercomares: structures which agglutinate around the granellare, further augmented by scavenged minerals and the microscopic remains of other organisms, such as sponges. The shell inside a stercomare is termed a test.
Out of the mud comes a high-rise. Stercomares are a habitat for worms, clams, sea stars, and crustaceans.
By this, xenophyophores are benthic beavers: creating a conducive environment for other species. Besides building stercomares, xenophyophoric foraging stirs the sediment, releasing nutrients that others appreciate.
Xenophyophores prolifically live in all the oceans; indispensable agents for engendering diversity in benthic ecosystems. A single cell can make a difference in the world.
Slime mold refers to protists that reproduce by spores. Slime molds were formerly fungi, but taxonomically slithered into protist territory.
The classification of slime molds remains more than a little slippery. As a group, slime molds are polyphyletic: the last common ancestor of the multitude of existing slime mold species that was not itself a slime mold. That makes slime molds an oddly diverse group.
While slime molds are protists, molds are fungi. There are several similarities, and a few differences. Both produce motile zoospores. Under certain circumstances, both slime molds and molds congregate and coordinate as a singular organism.
Fungal molds grow via hyphae: multicellular filaments. A connected network of hyphae – a mycelium – comprises a single organism. Slime molds, such as plasmodia, are creepy in their own way.
The major difference between the fungus and the protist is in the texture. Mold is dusty, owing to a profusion of spores on the mycelium. Slime molds are instead so watery as to slide into slimy.
Slotting slime molds into the tree of life is not the only slippery thing about them. Slime molds have no identifiable intelligence system, but their behavior is far from brainless.
Slime molds are smarter than they look (which, in of itself, is not much of an accomplishment, considering how they look). Slime molds can solve mazes and anticipate periodic events.
This simple organism has the ability to find the minimum-length solution between 2 points in a labyrinth. The cell is capable of ‘intelligent’ behavior, even in complicated situations in which it is difficult to optimize survival tasks. ~ Japanese biologist Toshiyuki Nakagai et al
A slime mold leaves behind a slime trail behind as it travels, which it can use to tell where it has already been. This helps it navigate to food supplies.
In reaching out for food, slime molds create complex communication networks; as complex and as efficient as human highway systems. A slime mold also recognizes and reacts to trails left by other slime molds.
Slime molds are nutritionally aware. They do best on a diet that is 1/3rd carbs and 2/3rds protein. When given a choice, they consistently choose food that has an optimal balance of nutrients.
Slime molds have long-term memory. They learn patterns and anticipate periodic events. And they pass their knowledge on.
Slime molds exhibit transfer of learned behaviour during cell fusion. ~ French biologist Audrey Dussutour & American biologist David Vogel
Slime molds demonstrate memory and problem-solving without having any identifiable physical structure that supports these abilities. Slime shows that intelligence is all in the mind.
Some solitary amoebae are protozoan, but social amoebae fall into the fold of slime molds. Certain so-called non-social amoeboid protists are also slimes. One is plasmodia.
A plasmodium is something of a supercell (syncytium): a cytoplasmic blob with up to thousands of structures that resemble cell nuclei, albeit of many different genetic origins (a heterokaryon). This happens as the product of plentiful pairwise fusions of amoeboid individuals that form a plasmodium. These are oversexed beasts indeed.
(Plasmodium is a genus of unicellular eukaryotic obligate parasites – same name (except capitalized and italicized), entirely different organism.)
While considered non-social, a plasmodium is actually an aggregation to an extreme of sociality: a single species symbiorg, albeit with extreme genetic diversity. Hence a plasmodium is an organic congregation acting as one. The notion of an individual having a single genome is completely lost on a plasmodium (not that any multicellular organism has a single genome either; that’s just a popular conceptual fiction among geneticists).
Heterokaryosis (diverse genetics) is also common in fungi and lichen. Syncytium (a multinucleate cell) is a normal cell structure for many fungi. Some plants too have syncytial cells.
The more one learns about the lives of these early life forms, the easier it is to understand the classification confusion that biologists face. These little ones share many similarities in different facets of existence.
Plasmodia creep along a forest floor, amoeba-like, feeding on yeasts, bacteria, and decaying vegetation. The creep is caused by coordinated wave movement, typically from the rear of the plasmodium forward.
When food runs low, a plasmodium prepares itself to reproduce. It creeps to a place that is sunny and dry. The top of a log is suitable.
The plasmodium reshapes its slimy self into several sporangia: fruiting bodies that are a stalk with a bulb on top, somewhat resembling a mushroom. The bulb is a protective incubator for the spores stored inside.
A wind blows over a bulb, opening it; releasing the spores to ride the breeze. The spores alight on the ground to await a wetting or are moistened in the air. Life begins anew as an amoeboid with a plasmodial future.
In good times, social amoeba live as single-celled organisms (myxamoebae), feasting on soil bacteria. When food runs short, tens of thousands band together to form a slug-like multicellular cluster, which then slithers away in search of a more bountiful patch of dirt.
During slug time, some of these social amoebae become specialized cells, roving around and vacuuming up invading toxins and unwelcome bacteria; in effect, an immune system.
The cells recognize foreign bacteria using a protein called toll/interleukin-1 receptor A (TirA). TirA is closely related to the protein that animals use in their immune systems to identify bacteria, indicating an evolutionary connection between amoeba and later, larger multicellular life.
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Dictyostelium discoideum (Dicty) is an ordinary soil amoeba that feeds on bacteria. Dicty form biofilms which enhance survival prospects for its members. A Dicty biofilm is commonly, and indelicately, called a slime mold.
Dicty life begins as a spore, released from a mature sorocarp (fruiting body). When conditions are favorable – warm and moist – individual cells (myxamoebae) hatch from their spores.
Myxamoebae are attracted to their prey by the smell of the folic acid which the bacteria secrete as a waste product. Once fat and happy, myxamoebae multiply by mitosis (cell division).
Farming occurs in societies. ~ Dutch evolutionary biologist Duur Aanen
Some Dicty carry around bacteria that they are fond of eating, with which they seed the soil. After the bacteria multiply, the biofilm amoebae selectively harvest the bacteria for food, leaving some the crop for later generations to enjoy.
Dictyostelium farmers suffer a reproductive cost but also gain beneficial capabilities, such as carriage of bacterial food (proto-farming) and defense against competitors. ~ American bacteriologist Susanne DiSalvo
Dicty farming gives the impression that the amoebae are in charge. Instead, bacteria are running the show; but not the ones being farmed for food (e.g., Klebsiella).
Dicty don’t farm until they are infected by Burkholderia bacteria. Via mind control, Burkholderia incite Dicty to take up farming.
The symbiosis benefits all 3 parties. The edible bacteria get dispersed, and at least some survive. Burkholderia also get around by driving their Dicty. By farming, Dicty survive when they might otherwise starve. If times get lean, Burkholderia may decide to dine on their host before moving on. It’s a microbe-eat-microbe world.
Burkholderia are the drivers, benefiting by exploiting new terrain, and sometimes harming their vehicle. ~ Susanne DiSalvo
Other such cultivating creatures include ants, termites, beetles, a salt marsh snail, and some damselfish. All are social species. Unlike Dicty, other farmers practice agriculture without being manipulated to do so.
When food runs low, myxamoebae aggregate to produce a next generation of spores. Starvation initiates latent metabolic pathways which produce glycoproteins and adenylyl cyclase: chemicals that engender colony formation.
Glycoproteins foster cell-to-cell adhesion. Adenylyl cyclase catalyzes conversion of ATP into cyclic adenosine monophosphate (cAMP). cAMP is secreted by the amoebas to cajole neighboring cells into congregating, whereupon they stick together as they bump into each other, thanks to the glycoprotein adhesion.
Once aggregated, the blob tips over, lying flat on the ground. A slug is born.
From here on out, the amoebae work as one. Cells differentiate into distinct functions, thus belying their protist label.
About 1% of the amoeba cells take on an immune system function: crawling through the slug in search of infectious bacteria. Like later-evolved macrophages, these amoebae police devour invaders, then excrete themselves from the slug, taking the pathogen with them, and dying in the line of duty. Such altruism is essential to the slug’s health.
Self-sacrifice in Dicty owes to their being able to recognize who their kin are. Slime molds are made of related amoeba.
The slug creeps to a well-lit spawning showplace, where it rearranges itself, using signaling pathways for coordination. One portion forms into a stalk of a fruiting body. Another portion turns into spores.
The initial fruiting body formation (sorocarp) is termed a “Mexican hat.” 20% of the amoebae in a sorocarp are doomed to starvation; a societal sacrifice. The kabuki of becoming a mature Mexican hat takes 8 to 10 hours.
Once the hat is fully formed, at a towering 1–2 millimeters tall, spores are released, initiating the life cycle once again. Farming Dicty tuck favored bacteria into their spores, so they are ready to sew a fresh crop when life begins anew.