Eukaryotic Microorganisms: Types and Functions

Eukaryotic microorganisms are single-celled or microscopically small organisms whose cells contain a membrane-bound nucleus and internal compartments, setting them apart from bacteria and archaea. This group spans an enormous range of life forms: photosynthetic algae that generate a large share of the planet’s oxygen, predatory amoebae that control bacterial populations, yeasts that ferment bread and beer, and parasites responsible for diseases like malaria. Far from being simple, these organisms display cellular complexity that rivals and sometimes exceeds that of the cells in your own body, and they drive planetary-scale processes most people never think about.

How Many Kinds Are There

The sheer diversity of eukaryotic microorganisms has long frustrated attempts at tidy classification. A landmark effort in the mid-2000s proposed six broad clusters, or “supergroups,” of eukaryotic life. Multicellular animals and fungi emerged from within one of those clusters (Opisthokonta), land plants from another (Archaeplastida), and brown algae from a third (Stramenopiles).¹ That framework was useful, but it has not held up cleanly. Ongoing genetic analyses have reshuffled the tree considerably: old supergroups have been split, formerly orphaned lineages have been folded into new larger groupings, and entirely new supergroup-level branches have been added.² The upshot is that microbial eukaryotes, collectively called protists when we need a catch-all term, are far more genetically diverse than all animals put together. Every year, environmental DNA surveys turn up lineages that have no close cultured relatives.

A useful way to think about protists is not by their formal taxonomy but by what they do: some photosynthesize, some hunt, some decompose organic matter, some cause disease, and many do several of these things at once. That functional lens is more practical for understanding why they matter.

Photosynthetic Microalgae and the Global Carbon Budget

Microscopic algae are responsible for roughly half of all photosynthesis on Earth. Among them, diatoms stand out. These single-celled organisms build intricate glass-like shells out of silica and are found in virtually every body of water, from ocean surface layers to freshwater lakes. Recent global estimates put diatoms’ contribution at about 38% of the variation in total marine primary production, making them the single largest phytoplankton group by that measure, ahead of cyanobacteria at 23% and green algae at 22%.³ Beyond fixing carbon dioxide into organic matter, diatoms are major conduits for moving both carbon and silicon from the sunlit surface to the deep ocean, a process that influences climate on geological timescales.⁴

How diatoms build their silica shells is a story of remarkable biological engineering. The process has traditionally been understood as entirely intracellular: the cell precipitates silica inside specialized internal compartments and then deposits the finished pieces. But recent cryo-electron imaging has shown that, at least for the long spiny extensions many species produce, silica formation can happen outside the cell body entirely.⁵ The variety of nanoscale architectures diatoms create is unmatched in nature, and the ability to convert genetic information into reproducible physical structure at that scale has attracted interest from materials scientists and engineers.⁶

Grazers That Keep Ecosystems in Check

Not all protists make their own food. Many are voracious predators. Phagotrophic protists, those that engulf and digest prey, are the dominant grazers of bacteria in aquatic ecosystems. Tiny flagellates smaller than 20 micrometers routinely consume a substantial portion of bacterial production, selectively cropping the fastest-growing cells and thereby shaping which bacterial species thrive.⁷ They also graze heavily on phytoplankton: estimates suggest protists routinely consume anywhere from a quarter to all of daily phytoplankton production in various ocean regions, even during large diatom blooms.⁷

The ecological consequences go beyond simple population control. When protists digest bacteria and algae, they release dissolved nutrients like nitrogen and phosphorus back into the water, fueling new rounds of growth. They also release dissolved trace metals, including iron, which limits productivity across vast stretches of the open ocean.⁸ Grazing even extends into the deep, dark ocean, where phagotrophic protists contribute to carbon turnover well below the sunlit surface.⁹ Without protist grazers, microbial communities would look profoundly different, and nutrient cycling in aquatic systems would slow dramatically.

Mixotrophs That Blur the Line

One of the more surprising findings in recent decades is how many protists refuse to be categorized as strictly “plant-like” or “animal-like.” A substantial number of planktonic protists engage in mixotrophy: they photosynthesize when light and nutrients are available, but they can also engulf and eat other cells when conditions change.¹⁰ These organisms are dominant members of open-ocean communities, yet their dual nature makes them hard to account for in models of ocean productivity. A mixotrophic protist might be a net producer of organic carbon in the morning and a net consumer by afternoon, depending on factors that researchers are still working to characterize.¹¹ The practical result is that traditional categories of “phytoplankton” (producers) and “zooplankton” (consumers) are far blurrier at the microbial scale than textbook diagrams suggest.

Microscopic Fungi and the Yeast You Already Know

Yeasts are eukaryotic microorganisms that belong to the fungal kingdom. The best-known species, baker’s yeast (Saccharomyces cerevisiae), has been used for millennia in bread, beer, and wine production. It is also one of the most extensively studied organisms in biology, serving as a model for understanding how cells grow, divide, and respond to nutrient availability. Detailed metabolic profiling of yeast grown under different nutrient-limiting conditions has revealed how tightly individual metabolites track the specific nutrient that is scarce. When nitrogen runs short, amino acids plummet while nucleotides stay high; when phosphorus is the bottleneck, the pattern reverses.¹²

Beyond brewing and baking, yeasts and other microscopic fungi are increasingly important in biotechnology for producing pharmaceuticals, industrial enzymes, and biofuels. Their rapid growth and relatively simple genetics make them amenable to engineering in ways that larger organisms are not.

Cellular Tricks That Multicellular Organisms Lack

Eukaryotic microorganisms have evolved forms of cellular complexity that you will not find in the cells of a plant or animal. One striking example is nuclear dualism in ciliates. Most eukaryotic cells have a single nucleus, but ciliates maintain two genetically distinct nuclei in every cell.¹³ The smaller micronucleus stays mostly silent during day-to-day life; it stores the germline genome and is used during sexual reproduction. The larger macronucleus handles all the active gene expression the cell needs for growth and survival.¹⁴ These two nuclei develop from the same precursor and share the same cytoplasm, yet they differ in ploidy, chromosome structure, and gene activity. In some ciliate lineages, the macronucleus chops its chromosomes into thousands of tiny gene-sized fragments, while in others the chromosomes remain longer.¹⁵ It is as if each cell runs two entirely separate operating systems side by side.

Movement in eukaryotic microbes is equally inventive. Amoebae crawl by extending pseudopods, projections of the cell membrane pushed outward by rapidly assembling networks of actin protein filaments. The leading edge of a moving amoeba builds branched actin scaffolds that push the membrane forward, while the rear of the cell uses a different arrangement of linear actin filaments to contract and pull the cell body along.¹⁶ Interestingly, this is not the only way amoeboid movement can work. Sperm cells in some nematode worms crawl in an amoeba-like fashion despite containing almost no actin at all, which means they have found an entirely different molecular solution to the same locomotion problem.¹⁷

When Eukaryotic Microbes Cause Disease

Some of the most devastating human diseases are caused by eukaryotic microorganisms. Malaria, caused by Plasmodium parasites transmitted through mosquito bites, has shaped human history and still kills hundreds of thousands of people each year. Part of why Plasmodium is so difficult for the immune system to defeat is its arsenal of evasion strategies. The parasite constantly varies the proteins it displays on the surface of infected red blood cells, making it a moving target for antibodies. It hides inside cells that lack the surface markers immune cells use for recognition. And it actively interferes with phagocytic immune cells by producing a byproduct of hemoglobin digestion called hemozoin that impairs their function.¹⁸ These strategies of antigenic variation, intracellular hiding, and immune suppression have evolved over millions of years of host-parasite coevolution.¹⁹

On the fungal side, Candida albicans is a normal resident of human skin and mucous membranes that can turn pathogenic when the immune system is weakened. Its primary trick is shape-shifting: it can switch between a rounded yeast form and elongated filamentous forms called hyphae. That yeast-to-hypha transition is closely linked to its shift from harmless commensal to tissue-invading pathogen.²⁰ Candida also secretes enzymes that break down host tissues, forms resilient biofilms on medical devices, and uses phenotypic switching to adapt to different body sites.²¹

Talking Without Mouths

For a long time, coordinated chemical signaling based on population density, known as quorum sensing, was considered a bacterial specialty. It turns out eukaryotic microbes do it too. In Saccharomyces cerevisiae, aromatic alcohols serve as density-dependent signals that regulate whether cells grow as individual yeasts or switch to filamentous forms and build biofilms. Candida albicans uses a different molecule, farnesol, to suppress hyphal growth, and another, tyrosol, to promote it, effectively balancing its shape-shifting based on how many neighbors are around.²² Even African trypanosomes, the parasites that cause sleeping sickness, use density-dependent peptide signals to switch from a rapidly dividing form to a transmission-ready form when their numbers get high enough in the host bloodstream.²²

Even photosynthetic protists get in on the act. The green alga Chlamydomonas reinhardtii increases its swimming speed in response to a low-molecular-weight compound that accumulates as cell density rises, a behavior that looks remarkably like bacterial quorum sensing. The signal molecule works across related species, suggesting the system is evolutionarily conserved.²³

Stolen Genes and Borrowed Skills

Bacteria are famous for swapping genes horizontally between unrelated species, but eukaryotic microbes have picked up this trick as well. Blastocystis, the most widespread eukaryotic parasite of the human gut, appears to have acquired a significant number of genes from prokaryotes, acquisitions that contributed to its metabolic adaptation to life in the intestinal environment.²⁴ In another example, horizontal gene transfer has spread the ability to make carotenoid pigments across multiple unrelated lineages of heterotrophic protists, an ability that normally belongs to photosynthetic organisms or bacteria. Repeated transfers across divergent eukaryotic groups enabled what amounts to parallel evolution of the same biochemical pathway in organisms that share no recent common ancestor.²⁵

Viruses That Control Protist Populations

Viruses are the most abundant biological entities in the ocean, and many of them specifically infect protists. Large DNA viruses known as nucleocytoplasmic large DNA viruses are major regulators of oceanic protist populations, capable of terminating algal blooms and redirecting carbon flow through ecosystems.²⁶ The interactions between these viruses and their protist hosts are far from fully mapped: many protist lineages have never had their viral parasites characterized. A deeper understanding of virus-protist dynamics is expected to reveal new layers of control over biogeochemical processes in the ocean.²⁷

Biofuel From Microalgae

Microalgae have become one of the most talked-about feedstocks for renewable biofuel. Their appeal rests on a few practical advantages: they do not require arable land and so do not compete with food crops, and they produce oil at rates estimated to be at least 15 to 20 times higher per unit area than land-based oil crops.²⁸ Their high photosynthetic efficiency and rapid growth rate make them a promising alternative to fossil-derived fuels.²⁹ The use of genetic editing and omics technologies has improved lipid accumulation in engineered strains, and researchers are particularly interested in strains that can tolerate high concentrations of inorganic carbon and resist environmental stress, traits that would make large-scale outdoor cultivation more feasible.³⁰ Despite decades of work, commercially competitive microalgal biofuel at scale remains a challenge, largely because of the cost of harvesting and extracting lipids from dilute aqueous cultures.

How Eukaryotic Microbes Survive Extremes

Extreme environments are not solely the province of bacteria and archaea. Eukaryotic microorganisms have colonized hot springs, polar ice, hypersaline lakes, and deep-sea hydrothermal vents. Thermophilic red algae like Cyanidioschyzon merolae survive near-boiling acidic waters by relying on heat shock proteins that stabilize other proteins as they begin to unfold, along with thermostable enzymes adapted to function at high temperatures.³¹ At the opposite thermal extreme, psychrophilic eukaryotes use ice-binding proteins, a diverse collection with different evolutionary origins, that prevent ice crystals from growing large enough to rupture cells.³¹

Tardigrades, microscopic animals found in mosses, sediment, and marine environments, push survival to a further extreme. When conditions become hostile, they enter a state called cryptobiosis, in which metabolism effectively stops. In the specific case of desiccation, tardigrades curl into a dried structure called a tun and can survive loss of nearly all their body water.³² They can recover from this state at any life stage, and the process involves a wholesale shutdown of DNA replication and protein-synthesis machinery.³³ Recovery rates depend on age and social context: younger tardigrades bounce back more reliably, and groups tend to recover better than isolated individuals.³⁴

Discovering What We Cannot Culture

A persistent challenge in microbial biology is that most microorganisms refuse to grow under laboratory conditions. The same is true for eukaryotic microbes: environmental DNA surveys routinely detect lineages that have no cultured representative, a realm sometimes called eukaryotic “dark matter.” Single-cell genomics and transcriptomics are changing this. By isolating individual cells directly from seawater and sequencing their genomes, researchers have revealed the identities and ecological interactions of organisms that would otherwise remain invisible. Early applications of this approach to marine picobiliphytes, a group of tiny algae, showed that even single cells can yield enough genomic data to place organisms into distinct evolutionary clades and identify who they associate with.³⁵ The expectation is that these techniques will rapidly expand our picture of eukaryotic diversity, much as metabarcoding did in the previous decade, and help connect genomic identity to ecological function for lineages we currently know only as anonymous DNA sequences.³⁶

How Endosymbiosis Built the Eukaryotic Cell

The very features that define eukaryotic cells, mitochondria and (in photosynthetic lineages) plastids, trace back to ancient events in which one cell engulfed another and, instead of digesting it, kept it as a permanent internal partner. Both mitochondria and plastids arose from a single such endosymbiotic event each, and the parallels between how these two organelles were reduced and integrated into their host cells are remarkably consistent.³⁷ In a sense, every eukaryotic microorganism alive today is a chimera, carrying the legacy of a bacterial ancestor inside every cell. That ancestral merger set the stage for the extraordinary diversity of forms, functions, and ecological roles that eukaryotic microbes fill across the planet.

Lichens offer a present-day window into how such partnerships form. A lichen is a symbiosis between a fungus and a photosynthetic partner, usually a green alga or cyanobacterium. Building a lichen involves staged biochemical negotiations: fungal lectins and algal peptides mediate early contact, while ongoing exchange of sugars, phytohormones, and antioxidant protection sustains the partnership through its lifetime.³⁸ It is tempting to see lichens as a snapshot of the kind of intimate cross-kingdom collaboration that, billions of years ago, gave rise to the eukaryotic cell itself.