[1][2] Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting large numbers of eukaryotic microbial communities have yet to be discovered.

Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.

[11] "Marine protists are a polyphyletic group of organisms playing major roles in the ecology and biogeochemistry of the oceans, including performing much of Earth's photosynthesis and driving the carbon, nitrogen, and silicon cycles.

[12] The ocean represents the largest continuous planetary ecosystem, hosting an enormous variety of organisms, which include microscopic biota such as unicellular eukaryotes (protists).

Despite their small size, protists play key roles in marine biogeochemical cycles and harbour tremendous evolutionary diversity.

[13][14] Notwithstanding their significance for understanding the evolution of life on Earth and their role in marine food webs, as well as driving biogeochemical cycles to maintain habitability, little is known about their cell biology including reproduction, metabolism and signaling.

Even so, in the decade to 2020, genome [15][16][17] and transcriptome sequencing initiatives [18] have resulted in nearly 120 million unigenes being identified in protists,[21] which is facilitating the development of genetic tools for model species.

A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other.

[33] It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size during blooms.

[48] Algae is an informal term for a widespread and diverse group of photosynthetic protists which are not necessarily closely related and are thus polyphyletic.

[55] Another single-celled algae, Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern.

Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an invasive species known colloquially as killer algae.

[63] Dead diatoms drift to the ocean floor where, over millions of years, the remains of their frustules can build up as much as half a mile deep.

[64] Diatoms have relatively high sinking speeds compared with other phytoplankton groups, and they account for about 40% of particulate carbon exported to ocean depths.

[67][62] Remote sensing data suggests there was a global decline of diatoms between 1998 and 2012, particularly in the North Pacific, associated with shallowing of the surface mixed layer and lower nutrient concentrations.

The luminescence, sometimes called the phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.

[87][88] Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.

[89][90] Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy.

Radiolarians are unicellular predatory protists encased in elaborate globular shells, typically between 0.1 and 0.2 millimetres in size, usually made of silica and pierced with holes.

[93] They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.

[99][100] The dominant pelagic grazers of phytoplankton are typically associated with distinct operating modes of the food web compartments and nutrient cycling.

[101][102] Grazing on particulate primary production in the global ocean surface is ~10–15% for mesozooplankton and 59–75% for microzooplankton,[103][104][105][106] with estimates for coastal and estuarine systems usually in the a lower range.

[108] Besides their significant role in carbon transfer, ciliates are also considered high quality food, as a source of proteinaceous compounds with a low C:N ratio in comparison to phytoplankton.

[118] One of the best examples is that the origin of eukaryotes is grounded in the interaction-events of endosymbiosis; giving rise to mitochondria, chloroplasts, and other metabolic capacities in the eukaryotic cell,[119][120][121][122] Microbial interactions guarantee ecosystem function, having crucial roles in, for instance, carbon channeling in photosymbiosis, control of microalgae blooms by parasites, and phytoplankton-associated bacteria influencing the growth and health of their host.

In 1851, while on board HMS Rattlesnake in the Pacific Ocean, Thomas Huxley discovered small yellow–green cells inside the conspicuous planktonic radiolarians which he thought were organelles.

[127] Since these early studies, hundreds of others have reported microbial interactions by using classic tools, mainly microscopy, but this knowledge has not yet been gathered into one accessible database.

In B, B, and D the following abbreviations for supergroups are used: Ar Archaea, Ba Bacteria, Rh Rhizaria, Al Alveolata, St Stramenopiles, Ha Haptista, Cy Cryptista, Ap Archaeplastida, Ex Excavata, Ob Obazoa, Am Amoebozoa, Cu CRuMS, An Ancryomonadidae, Is Incertae sedis.

These glass structures have accumulated for over 100 million years leaving rich deposits of nano and microstructured silicon oxide in the form of diatomaceous earth around the globe.

However, in 2018 it was shown that reflection of ultraviolet light by nanostructured silica protects the DNA in the algal cells, and this may be an evolutionary cause for the formation of the glass cages.

In the diagram, (A) represents accelerated photosynthesis including carbon concentrating mechanisms (CCM) and enhanced light uptake via scattering of scarce photons for deep-dwelling species.