Take two breaths. For one of them, you can thank plankton, in particular the single-celled photosynthetic drifters that comprise the phytoplankton of the world's oceans. These elegant, microscopic creatures perform half of the photosynthesis and consequent oxygen production on Earth.
Here, two species of diatoms, a type of phytoplankton: Odontella sinensis on the left; Odontella mobiliensis on the right.
These diatoms measure 100 to 200 microns (0.1–0.2 mm). The small particles within the cells are chloroplasts.
Foraminifera, by contrast, catch their food. Some live on the seafloor while others are planktonic, meaning they drift through the water.
Foraminifera deploy their numerous membrane extensions, called pseudopodia, to find, capture and envelop prey. They often feed on bacteria, other protists, and larvae.
A foraminiferan is a single, large cell often subdivided into several chambers by intracellular calcium carbonate shells called tests. A multitude of string-like membrane extensions pass through holes in the test.
These thin pseudopodia probe the environment, move the foraminifera over surfaces, and engulf prey.
In the Globigerinoides bulloides foraminiferan at left, the pseudopodia are retracted. At right, they are fully extended.
Thalassionema nitzschioides is a diatom, a major group of algae and one of the most common types of phytoplankton.
The cells, each measuring 10 to 20 microns, are joined together in chains by mucilaginous links.
Radiolarians are zooplankton, drifters that feed on other small organisms.
At left, a large Thalassicolanucleata radiolarian with a central capsule enclosing the nucleus. At right, a colonial collodarian radiolarian comprised of many individual cells, each with a central capsule, sharing a common jelly.
In both images symbiotic micro algae are visible as small ochre-colored dots.
This small jellyfish is called Oceania amata. Its closely related cousin, Turritopsis dohrnii, was recently featured in the media as an example of an organism that may achieve immortality.
For now, Turritopsis dohrnii is the only known animal that can develop in reverse, transforming from a medusa back to a polyp.
This is the free-swimming reproductive form of the colonial cnidarian Abylopsis tetragona. At the top, a lobed swimming bell called an oleocyst contains droplets of oil.
On either side is a reproductive organ: an empty male gonophore (left), and a female gonophore with eggs inside (right). When gametes are expelled, sperm are attracted by a chemical cue and swim rapidly to one pole of the egg (bottom left) in order to fertilize it.
This male Sapphirina copepod reflects and diffracts light through tiny plates in the epidermal cells covering its surface.
Depending on the orientation of the animal, its flat body switches from fully transparent to brightly colored.
A female Phronima, left, holds her progeny against the wall of the barrel — a structure built using leftovers from its prey. She feeds the young for two or three weeks, making sure they stay bunched together inside. A group of young Phronima, right, escaped from the barrel, but were caught by their mother and brought back inside.
Phronima juveniles, bottom, are able to move around on the wall of the barrel, but they need to stay grouped, with heads facing outward, as if held together by a mutual force of attraction.
Fun fact: The cartoonist Moebius was inspired by Phronima's form, and his drawings were used to create the monster in the Hollywood blockbuster Alien.