Marine microorganisms

Marine microorganisms are tiny living things that live in the saltwater of seas and oceans, or in the mix of fresh and salt water in coastal areas called estuaries. These tiny creatures are so small that you need a microscope to see them. They include many different types of life, such as bacteria, tiny plants called algae, and some very small animals.

Even though they are small, marine microorganisms are very important. They make up a big part of the living material in the ocean and help keep our planet healthy. They help recycle nutrients, produce oxygen, and control carbon levels all around the world. Without them, life in the ocean—and on land—would be very different.

Scientists are still learning about these tiny ocean inhabitants. In 2016, they discovered a set of genes that are shared by all life on Earth, including marine microorganisms. Even today, much about what they do and how they behave remains a mystery, showing how much we still have to learn about the microscopic world under the waves.

Overview

Marine microorganisms are tiny living things that live in the salt water of oceans and seas. They are so small that you need a microscope to see them. These tiny creatures include bacteria, tiny plants called algae, and other simple life forms. They help keep our planet's ecosystems healthy by recycling nutrients and providing food for other sea creatures.

Microbial mats, which are layers of tiny life forms, are some of the earliest known forms of life on Earth. These mats can create structures called stromatolites as they move upward to avoid being covered by sand. Even though we have learned a lot about these tiny sea creatures, there is still much we do not know about them. They are found almost everywhere, from the surface of the ocean to the deepest parts, like the Mariana Trench, and they help control important processes in the world's oceans.

Marine viruses

Main article: Marine viruses

A virus is a tiny living thing that needs to get inside the cells of other organisms to grow. Viruses can infect animals, plants, and even small organisms like bacteria. When they are not inside a cell, viruses look like small particles made of genetic material, a protective coat, and sometimes a layer of fats.

Viruses are found everywhere that there is life, and they help keep nature balanced. They help move genes between different species, which makes life more diverse. Some very large viruses, called giant viruses, were found in the deep ocean. They can be much bigger than typical viruses. Scientists are still learning more about these giant viruses.

Prokaryotes

Main article: Marine prokaryotes

Bacteria and archaea are very small living things called prokaryotes that live in the ocean. Bacteria can be shaped like spheres, rods, or spirals. They are some of the oldest life forms on Earth. They can live almost anywhere, even in very hot or very cold places. Unlike animal cells, bacterial cells do not have a nucleus or other parts inside membranes.

Archaea are another group of tiny ocean organisms. They look like bacteria but have some different traits. Some archaea can live in very salty water or very hot places. Both bacteria and archaea help keep the ocean environment healthy and balanced.

Eukaryotes

All living things are either prokaryotes or eukaryotes. Life first started as simple prokaryotes and later evolved into more complex eukaryotes. Prokaryotes include bacteria and archaea, while eukaryotes include protists, plants, fungi, and animals. Protists are usually single-celled, while plants, fungi, and animals are usually made of many cells.

Marine protists are eukaryotes that are not plants, fungi, or animals. They are usually single-celled and very small. Protists can get food in different ways, like plants, animals, or fungi. They are found in oceans and river sediments, and scientists believe there are many more types yet to be found. Some protists, like diatoms, help make much of the world's oxygen. Others, like seaweed, can grow very large.

Marine fungi are also important. Over 1,500 types of fungi live in the ocean. They live on algae, animals, or dead material, and they help break things down and cycle nutrients. Mycoplankton, a kind of marine fungi, helps keep the ocean's food web healthy.

Marine microanimals include tiny creatures like copepods, which are small sea animals, and tardigrades, also called water bears. These small animals are important for keeping the ocean balanced and supporting larger sea life.

Protists according to how they get food
Type of protist		Description	Example		Some other examples
Plant-like	Algae (see below)	Autotrophic protists that make their own food without needing to consume other organisms, usually by using photosynthesis		Green algae, Pyramimonas	Red and brown algae, diatoms and some dinoflagellates. Plant-like protists are important components of phytoplankton discussed below.
Animal-like	Protozoans	Heterotrophic protists that get their food consuming other organisms (bacteria, archaea and small algae)		Radiolarian protist as drawn by Haeckel	Foraminiferans, and some marine amoebae, ciliates and flagellates.
Fungal-like	Slime moulds and slime nets	Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed		Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel	Marine lichen
Mixotrops	Various	Mixotrophic and osmotrophic protists that get their food from a combination of the above		Euglena mutabilis, a photosynthetic flagellate	Many marine mixotrops are found among protists, particularly among ciliates and dinoflagellates

Protists according to how they move
Type of protist		Movement mechanism	Description	Example		Other examples
Motile	Flagellates		A flagellum (Latin for whip) is a lash-like appendage that protrudes from the cell body of some protists (as well as some bacteria). Flagellates use from one to several flagella for locomotion and sometimes as feeding and sensory organelle.		Cryptophytes	All dinoflagellates and nanoflagellates (choanoflagellates, silicoflagellates, most green algae) (Other protists go through a phase as gametes when they have temporary flagellum – some radiolarians, foraminiferans and Apicomplexa)
	Ciliates		A cilium (Latin for eyelash) is a tiny flagellum. Ciliates use multiple cilia, which can number in many hundreds, to power themselves through the water.		Paramecium bursaria click to see cilia	Foraminiferans, and some marine amoebae, ciliates and flagellates.
	Amoebas (amoeboids)		Amoeba have the ability to alter shape by extending and retracting pseudopods (Greek for false feet).		Amoeba	Found in every major protist lineage. Amoeboid cells occur among the protozoans, but also in the algae and the fungi.
Not motile		none			Diatom	Diatoms, coccolithophores, and non‐motile species of Phaeocystis Among protozoans the parasitic Apicomplexa are non‐motile.

Primary producers

Main article: Marine primary production

See also: Evolution of photosynthesis

Primary producers are tiny living things that can make their own food instead of eating other animals or plants. They are the first step in the food chain for other sea creatures that need to eat to get energy.

Some of these tiny sea creatures are special bacteria that gather around hot spots in the ocean and use chemical processes to make food. But most use sunlight to make food from carbon dioxide in the water. This process gives us oxygen, which helps keep many sea animals alive.

The main marine primary producers are tiny blue-green bacteria called cyanobacteria, different kinds of algae, and marine plants. These tiny plants float in the ocean and give energy to many animals. They also help keep our planet healthy by recycling water and carbon.

Marine microplankton

Plankton are tiny living things that float in oceans and lakes. They can't swim against the current, so they move with the water. Plankton are very important because many bigger animals, like fish and whales, eat them.

Phytoplankton are a type of plankton that make their own food using sunlight. They are important because they create much of the oxygen we breathe and help control carbon dioxide in the air. Phytoplankton can sometimes grow very fast and create big groups called blooms. They come in many shapes and sizes, including tiny plants and some types of bacteria. One common type is diatoms, which have glass-like shells. Another type is coccolithophores, which have tiny plates made of chalk.

Mixotrophic plankton that combine phototrophy and heterotrophy – table based on Stoecker et al., 2017
General types	Description		Example		Further examples
Bacterioplankton	Photoheterotrophic bacterioplankton			Vibrio cholerae	Roseobacter spp. Erythrobacter spp. Gammaproteobacterial clade OM60 Widespread among bacteria and archaea
Phytoplankton	Called constitutive mixotrophs by Mitra et al., 2016. Phytoplankton that eat: photosynthetic protists with inherited plastids and the capacity to ingest prey.			Ochromonas species	Ochromonas spp. Prymnesium parvum Dinoflagellate examples: Fragilidium subglobosum,Heterocapsa triquetra,Karlodinium veneficum,Neoceratium furca,Prorocentrum minimum
Zooplankton	Called nonconstitutive mixotrophs by Mitra et al., 2016. Zooplankton that are photosynthetic: microzooplankton or metazoan zooplankton that acquire phototrophy through chloroplast retentiona or maintenance of algal endosymbionts.
	Generalists	Protists that retain chloroplasts and rarely other organelles from many algal taxa			Most oligotrich ciliates that retain plastidsa
	Specialists	1. Protists that retain chloroplasts and sometimes other organelles from one algal species or very closely related algal species		Dinophysis acuminata	Dinophysis spp. Myrionecta rubra
		2. Protists or zooplankton with algal endosymbionts of only one algal species or very closely related algal species		Noctiluca scintillans	Metazooplankton with algal endosymbionts Most mixotrophic Rhizaria (Acantharea, Polycystinea, and Foraminifera) Green Noctiluca scintillans
aChloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm.

Marine sediments and microfossils

Further information: Marine sediments, microfossils, and protist shells

The ocean floor is covered with sediments from two main sources: land and tiny ocean creatures. Land-based sediments, called terrigenous sediments, make up about half of the ocean's sediments. They come from rocks on land worn down by water, wind, or ice, and then carried into the ocean.

The other half of ocean sediments come from the remains of tiny ocean creatures, called biogenous sediments. These include the shells of single-celled organisms like diatoms and foraminifers. When these tiny creatures die, their shells pile up on the ocean floor. Some of these special mixtures are called "ooze" because they come from these tiny creatures.

Main types of biogenic ooze
type	mineral forms	protist involved		name of skeleton	typical size (mm)
Siliceous ooze	SiO2 silica quartz glass opal chert	diatom		frustule	0.002 to 0.2		diatom microfossil from 40 million years ago
		radiolarian		test or shell	0.1 to 0.2		elaborate silica shell of a radiolarian
Calcareous ooze	CaCO3 calcite aragonite limestone marble chalk	foraminiferan		test or shell			Calcified test of a planktic foraminiferan. There are about 10,000 living species of foraminiferans
		coccolithophore		coccoliths			Coccolithophores are the largest global source of biogenic calcium carbonate, and significantly contribute to the global carbon cycle. They are the main constituent of chalk deposits such as the white cliffs of Dover.

Marine microbenthos

See also: Seabed § Sediments, bioturbation, and bioirrigation

Marine microbenthos are tiny living things that live near or on the seafloor, or inside or on the top layers of seafloor dirt. The word benthos comes from an old word meaning "depth of the sea". You can find microbenthos everywhere on the seafloor, from shallow areas to deep waters. They are especially many where the seafloor dirt is rich.

In shallow waters, places like seagrass meadows, coral reefs, and kelp forests give microbenthos good homes to live. In areas where sunlight reaches, a type of microbenthos called benthic diatoms are common because they can make their own food using sunlight. In places where the ocean meets the land and the water level changes with the tides, called intertidal zones, the tides control when and where microbenthos can live.

Some of these tiny creatures, like Elphidium and Heterohelix, have shells that scientists study to learn about Earth’s past climate. By looking at the chemistry of these shells, researchers can figure out what the ocean was like long ago. Even after these tiny creatures die, their shells fall to the seafloor and tell us stories about the past.

Marine microbiomes

Main article: Marine microbiomes

Further information: marine microbial symbiosis and holobiont

Marine microbiomes are very small living things that live in the ocean. They can work together with bigger animals in a special way called symbiosis. This means they help each other.

One example is corals, which are animals that build reefs. Corals have tiny plants called zooxanthellae living inside them, along with helpful bacteria and viruses. Together, they form a group called a holobiont, which works as one team to stay healthy. Other animals like sponges and seagrass also have their own teams of tiny helpers.

Marine food web

Further information: marine food web

Marine microorganisms are very important in the ocean's food web. They help keep things balanced by recycling materials so that many tiny plants and animals can survive. One way they do this is through a process called the viral shunt. This process breaks down big pieces of organic material into smaller pieces that microorganisms can use.

One special type of bacteria, Pelagibacter ubique, is the most common bacteria in the ocean. It plays a big role in the Earth's carbon cycle by helping move carbon through the ocean.

Niche communities

Black smoker in the High Rise portion of the Endeavour Hydrothermal Vents.

Bacteria can be very helpful. The Pompeii worm, an extremophile, lives only at hydrothermal vents and has a protective cover of symbiotic bacteria.

Sea ice microbial communities are tiny living things that live within and around sea ice at the poles. They can survive changes in salt, light, temperature, and food. Many of these tiny plants and animals are found in the lower parts of the ice.

Hydrothermal vents are places where the Earth's plates move apart, allowing ocean water to heat up deep underground. This hot water comes back out, carrying minerals and chemicals. These areas support special microscopic life, including tiny organisms that live in mats, float freely, or live close to animals. Since there is no sunlight, these communities use chemical processes for energy.

Viruses are also found around hydrothermal vents and play an important role in these ecosystems. They are very common in the ocean and carry lots of genetic information. In these extreme places, viruses may help microbes survive by sharing genes.

Deep biosphere and dark matter

See also: deep biosphere, rare biosphere, microbial dark matter, SLiME, and marine cryptic interactions

The deep biosphere is the world of tiny living things far below the surface. It goes down at least 5 kilometers under land and even deeper under the ocean, where it can get very hot—over 100 °C!

Unlike life on the surface, deep-life does not eat plants or use oxygen. Instead, it uses simple chemicals from rocks, like hydrogen and methane. Because there is very little energy deep down, these tiny creatures live very slowly—sometimes taking thousands of years to grow.

Scientists found that much of all tiny life on Earth lives deep underground. In later years, they discovered life very far below the surface that eats rocks. Researchers also found tiny life that had been almost frozen in time for many millions of years.

Many of these deep-life creatures cannot be grown in labs, so scientists call them microbial dark matter. Even though we know they exist, we still have much to learn about them. Researchers study their DNA from samples taken from the environment.

Microbial diversity

Bacteria are some of the oldest and most different kinds of tiny living things in the sea. They live together with other tiny creatures called Archaea and Fungi. Scientists once thought there were about 3 million kinds of living things on Earth. But now they think there could be as many as 1 trillion kinds of tiny sea creatures, and many of these are still unknown.

Sampling techniques

Scientists use special tools to collect tiny sea creatures from the ocean. One tool is called a plankton net. It helps catch small organisms floating in the water. Another tool is the video plankton recorder. It uses cameras to record these tiny creatures without catching them. These tools help scientists learn about microscopic life in our oceans.

Main articles: niskin bottle, plankton net, video plankton recorder, and continuous plankton recorder

Identifying microorganisms

Scientists study many tiny sea creatures, called plankton, using special cameras and tools that read their DNA, called sequencing techniques. In the past, people tried to group these tiny creatures by looking at their shapes, but that was hard because many don’t have clear shapes like animals or plants. Now, scientists look at the DNA of these creatures to learn more about them.

New tools let scientists study the DNA of tiny sea creatures right from the water, without needing to grow them in a lab. This has helped us learn more about how many different kinds of tiny creatures live in the ocean and how they help keep the ocean healthy.

Methods of identifying microorganisms
Chromogenic media	Microscopy techniques	Biochemical techniques	Molecular techniques
Traditional media	Bright field Dark field SEM TEM CLSM ATM Inverted microscopy	Spectrometry     – FTIR     – Raman spectrometry Mass spectrometry     – GC     – LC     – MALDI-TOF     – ESI Electrokinetic separation Microfluidic chip Propriety methods     – Wickerham card     – API     – BBL Crystal     – Vitek     – Biolog	PCR Real-time qPCR Rapid PCR PCR sequencing RFLP PFGE Ribotyping WGS MALDI-TOF MS

Using omics data

Further information: Omics, environmental DNA, metabarcoding, metagenomics, and metatranscriptomics

Omics is a way scientists study living things. It includes areas like genomics and proteomics, all ending in "_-omics." These help scientists look at many tiny parts of living things at the same time. For example, functional genomics studies what genes do in an organism.

Scientists use omics methods to learn about tiny sea creatures and their friends. This helps them understand the microbiome — all the tiny living things that live together. Methods like metagenomics, metatranscriptomics, metaproteomics, and metabolomics give clues about what these tiny creatures are doing.

Anthropogenic impacts

Marine microorganisms help balance the amount of carbon in our atmosphere. They take carbon from the air and store it, then release it back later. This is part of the carbon cycle.

When humans burn fossil fuels, we add carbon to the air faster than nature can take it away. This upsets the balance and leaves more carbon in the atmosphere. Scientists say it is important to study these tiny life forms to help protect our planet and fight climate change.

History

Marine microorganisms called cyanobacteria first appeared in the oceans about 2 billion years ago during the Precambrian era. They could make oxygen through a process called photosynthesis. This changed Earth's environment and helped shape the development of plants, animals, and many other living things. These tiny ocean creatures were first seen in 1675 by a Dutch scientist named Antonie van Leeuwenhoek. He used a simple microscope to observe them.