Like their land counterparts, marine plants are the building blocks of their ecosystem. They provide not only food but shelter for the animals and organisms that live in waters around the world, and without them, there would be nothing to sustain that life. Most plants live in the sunlit zone, the top layer of the waters that allows enough light for photosynthesis. It's there that you'll find some of the world's oldest living structures.
Diatoms are microscopic, single-celled organisms that exist in waters all over the world. Most are free-floating, but some can attach themselves to rocks, soils, and to the sides of the animals they share their marine environment with.
In spite of their microscopic size, they're one of the largest food sources for marine animals from fish to whales. Diatoms are part of a larger group known as plankton, and reproduce both sexually and asexually.
Diatoms also come in two main shapes: centrate (circular) and pennate (oval-shaped). Generally, the circular forms are found in greater percentages in salt water habitats, while the oval diatoms dominate in freshwater.
Both types date back to the Cretaceous period in the fossil record. This makes them between 144 and 65 million years old, having developed alongside the last of the dinosaurs.
Dinoflagellates make up another important component of plankton. While, like diatoms, they also contain chlorophyll they differ in that they have flagella on their cell structure. This flagella gives them a distinctive motion when they swim. They also have cellulose in their structure; this tabulation is the primary means for identifying them.
In addition to being a large producer in the marine ecosystem, they're also one of the most dangerous. They produce a neurotoxin that can cripple an ecosystem when it is introduced in abundance, and that unfortunately happens often. With warmer weather and more sunlight comes a population boom in the dinoflagellate population, easily visible to the naked eye when it turns the water a distinctive red. The neurotoxin can be both fatal and non-fatal, and can affect humans when they eat shellfish that have been exposed to it.
Without dinoflagellates, the creation of coral reefs would take much longer than it does. The dinoflagellates create a symbiotic relationship with coral after being ingested, producing carbohydrates that greatly aid in the formation of reefs.
There are nearly 2,000 types of dinoflagellate, and they date back to the Precambian period.
Blue-green algae is also classified as a type of bacteria, and it can be found anywhere in the world. Along with diatoms and dinoflagellates, they are also considered a part of plankton.
The algae thrives in warm climates and in places were the water contains nutrients similar to those needed by land-dwelling plants, especially including nitrogen and phosphorus. Population booms can result in the algae covering bodies of water--especially those with no current or tide--like a green carpet.
This is especially dangerous, as blue-green algae produces a toxin that is not only poisonous to humans and animals, but has been documented to kill some that have drank water infested with the algae. Those who ingest the toxins suffer paralysis before death, and there is currently ongoing research into how to control these algae blooms.
Seagrasses are frequently found in coastal areas, where they root into the seabed and use the ample sunlight in the shallows for photosynthesis. Because they rely on enough sunlight making it into the water, typically they are only found where the waters are clear.
Clusters of seagrasses are a vital part of the marine ecosystem. They provide food and shelter for many animals, including young fish and crustaceans that grow up in the safety of the grasses. They are a food source for the same fish and for other animals as large as the manatee, and have been found to stabilize the soils on the sea floor and to prevent erosion from the tides.
There are approximately 52 species of seagrasses worldwide, and they have become a telling sign of climate change and the degradation of water quality.