Marine biology is the science of saltwater and everything that lives, moves, and filters through it. The word "marine" in this sense refers specifically to saltwater that you find in oceans, not freshwater, which is found in lakes.
When you look at a globe, you can see that nearly three quarters of the Earth's surface is covered with water. Scientists believe life on Earth got its start in the ocean, and gradually adapted to life on land. Some land-based organisms eventually returned to the water, like dolphins and whales. Tiny ocean plants, called phytoplankton, produce most of the oxygen in the air we breathe. When producing oxygen, these plants soak up carbon dioxide, removing this gas from the air.
The ocean is always in motion: currents move water around the globe. Water evaporates and rises into the atmosphere where it will eventually fall as rain and snow. Cold water sinks in the ocean, warm water rises, and this constant movement distributes heat and nutrients around the globe.
In this course, you will learn about life in the ocean depths, at the Polar extremes, in coral reefs, estuaries, and in the open sea. You will learn about plants large and small, marine birds, reptiles, invertebrates and fish. You will learn how all these organisms connect with each other in the marine biome, and what threats are facing these ecosystems.