Like plants on land, true plants found in the ocean flower, put down roots, and reproduce through seeding and pollination. These plants, from the tiniest phytoplankton to numerous varieties of seagrasses found in most parts of the world, share the vascular system of terrestrial plants, relying on sunlight and carbon dioxide to survive, and drawing nutrients from the water in which they live.
Unlike seagrasses, algae are not considered true plants, because they do not flower or put down roots, and lack the vascular system used by plants to draw nutrients from the environment. The life cycles of algae may be simple or complex. Some, such as bladderwrack algae, have a simple life cycle that resembles that of the true plants: mature individuals develop reproductive elements that release sperm and eggs into the water. Once fertilized, a new male or female plant germinates and grows.
Other types of algae, such as the very large variant known as seaweed, have a more complex, or multigenerational, life cycle in which reproduction depends on alternating generations of spores and gametes. This cycle begins with a form of the organism called a sporophyte, which produces spores that mature into a different form called a gametophyte. This form produces not spores, but gametes (sperm or eggs). When gametes merge, a sporophyte is created, and the cycle begins again.
The life cycles of marine plants provide significant information about the effects of climate change and human-caused damage to the marine environment. Since these organisms form a basic part of the ocean food chain, changes in the quality, amount and location of marine plants and algae can have a widespread effect on other ocean dwellers. For example, a reduction in the amount of phytoplankton due to changes in water temperatures or toxins can have a ripple effect that affects creatures ranging from shrimp to whales.