Algae are simple organisms that make food from sunlight by photosynthesis, but lack the roots, stems, and leaves of proper plants. Algae are found in all water environments, and some can live on land, forming a thin, greenish layer on damp surfaces. Algae make up most of the oceans’ PHYTOPLANKTON – microscopic life forms photosynthesizing at the ocean surface. Larger marine algae called seaweeds are made of many cells, with structures called fronds that look similar to plant leaves.
Kelps are varieties of large, dark green or brown seaweeds that grow in cold seas around the world. One type, called giant kelp, can reach up to 60 m (196 ft) from seabed to surface. Giant kelp can form magnificent underwater forests. Kelp beds provide an important habitat for other marine life, including snails, crabs, sea urchins, seals, and sea otters.
All seaweeds contain green chlorophyll for photosynthesis, but some types have extra pigments which make them appear brown or red. Different seaweeds survive at different tidal zones on the seashore – the longer they can survive being exposed by the tides, the higher up the beach they can live.
Lakes, ponds, and ditches can be smothered by algal growth when there are too many nutrients in the water. Thick layers of algae cover the water’s surface, cutting off sunlight to the plants and algae below and killing them. As these organisms rot, oxygen in the water is used up, and much of the life below the water’s surface dies.
Microscopic algae that float in the oceans, using the energy of sunlight to make food and grow, are called phytoplankton. Together with zooplankton – tiny animals and animal-like organisms – they float near the water’s surface. In the right conditions, phytoplankton can multiply rapidly, turning water green or red. They are the ultimate source of food for almost all marine life.
Algal blooms can happen in the sea when nutrients such as fertilizers or sewage make life too easy for phytoplankton. Algae such as Noctiluca scintillans can turn the sea red, poisoning animals such as shellfish with toxins which would normally be more dispersed. Noctiluca means “night light” – this phytoplankton can glow in the dark, creating flickers of light on the sea’s surface.