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.
