Algae are aquatic organisms that produce their own food from sunlight, carbon dioxide and water, through photosynthesis. They range in size from microscopic single-celled algae to the giant kelp, which can grow to over 60 m (200 ft) long. Most live free and independent lives, but some algae form symbiotic relationships. Symbiosis is a mutually beneficial relationship such as the relationship between bees (who gain food) and flowers (which the bees pollinate).
LICHEN
One common example of a symbiotic relationship is that between algae and fungi. This combination of the two types of organism is so close that it is known by a single name: lichen. There are very few scientists researching lichens, and so there are many unanswered questions about them. Many scientists believe the algae benefit from the relationship by being protected from drying out, and the fungi benefit by being provided with food derived by photosynthesis, while others see it as a parasitic relationship with little or no benefit to the algae.
CORAL
Another well-known symbiotic relationship is that between single-celled algae called
zooxanthellae and reef-forming corals. In this relationship, the algae provide the food (especially carbon, which is used in building the structure) through photosynthesis, and the coral provides protection and nutrients for the algae. The symbiotic relationship is most obvious when the algae die off through changes in temperature, pollution, or when there is too little light. With the algae gone, the coral looks bleached’ because the algae and their pigments have disappeared.
SPONGES
Some sponges have a symbiotic relationship with algae. As with coral, the algae benefit by being protected within the sponge, and the sponge benefits because of the food and oxygen produced by the algae during photosynthesis.
SEA ANEMONES AND JELLYFISH
Another way in which some algae (zooxanthellae, as in coral) have found protection is in a symbiotic relationship with some sea anemones and jellyfish. In this relationship the sea anemones and jellyfish benefit by gaining food, and the algae gain protection and nutrients.
GREEN SEA SLUGS
Some sea slugs have the advantage of being green, which acts as a camouflage against a background of green seaweeds, but an even greater advantage is that they are capable of photosynthesis. The green colour comes from algal chloroplasts. This is a remarkable example of symbiosis because it is intracellular, with the algae’s cell structure being destroyed when it is incorporated into the sea slug’s skin, and yet the algae remain viable for several months. Again, the algae gains protection, but the relationship seems extremely one-sided, with the sea slugs being the big winners.
ALGAE AND BACTERIA
Evidence is emerging that some algae have a symbiotic relationship with bacteria, and through this relationship the algae gain Vitamin B12, which is an essential vitamin.
Single-celled algae are tiny and vulnerable organisms that must be kept moist at all times, and they have developed a number of ways of making sure they are kept protected and moist. At least one of these algal symbiotic relationships can be seen from space: the Great Barrier Reef.
