The Role of Symbiotic Algae in Marine Invertebrates
by J. Charles Delbeek M.Sc.

Contrary to popular belief, coral reefs are relatively nutrient poor
areas. Phosphate, nitrogen, iron and other essential nutrients are
barely detectable in the clear waters; clear because so little is
growing in it (Benson, 1984). To compensate for this lack of
nutrients, many invertebrates have developed symbiotic relationships
with algae which they hold in their tissues (they are usually
ingested by the animal in their larval stages; dead algal cells are
expelled). These algal cells produce energy which is used by the
host. The host then produces ammonia which the algae utilize as an
energy source. Freshwater animals, such as Hydra, Spongilla and
Paramecium, contain a green alga known as Zoochlorella, while marine
forms generally contain the brown dinoflagellate, Gymnodium
microadriaticum, commonly termed zooxanthellae ("xanthos" =
yellowish-brown) (Gordon, 1977). Zooxanthellae are found in the reef
building corals, sea anemones and giant clams; sponges and some
ascidians (Sea Squirts) utilize different types of symbiotic algae
but their role is essentially the same (Gordon, 1977).In this article
we will be concerned only with the marine algal symbionts.

The Biochemistry of Zooxanthellae in Reef Building Corals
Zooxanthellae are most common in the reef building corals, (hard
corals with zooxanthellae are called hermatypic) consequently, since
algae require abundant light, reef building corals are restricted to
shallow waters (<90 m) (Barnes, 1974). Although corals can exist
quite well by feeding on plankton, the presence of zooxanthellae
makes them, in effect, super-corals (Benson, 1984). The many
interactions between the corals and their symbiotic algae are only
now beginning to be understood and there is still much to be learned.

Zooxanthellae are found in the second layer of cells below the
epidermis; one algal cell per animal cell. They are important
components of reef building corals providing them with nutrients,
removing waste and contributing to the production of calcium
carbonate skeletons. Algae utilize light energy to fixate
bicarbonate, a form of carbon dioxide, into carbohydrates (simple
sugars). The carbohydrates released by the algae are in the form of
glycerol and glucose; the amino acid alanine is produced also. This
requires the presence of certain nutrients, mainly nitrogen and
phosphate. As a source of nitrogen, zooxanthellae utilize the ammonia
produced by the coral (Barnes, 1974; Gordon, 1977). The nitrogen and
phosphate produced by the coral is a result of the metabolism of the
zooplankton on which the coral feeds (Barnes, 1974). The
zooxanthellae give up about 80% of their photosynthetic products to
the coral. This is facilitated by the action of the digestive enzymes
of the coral on the cell walls of the algae which cause them to
become "leaky" and pass their photosynthetic products to the coral.
The alanine produced by the zooxanthellae is used by the coral to
make more complex proteins, while the carbohydrates provide energy
for work and tissue growth (Benson, 1984).

In summary, using light energy, zooxanthellae convert carbon dioxide
(from bicarbonate taken from seawater and the carbon dioxide
produced by cellular respiration of the coral tissue) into
carbohydrates and alanine. These products are then passed on to the
animal tissue which subsequently provides a source of nitrogen
(ammonia) and phosphate to the algae. Without the zooxanthellae, the
coral host would soon suffocate in it's own wastes.

As was mentioned earlier, zooxanthellae also contribute to the
production of coral skeletons. Corals which are deprived of their
zooxanthellae, or are kept in the dark, deposit calcium at a much
slower rate than normal. It is thought that algal photosynthesis may
increase the calcium carbonate production by removing carbon dioxide
and driving the following reaction to the right:

Ca(HCO3)2 <--> CaCO3 + H2CO3 <--> H2O + CO2 (Barnes, 1974)

It is this ability to rapidly deposit calcium carbonate which has
helped the corals to become the dominant animals on the reef.

There are some hard corals which do not have symbiotic algae
(ahermatypic). These corals are either deep water corals or corals
which are found in caves or coral grottoes. The most commonly
encountered ahermatypic hard coral in the aquarium is the Orange
Flower Coral (Tubastrea spp.). Subsequently, if you purchase such a
specimen, be sure to keep it away from strong light and ensure it has
a strong current flow to encourage the polyps to open.

Zooxanthellae and Soft Corals
Many of the soft corals, such as leather corals (those commonly
offered for sale belong to the genus Sarcophyton) and gorgonians,
also contain zooxanthellae. Therefore, when keeping these types of
corals, you should ensure that they receive plenty of light. However,
there are several deep-water soft corals which do not have
zooxanthellae and, hence, do not need as much light as their
shallow-water cousins. Among these are the brightly coloured
Dendronephthya which can be red, white or yellow in colour
(Alderslade et al, 1984).

Zooxanthellae and Anemones
As has been mentioned in a previous article on mushroom anemones,
zooxanthellae are found in the majority of anemones. The relationship
between zooxanthellae and anemones is similar to that described for
hard corals (Barnes, 1974).

Zooxanthellae and Giant Clams 
The giant clam, Tridacna, also harbours the same type of
zooxanthellae as do the corals. In their case though, the
zooxanthellae are only located in the mantle tissue. The clam will
expand and contract its mantle as light intensity changes, depending
on their need to eliminate ammonia (Benson, 1984). Usually those
clams with a brown mantle do not require as much light as those with
blue mantles and are generally considered to be easier to keep in the
aquarium. The blue forms represent those that live in shallow-water
areas, the blue pigment acting as a light filter just as in mushroom
anemones. Therefore, they require immense quantities of light to
maintain their bright colours. Clams are not as dependent on their
zooxanthellae as are the corals (Gordon, 1977). It is thought that
the large size attained by some clams is due to the presence of
zooxanthellae (Gordon, 1977).

Zooxanthellae and Sponges
Sponges too, contain zooxanthellae, however these symbiotic algae are
somewhat different than those found in corals and sponges. The algae
used in sponges are the blue-green Cyanophyta which possess a
red-coloured pigment that can absorb energy from the sun and pass it
on to the chlorophyll of the algae for photosynthesis (Benson, 1984).
This pigment results in many sponges being either a distinctive red-
brown or purple in colour (Wilkinson, 1984). The blue-green algae
function well at low light levels therefore many of the symbiotic
sponges live in deeper water away from the more dominant reef
building corals. In order to trap as much light as possible, many
sponges have changed their shape from spherical or tubular shapes
into dish, cup, fan, or encrusting forms, while deeper water forms
also increase the amount of red pigment to trap more light
(Wilkinson, 1984).

Light and Zooxanthellae
The zooxanthellae of hard corals are brown in colour which is the
best colour for absorbing blue light. If you have ever been lucky
enough to SCUBA dive in a tropical ocean, or have seen pictures from
these areas, you may have noticed that the water is very blue. This
is due to the absorption of the longer wavelengths of light (red and
yellow) within the first few metres of water. Therefore it is the
blue light which extends farthest into the sea (Benson, 1984). For
this reason, zooxanthellae have adapted to make the most use of blue
light. Just as in terrestrial plants, algae adapt to decreasing
levels of light (such as encountered at greater depths or under
overhangs) by increasing the amount of chlorophyll in their
chloroplasts (Benson, 1984).

In conclusion, ecological, biochemical and physiological data
indicate that symbiotic algae are of major importance in the
nutrition and growth of coral reefs. They are important not only to
the reef building corals, but also to other important reef dwelling
animals such as sea anemones, giant clams and sponges. Symbiosis
between animals and algae appears to be a highly successful
adaptation for solving nutritional problems in nutrient-poor areas
(Gordon, 1977).

Zooxanthellae in the Aquarium
From the above information, there are a few points which are well
worth noting with respect to aquarium management:

1. Try to find out exactly what species of invertebrate you are
buying and where they normally occur on the reef (i.e. shallow- or
deep-water).

2. Observe the colour of your specimens. In corals, brown colours
indicate the presence of zooxanthellae. Red, yellow or white soft
corals usually lack zooxanthellae (Sarcophyton soft corals are
shallow-water corals and need a lot of light, while Dendronephthya
are deep-water corals and do not need to be given a lot of light). Red
sponges come from deep water, or from underneath coral overhangs and
therefore do not require as much light.

3. Zooxanthellae require carbon in the form of bicarbonate for
photosynthesis, so make sure your pH is well above 8.0. Your corals
and anemones will let you know when the pH falls too low, they are
much more accurate than any test kit! They will not expand as much,
and they will begin to loose their colours due to the death of the
zooxanthellae.

4. Try not to disturb your corals too much when transporting them,
corals tend to expel their zooxanthellae if greatly disturbed
(Benson, 1984).

5. Make sure you have light which has greater amounts of blue than
red. Again your zooxanthellae bearing animals will let you know if
they are not receiving enough light. They will tend to loose colour
and, in the case of mobile animals, will gather near the surface to
get as much light as possible.


References
Alderslade, P.N., Lovell, E.R. and C.C. Wallace 1984. Corals. In.
Reader's Digest Book of the Great Barrier Reef. Mead and Beckett
Publ., Sydney.

Barnes, R.D. 1974. Invertebrate Zoology. W.B. Saunders Co., Toronto.

Benson, A.A. 1984. Symbiosis. In. Reader's Digest Book of the Great
Barrier Reef. Mead and Beckett Publ., Sydney.

Gordon, M.S. 1977. Animal Physiology. Macmillian Publishing Co., N.Y.

Wilkinson, C.R. 1984. Sponges. In. Reader's Digest Book of the Great
Barrier Reef. Mead and Beckett Publ., Sydney.