Zooxanthellae are single-celled algae that live symbiotically inside the tissues of corals and other invertebrates. They produce food for their hosts through photosynthesis. This mutualistic relationship is essential for coral reef formation, as it allows corals to grow skeletons and build reef structures. However, environmental stresses like high temperatures can cause corals to expel the zooxanthellae, leading to coral bleaching. While bleached corals can sometimes recover by regaining their symbiotic algae, severe or prolonged bleaching often damages or kills coral reefs.

2. What are zooxanthellae?
Zooxanthellae

3. • Zooxanthellae are
single-celled plants
that live in the tissues
of animals. These
organisms are part of a
group of
dinoflagellates that are
most often found as
plankton.
(phytoplankton)

4. • Like most plants, phytoplankton are
able to convert the sun's energy into
food through a process called
photosynthesis, so to survive they
are only found in the upper layers of
the sea and lakes where sunlight
can penetrate.

5. Zooxanthellae in Gastrodermal Cells of
Corals
"Zooxanthellae are very small that they can only be seen through
electron microscope"

6. Zooxanthellae:
Symbiodinium microadriacticum

8. Zooxanthellae is best known for a
mutualistic relationship they
have with reef building corals

9. The coral reefs have formed as the result of the
special symbiotic relationship which exists between polyps and
the zooxanthellae inhabiting their cells.

10. Corals provide a protected environment and
compounds needed for photosynthesis to
zooxanthellae.
In turn, zooxanthellae provide food as products
of photosynthesis to coral.
This gives corals a boost of nutrients, so they can
secrete the calcium carbonate skeleton that serves
as the foundation for coral reef.
This important relationship represents a highly
efficient exchange of nutrients in a nutrient-poor
environment.

12. Benefits to the Coral Host
• Higher rates of calcification
(skeleton deposition).
• Energy and materials for
growth, repair, and
reproduction.
• Removal of chemical wastes
from animal metabolism
(nutrient recycling).
• Receive chemicals that absorb
damaging ultraviolet light.

13. Benefits to the Algal Symbiont
• Chemical wastes from animal
metabolism are important
inorganic nutrients for plants
(nutrient recycling).
• Surrounding animal tissues
can concentration substantial
amounts of ultraviolet light
absorbing compounds.
• Zooxanthellae, nonmotile
single cells, get a place to live
in the crowded reef
environment.

14. Corals are not the only hosts of
zooxanthellae. Some other
reef hosts include:
• Anemones
• Giant clams
• Gorgonian corals or sea fans
• Soft corals
• Sea whips
• Nudibranchs
• Jellyfish

15. Zooxanthellae live in other protozoa (foraminiferans and radiolarians)
and in some invertebrates.

16. Transmission of Zooxanthellae into
Host Tissues
• Maternal (direct) transmission.
• Environmental (indirect)
transmission

17. Maternal Transmission

18. Environmental Transmission

19. •Polyps can acquire Zooxanthellae by direct ingestion. However, their hosts do
not digest them. In other cases, zooxanthellae may be transmitted by coral eggs
and planulae. Zooxanthellae reproduce asexually by budding (one individual
splitting into multiple descendants.

21. Healthy Bleached
coral coral
Corals bleaching occurs when environmental stress
adversely affects the mutually beneficial relationship
between the host organism and its zooxanthellae.

22. • Coral bleaching occurs when zooxanthellae
densities within coral tissue become low or
the concentration of photosynthetic pigments
within each zooxanthella decline.
• Color loss also comes from reduced
concentrations of pigments produced by the
cnidarian itself.

23. What can stress a coral?
- High light or UV levels
- Cold temperatures
- Low salinity and high
turbidity from coastal
runoff events or heavy rain
- Exposure to air during very
low tides
- Major: high water
temperatures

24. Thermal stress
- Corals live close to
their thermal
maximum limit
- If water gets 1 or 2 C
higher than the
summer
average, corals get
stressed and bleach

25. Healthy Bleached
coral with coral with
algae no algae
• - As a stress response, corals expel the symbiotic
zooxanthellae from their tissues
• - The coral tissue is clear, so you see the white
limestone skeleton underneath

26. Can corals recover?
- Yes, if the stress doesn’t last
too long
- Some corals can eat more
zooplankton to help survive
the lack of zooxanthellae
- Some species are more
resistant to bleaching, and
more able to recover
- Corals may eventually regain
color by repopulating their
zooxanthellae

27. Thank you