Leonard HoZooxanthellaeMarch 8, 1998

The topic tonight: zooxanthellae. I will attempt to be as in

depth as possible in the alloted time.

I found this topic very difficult to approach because of the

varying degree of knowledge amongst hobbyist about zooxanthellae.

Most everyone knows zooxanthellae are the associated algae found

within corals. But more important then what they are is how and why they

benefit their symbiotic hosts.

Actually, the scientific understanding of zooxanthellae is still

rather rudimentary. There are still many unanswered questions.

Tonight, I'll present some of the presently known facts and

leading theories regarding zooxanthellae. I shall begin with a few

fundamental backround information.

Zooxanthellae: term for any dinoflagellates which participate in

symbiosis with coral, clams, or sponges.

Dinoflagellates are unicellular organisms of the Kingdom Protista.

There are numerous different species.

Some obtain energy and carbon needs solely from ingestion of

organic sources (animal like chemoheterotrophs)

Other dinoflagellates obtain their sole energy requirements

through photosythesis and carbon through CO2 (plant like photoautotrophs).

Zooxanthellae are photoautotrophic dinoflagellates.

Zooxanthellae are brownish gold in color (best color for

photosynthetic spectral absorption). They are of genus Symbiodinium, and

unknown number of species exists.

Zooxanthellae are slightly modified dinoflagellates. They occur

independently in open water where they solely exists without a host,

carrying on photosynthesis and taking from the water whatever is required

for life processes.

Chemoreception seems to be one way of locating host corals. .Upon

entering corals, they lose their motility by dropping their two flagella.

Flagella are protein based projections attached to its cell wall which spins

like a rotor to propel the dinoflagellates in open water.

Zooxanthellae enter corals through ingestion by the coral's

polyps. Once within, they are incorporated within the second tissue layer

(the gastroderm) where they proceed with their photosynthesis. How they

evade digestion by corals is uncertain.

In coral species that produce planulae, the zooxanthellae are

passed on by the parents to offsring. In layments terms, the baby coral was

given zooxanthellae by its parents. In asexual reproduction (e.g. budding,

binary fission, etc.) zooxanthellae are obviously present in offspring.

In species that broadcast spawn, the planulae take zooxanthellae

from the water during their meroplanktonic stage or usually shortly after

settling.

The reason why broadcast spawn do not carry zooxanthellae from

their parents is the the offsprings travel great distances before settling.

Broadcasting is used for species dispersal, and because the offspring is

dispersed over varying locations, they ingest zooxanthellae that match the

light conditions of any given location.

Clams of the genus Tridacna also participate with zooxanthellae in

symbiosis, but in a completely different manner then do corals.

The digestive system of Tridacnas are highly modified/simplified

in order to accommodate this symbiosis. Zooxanthellae are taken up by

Tridacnas within the first 8 days following fertilization.

The zooxanthellae enters via the gills, then travels through a

unique symbiont channel system where they are then carried to tubules nodes

in the mantle (Norton). There is a dissenting hypothesis that states the

zooxanthellae reside in the clam's bloodlymph (Yonge).

The orientation of Tridacna clams, as I'm sure you are aware of,

is modified and very unique as well. This genus rotates its body 180 degree

with respect to its shell, always exposing its zooxanthellae-rich mantle up

towards the light.

The exact benefits of zooxanthellae are not known, but here are

the some of dominant, developed hypothesis:

Zooxanthellae provide a possible nutrient source and subsequently

facilitates the excretory processes of hermatypic corals. This is an

example of mutualistic symbiosis (a symbiosis in which both parties benefit).

Through permeases, the zooxanthellae can transport as much as 98%

of it's photosynthetic byproducts through its plasma membrane to the coral.

Permease are specific enzymes that permit certain compounds (e.g.

sugars, amino acids) to travel in and out of cells. Generally, the cell

itself regulates the importation and exportation of nutrient/waste by

activating specific enzymes to allow intake/expulsion of certain compounds.

Corals may facilitate zooxanthellae's transportation of

photosynthetic byproducts by "donating" its own enyzmes to the zooxanthellae

that make the zooxanthellae "leaky" (as described in TRA).

In this manner, corals are able to obtain the necessary components

for their sustenance and growth such as amino acids, fatty acids, and

carbohydrates from the zooxanthellae.

Likewise, zooxanthellae require certain components to carry on

photosynthesis. Aside from protection from predation, the zooxanthellae's

other main benefit is that it gathers necessary ingredients to carry on

it's photosynthesis and growth. In doing so, it consequently assists coral

excretory processes as well.

CO2 would be the first ingredient that comes to mind.

Zooxanthellae receive a constant supply of CO2 as a result of the coral's

respiration, and in turn the coral receives O2 (for respiration) from the

byproduct of photosynthesis performed by zooxanthellae.

A second hypothesized benefit of CO2 removal by zooxanthellae is

that chemical reactions are driven towards calcification in corals, thus

improving coral skeletal growth. CO2 removal by zooxanthellae is

hypothesized as the primary event responsible for increased growth in corals

containing zooxanthellae (Goreau and Goreau).

Zooxanthellae also removes phosphorous in form of phosphate from

corals, benefiting both coral and zooxanthellae. There are several

experiments done to indicate phosphate removal by zooxanthellae does occur.

Test have shown that ahermatypic corals (those that do not contain

zooxanthellae) excrete significantly more phosphate then do corals

hermatypic corals. (Yonge and Nicholls)

In a separate test, Tridacnas within a sealed container depleted

all the phosphorous within the water, while Spondylus, a clam genus lacking

zooxanthellae, did not. (Younge)

Zooxanthellae need phosphate (like true plants) to grow and

reproduce. Thus, it is self evident zooxanthellae benefit from the

phosphate corals produce as waste products.

Corals likewise benefit. CaCO3 precipitation can be shown to be

inhibited by organic phosphate. In recent years phosphate has been shown to

be a major inhibitor on the surface of crystals.

Aragonite, the crystal form of CaCO3, is therefore inhibited by

phosphate and deposition of calcium carbonate as the coral's skeletal matrix

(in form of aragonite in corals) is greatly hindered.

So, to summarize, because the zooxanthellae actively remove

phosphate from within and surrounding the coral, calcification is enhanced,

and corals, by theory and through observation, grow at increased rates.

Q: 4If you have any questions, either type /msg starfish and type

your question or double click on starfish and type your message.

Another benefit of this remarkable symbiosis is that zooxanthellae

aid lipgenesis (formation of lipids) in corals.

1/3 of the dry weight of corals and anemones is lipds, which

indicatates that it is probably a primary energy source for corals. Corals

have been demostrated to greatly elevate lip synthesis in presence of light.

Zooxanthellae use acetate (oxidized fatty acids) to form lipids.

The source of the acetate? The corals. Corals, by ingestion of food (e.g.

microplankton), intake fatty acids which they oxidize in the process of

digestion.

This acetate then converted to a narrow spectrum of saturated

fatty acids and exported to zooxanthellae as energy source for

photosynthesis, who then convert the these fatty acids to lipds which are

recycled to the coral as their energy source.

whew

So, to sum it all up in one sentence (albeit long sentence),

within this symbiosis, there exists a beautiful, complex recycling of

available nutrients between coral/clam and zooxanthellae in a nutrient-poor

reef setting, ensuring the survival of both.

This is about as tight as any relational interdependency between

animals (i.e. corals) and plants (i.e. zooxanthellae). Remarkable, wouldn't

you agree?

I tried my hardest to keep this speech nice and short. I'd like

to thank Charles Delbeek, A.J. Nilsen, Daneil Knop, and Steve Tyree for

their help. I can take about 10 minutes of study (tight schedule today).

Q: Can you tell us just how is "zooxanthellae" pronounced?

I can't. I've heard so many different pronouciations that I don't

know which is correct - LH

Q: How would one get zooxant. back into coral, clam. or upside-down

jellyfish that has lost its zooxant. algae???

by the polyps ingesting pelegic zooxanthellae and reincorporating

them back into their gastrodermis

may happen after bleaching

-LH

Q: I was wondering how polyps and the such atain zooxanthellae? I

had a group of Green button polyps die off because of PVC glue vapors so I

removed the polyps but there was a small amount of "tissue" left on the rock

and about 2 weeks later polyps started growing from the tisse and they still

have the Zooxanthellae. How did the Zooxanthellae stay in the tissue even

when there were no polyps left?

two possbilities:

your zooxanthid polyps could have retaken zooxanthellae (most

likely different strain) from your water (not likely, but can happen)

or ....

there still was zooxanthellae remaining in whatever tissue that

survived. In minute numbers, you probably didn't notice the slight brwon

coloration

these remaining zooxanthellae will assexually reproduce and

repopulate -LH

Q: What is it that balances the growth of Zoozanthellae in a reef

environment?

answer will sound simple, but like everything else, availibility

of nutrients and energy source (e.g. light, phosphate, ammonia, etc.)

-LH

Q: So, if we introduce Zooxanthellae into our water, then in theory

it should boost coral healthy by providing the corals extra algae

nutrients... and perhaps increase growth rates?

no ..... corals that already have zooxanthellae will not readily

adopt new strains. One strain of zooxanthellae may not be beneficial to

another coral host. Besides, your coral will already regulate the

population of zooxanthellae by necessity - LH

Q: How do you introduce new zooxanthellae to an aquarium? It is not

like you can go to the lfs to get it.

You can't. And there is no need to. However, when you place

different species of zooxanthellae-bearing organisms, you get possibility of

introducting different strains in your tank. - LH

Q: How long can corals/clams/etc survive without their zooxant.

algae?? in a tank situation

not long :)

actually, there is a very slight possibility of repopulation after

bleaching, but it doesn't occur often and corals will die within weeks from

malnutrition

-LH

Q: Would dispersion type corals be more inclined to loose the

zooxanthella by improper lighting?

There really isn't a "dispersal type" coral. Generally, all

corals undergo sexual reproduction (broadcasting) and assexual reproduction.

They all have equal possibility of bleaching due to improper lighting,

though different species can be more sensitve (nondependent on reproductive

means) - LH

Q: Do zoox in the water column actually seek out the host?

theorized answer: yes ... through chemorecption (chemcial "tracking")

-LH

Q: i read in an article that light intensity is more important than

spectral quality

Q: as the zooanthella is adaptable to different wavelengths, is this

true?

zooxanthellae can adapt to a wide range of spectrum (brown absorbs

just about every color, but I do not agree that intensity is more important

then spectral quality. I'd put them on equal par

-LH

Q: Slightly off-topic: Are other dinoflagellates such as the

bio-luminescent ones safe to introduce into a reef tank?

those bio-luminescnet ones are the ones that cause the dreaded red

tides. Red tide blooms can spell death

so no

-LH

Q: are there zoox. floating around in our tanks?

like all plankton, yes, but I'd would not assume a very high

population exists in open water

-LH

Q: are zooanthalae less prominent in a brighter colored coral as

opposed to a darker (brown)?

a simplified response would be yes

darker pigmented corals usually come from deeper waters where they

are required to collect as much light as they can to survive

in shallow water species (with bright colored protective

pigments), too much zooxanthellae can result in oxygen poisoning within

tissue, etc. -LH

Q: How many different species of zooxanthellae are there?

unknown as of this time -LH

Q: If corals can regulate thier zoozanthelle does it matter what

type of lighting we use on our tanks

Yes, because certain spectrums are better recepted and used for

photosythetic purposes. Of course, intensity matters too. -LH

Q: If corals are heavily fed will this affect their zoox population?

I have not read of any experiments on this subject, but I would

give a hestitate no answer - LH

Q: Essentially, are you describing a situation that can be observed,

but is more or less beyond our control to regulate at the hobbyist level?

right or wrong?

There are few things we as hobbyist can do. Correct lighting

schemes, nutrient additions, water chemsitry maintanence, etc. The

zooxanthellae do their own stuff, but it is definately not out of the

hobbyist's hands - LH

Q: What lighting do you use in your tanks?

I always use intense halide lighting (my preference). 400W 6,500K

or greater Kelvin rating. Actinics to "color" up corals

lower wattage MH are fine too - LH

Q: In a coral that shows a lightening (not bleaching) in low-light

areas due to reduced zoox density, re-populate those areas if lighting was

increased in those areas?

corals usually don't lighten up in low light situations ....

interesting. I would not direct more light at it if it is lightening up,

becuase logically, to lose pigmentation means it is self regulating because

it had too much. Then again, there are numerous water parameters that could

lead to lose of coloration. Difficult to answer -LH

Q: Should we put darker corals in less light? Or will more intense

light cause Zooxant. from deep-water corals any problems??

darker (brown) corals usually (a dangerous generalization) do not

need as much light. More light will not cause zooxanthellae any problems,

but the coral may suffer from superoxidation within its tissue if exposed to

too much light due to zooxanthellae "overproduction"

acclimate slowly - LH

Q: what are best light spectrums?If using VHO, what mix of

bulbs/wattage?

Depends on the animal :) where it was collected, etc. I really

can't answer that one and feel comfortable with my answer

-LH

Q: I have 2 40w actinics and 2 20w triton tubes over a small tank,

40g, would adding 2 extra daylight tubes benefit?

anytime you add more light - gradually - would benefit. -LH

Q: I have a particular colony of acropora on which the lower (less

lit) branches are paler than the upper well-lit branches. Is this indicative

of a problem?

no. Nutrient transport within a colony of corals will regulate

the differences. You have trouble if the bottom starts receding, in which

case you don't have enough light intensity -LH

Q: Will toxins affect Zooxant. or corals first??

I can answer only about 3 more (I have an engagement at 6)

depends on the toxin, but usually coral first (dino's are not as

complex and senstive an organism) - LH

Q: After adding a 400 watt 6500k mh to a vho system, a flourescent

green nephia does not open like it use to is this normal what can be done

to help it?

too much light..... the coral is responding to this sudden

increase in light by deflating so as to reduce surface area exposed to

lighting. Reduce photoperiod or move coral to lower location -LH

Q: are pcs as good or better for corals if compared watt and

spectrum over vho and mh?

can you ask me another question :) I don't like lighting debates.

PC's are efficient, I'll say that much, but MH is still the most intense

lighting available on the market. -LH

Thanks for the talk Len!