Products exchanged between corals and zooxanthellae
Symbiodinium is a group of symbiotic unicellular algae that live in the tissue of corals and provide the energy necessary for building large reef structures of calcium carbonate skeletons. These photosynthesizing dinoflagellates, also known as zooxanthellae, reside in vacuoles within the gastrodermis of coral at densities of about 1 million cells per cm2. Symbiodinium is comprised of a number of phylogenetic divisions, or clades, (A, B, C, and D are the most common) separated according to genetic differences1.
Corals receive 34%-100% of their fixed carbon (sugars) from Symbiodinium in the form of glycerol and other simple sugars2, 3, 4 and may receive succinate and fumarate depending on the species of coral (or anemone) host5. (Anemones are related to corals and some harbor Symbiodinium.) Symbiodinium also provides the corals with lipids, proteins, and amino acids6. In return, corals serve as an ammonium, carbon dioxide, and phosphate source for the Symbiodinium. Ammonium and carbon dioxide, in particular, are typical animal waste products, while Symbiodinium, like all photosynthetic organisms, use carbon dioxide in photosynthesis.
Costs and benefits to algal symbiosis
|Benefit to Corals
Corals benefit from this relationship by gaining a source of sugar, oxygen and nutrients, so that they have more energy to put towards reproduction and calcification.
|Cost to Corals
In order to harbor productive algae symbiotic corals must live out in full sun and so must spend energy defending against UV stress and reactive oxygen molecules produced by the photosynthesizing Symbiodinium. There is also an energy cost associated with regulating algal populations in their vacuoles.
|Benefit to Symbiodinium
Corals supply carbon dioxide and nutrients, and provide a structure in which they can reside in full sunlight. This structure also provides protection from UV and a place in which they can asexually divide to develop and maintain a population.
|Cost to Symbiodinium
A significant amount of energy is translocated to the host. For example, samples of the coral Porites porites on a sunny day at 10m depth translocated 78% of the sugars they made to the coral host7. Their growth rate is also regulated by the host and they may be expelled by the host.
Summary of photosynthesis
Symbiodinium photosynthesize, which means they convert light, carbon dioxide (CO2), and water into sugars (glucose) and oxygen (O2), as described in the formula below:
Photosynthesis takes place within chloroplasts inside Symbiodinium (see figure 3). Photosynthesis is comprised of a light reaction and a dark reaction. In the light reaction, photosystem II (PSII) absorbs a photon. This photon excites an electron, and the excited electron is carried down the electron transport chain and “recharges” molecules such as ATP and NADH (Figure 4). ATP and NADH are used to convert carbon dioxide into sugars in the dark reaction. The light absorbing reaction center, PSII, is located in the thylakoid membrane (shown in figure 2), where it can pass electrons to enzymes that increase the pH on one side of this membrane. An ATPase then uses this pH gradient across the thylakoid membrane to “recharge” ATP.
In the presence of free amino acids within the host, Symbiodinium releases extra sugars to the coral and increases its photosynthetic rate8.
Measurements of coral photosynthesis: Pulse-amplitude modulated (PAM) fluorometry
The capacity for photosynthesis in a coral can be found by measuring oxygen production during photosynthesis or with pulse-amplitude modulated (PAM) fluorescence. Traditionally photosynthetic activity was measured as the amount of oxygen evolved by subtracting the amount of oxygen present in lighted conditions minus the amount consumed in the dark via cellular respiration.
More recently, chlorophyll fluorometers have become inexpensive and simple to use. Chlorophyll, the molecule that absorbs light for photosynthesis, can either use the absorbed light energy in photosynthesis or can re-emit that light as fluorescence. A fluorometer measures the maximum capacity for photosynthesis by comparing fluorescence before and after a flash of light. This method is usually carried out in the dark because in the sunlight, most of the chlorophyll molecules will already be used for photosynthesis, resulting in a reduced measurement of photosynthetic capacity. This technique has been optimized by the use of PAM, a signal-processing method of isolating light of specific frequencies to eliminate some of the “noise” from background signals.
To measure photosynthesis in corals, a sensor on a PAM fluorometer sends a flash of light and measures the fluorescence right at the surface of the coral. (For more information, see Maxwell and Johnson 2000)9.
General differences between autotrophy and heterotrophy
Corals have two primary ways to obtain nutrients. One way is autotrophy. The other way corals obtain nutrients is through heterotrophy (eating other organisms). Autotrophy provides the corals with glucose, which is a quick form of energy, while heterotrophy provides essential nutrients such as nitrogen and phosphorus which cannot be obtained through photosynthesis10.
What corals eat
Corals generally feed on organisms that vary in size from 0.2 µm to 4500 µm11. Corals are capable of grabbing organisms of greater size, but that appears to happen very rarely. There is a variety of zooplankton that fall within the general size range. Some examples include species of Copepods, such as members under the genus Macrosetella and Saphirella, and some species of Isopods. A study by Sebens et al. (1996)11 showed that corals can selectively choose which type of prey they capture.
The Polyp Structure and Its Role in Feeding
A single coral is made up of a large colony of separate structures known as polyps. Each polyp is connected to one another through the gastrovascular canal, which is used to exchange materials such as nutrients between polyps. The structure of each polyp includes an oral disc that acts like a mouth, a row of tentacles that surround the oral disc, and a gastrovascular cavity that is connected to the oral disc. Food is partially digested in the gastrovascular cavity and then sent to the gastrovascular canal.
The size of the polyp varies between species. The factors that influence polyp size include water flow and the food the coral feeds on. Most corals feed predominately at night because they photosynthesize during the day. However, the photosynthesis process does not preclude the coral from doing some heterotrophic feeding during the day.
How Corals Capture Their Prey
Food is captured by the individual coral polyps in a multitude of ways. One technique is called sieving, which works best for food that is bigger than the space between the individual tentacles. Basically, large food particles are caught by the tentacles working in tandem and pulled into the oral disc12. Another technique involves scanning and trapping, where each tentacle independently captures food and brings it to the oral disc. A third technique is called mucosal capture. Mucosal capture involves producing a web or net of mucus which is used to capture food particles as they float by. Then the corals use the cilia on their tentacles to pull the mucus net into their oral disc for digestion. A fourth technique involves direct absorption of dissolved organic matter13. Several of these feeding techniques involve the use of an organelle called a nematocyst. This interesting organelle fires a microfilament that has a spine located at its end. This spine can contain toxins that sting or paralyze the attacked prey. A change in osmotic pressure triggers a nematocyst to fire. These nematocysts sting or paralyze the coral’s prey in order for them to more easily maneuver the food to the coral’s oral disc. Nematocysts are also used for purposes other than feeding, such as defense.
The Benefits of Heterotrophy
Heterotrophic feeding is vitally important to corals. A study by Houlbrèque and Ferrier-Pagès (2009)14 showed that corals that feed heterotrophically have an increase in calcification, photosynthesis, and protein concentration. Another study by Grottoli et al (2006)15 showed that heterotrophic feeding can help corals be resilient to bleaching.
While some corals are ahermatypic (they do not form large reefs and lack zooxanthellae) and must get all of their nutrition heterotrophically, and there are plenty of dinoflagellates that are not in symbiosis with corals (e.g. those that cause redtides are free-living), the relationship between corals and their zooxanthellae is such that both are greatly benefitted by one another for certain metabolic functions.
Although photosynthesis and coral feeding improve each other, corals with better light capture ability are not necessarily better feeders. Figure 10 shows how corals with higher surface area to volume ratios (the better light capturers) tend to have smaller polyps (so they cannot catch as many food particles per polyp16. This relationship of polyp size and food capture does not hold in all cases because prey are able to avoid and escape capture11.
The benefit to zooxanthellae
Through heterotrophy, or coral feeding, photosynthetic rates can theoretically double14, because the number of zooxanthellae in a coral can increase, and so can the amount of chlorophyll (the green pigment required for photosynthesis, also found in leaves) and other pigments within each zooxanthellae cell. The zooxanthellae live within corals because corals provide a mostly controlled environment in which the algae can get nutrients for photosynthesis and physical protection within the coral itself.
The benefit to corals
Corals receive about 78% of the photosynthetic products produced by zooxanthellae7. The products include glucose, glucose-6-phosphate, glycerol, lipids, and some amino acids, including citrate, fumarate, aspartate, glutamate, glutamine, and lysine5. But these materials do not just diffuse between the coral tissue and the zooxanthellae; amino acids within the coral tissue stimulate the release of particular nutrients/photosynthetic products from zooxanthellae8.
Energy budgets and coral bleaching
Corals do not receive nutrition in equal amounts from their zooxanthellae and feeding efforts. In normal light conditions, photosynthetic products make up most of the energy provided for the corals, because they produce 140% of the carbon required by corals for daily maintenance and growth. Yet in the shade, zooxanthellae only produce about 60% of this amount, and corals must increase feeding to obtain the rest4.
When corals bleach, they lose most of their zooxanthellae. They do not necessarily die right away, but their ability to survive decreases after long periods with reduced zooxanthellae numbers during stressful environmental conditions. Some corals are able to make up for the loss of zooxanthellae by increasing their own feeding rates, but others don’t, continuing to rely on the limited photosynthetic products. Montipora capitata is an example of a coral that will increase heterotrophic input to its energy budget when bleached, but Porites compressa and Porites lobata will not15. The corals that can increase feeding when they are bleached are more likely to recover from bleaching events and remain growing on coral reefs. In the long run, however, corals depend on photosynthetic products from the zooxanthellae, because most of their energy comes from autotrophic rather than heterotrophic inputs17.
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