<a href=showthread.php?s=&postid=12349742#post12349742 target=_blank>Originally posted</a> by Meisen
MCsaxmaster, you make some good points and well-explained ones at that. Brought back a little Biochem rush for me. But I have questions! So agreed that photsynthesis is basically an instantaneous process (for our purposes here) and that probably, for most folks corals, their zooxanthellae get cooking right away. I don't know nor have I read more on how we get from Zoxanthellae producing sugar via photosynthesis to coral growing. Books say that the algal cells are "leaky" and leak sugar to their host. Is there more to it than that?
Good question. Yes, there is more to it than that, including significant remaining unknowns.
Zoox. produce fixed carbon in a variety of forms, just like all photosynthetic critters. One of the major products of photosynthesis is glycerol. All algae tend to be "leaky" and free-living dinoflagellates may lose up to 1/2 of their daily photosynthate due to accidental leakage (though usually much less than that). However, zoox. in hospite translocate far more than that percentage of their photosynthate to the corals. Depending on the methodology, folks have found anywhere from ~75-98% of the photosynthate produced by the zoox. is translocated to the corals. That is, to say the least, a lot. Imagine spending 75-98% of your income on rent
This high rate of translocation really can't be explained just by accidental leakage from the zoox. into the coral tissue. Rather, it seems inescapable that the corals are somehow inducing the zoox. to fork over their photosynthate. There's good evidence now that the so-called host release factor is dissolved free amino acids within the coral tissue. What seems to be happening is the coral host maintains the zoox. in an environment that appears to be desicating to the zoox. One of the responses to this by algae is to dump photosynthate so as to encourage osmosis (better to dump some food and live than hold onto it and die). Thus, the zoox. dump out most of their photosynthate into the coral tissues.
Most of what gets translocated is glycerol. There's very little nitrogen translocated, but the little bit that does get translocated is usually as alinine. Thus, the zoox. provide the coral with a whole lot of simple carbohydrates, but very little else nutritionally.
Of the photosynthate translocated to the coral, on average about 50% of it is lost daily as mucus. In other words, it doesn’t directly contribute to the metabolic needs of the coral, but is used for another important biological function. The other half of the photosynthate is used to fuel metabolism or stored as lipids for later use. These processes are governed by basic biochemistry, and are fairly similar amongst most organisms in numerous ways (Krebs cycle is Krebs cycle).
The proportion of the metabolic needs contributed by photosynthate varies from coral to coral depending on species, lighting conditions, stress, etc. Most healthy corals in sufficiently bright light (e.g., upper 20 m of the water column in clear water) can meet ~70-100%+ of their metabolic needs from translocated photosynthate with the rest coming from prey capture and uptake of dissolved organics.
Now, a point that I think is important to stress is that a coral that receives 100% of it’s daily metabolic needs in photosynthate and nothing else is in a losing battle. Remember, typically half of what the coral receives is uses to make mucus, and thus loses. To achieve mass balance, most corals would have to receive 200% of their daily metabolic needs in C to account for that (half to metabolism and half to mucus production). However, that doesn’t take into account C needed for tissue growth, for reproduction, defense, or repair of injuries Those are all significant demands. Thus, just to maintain good health by growing, reproducing, repairing injuries, producing mucus, and fueling normal metabolism most corals need on the order of 200-300% of their daily metabolic needs to cover all their bases.
If we consider the rate of C fixation by zoox. and subsequent translocation we see that most corals in shallow water can fix greater than 100% of their daily metabolic C needs, but less than 200%. Thus, most corals, even those given plenty of light, have to rely on prey capture and uptake of dissolved nutrients (prey capture being the more important source) just to break even and continue doing well. Even under the best case scenario, zoox. photosynthesis simply is not enough to cover all the bases most of the time.
We’ve also said essentially nothing about N or P thus far. Photosynthate is very poor in N (and P), as mentioned. It’s a bit like eating cotton candyâ€"it doesn’t cut it if that’s all they get. In order to get N and P for tissue growth, reproduction, tissue repair, etc. the majority of it has to come from prey capture and uptake of dissolved substances (prey capture being the more important source). For most corals, better than 50% of their N comes from prey/dissolved organics, and for many it is better than 70%.
Thus, a heck of a lot of the C needs the coral has are met by the zoox., but usually not all of them. Little of the N needs are met by the zoox. and instead that is mostly obtained from prey.
Photosynthesis enhances calcification in corals, but it isn’t entirely clear why. It may be that simply providing energy (since calcification is energetically expensive) is the key to this enhancement, but based on physiological work including timecourses of calcification in response to changes in light, it seems unlikely that this is the only factor playing a role. Since calcification is energetically expensive, it also consumes a great deal of O2. Zoox. produce O2 like mad in the light. In fact, the concentration of O2 in tissues can reach well over 3x the value in seawater in the light. The concentration can rise so high, and thereby encourage the production of reactive oxygen species, that symbiotic corals have greatly enhanced mechanisms to deal with this excessive O2 during the day (e.g., SOD). This does help ensure that O2 does not become limiting to metabolism/calcification during the day though. From what I’ve heard, a friend of a friend has found higher rates of dark calcification just by artificially bumping up the concentration of O2 in sea water. The zoox. might also be modulating the internal chemistry of the coral polyps in a way favorable to calcification. By removing alkalinity from the water within the coral polyps (for calcification) the pH falls, and in fact it can fall a lot (down to ~7.5 inside the gut in seawater of pH = 8.2 is common). Zoox. take up CO2 and fix it which raises pH within the coral polyps (pH~8.5 inside polyps compared to pH = 8.2 in SW is common). This may encourage calcification by creating a more hospitable environment in which to calcify. There are a few other ideas that have been proposed (e.g., uptake of nutrients), but those seem somewhat less likely IMHO.
There are still gaps in our understanding. We’ve been asking why photosynthesis enhances calcification for at least 50 years and still don’t have an unambiguous answer, though we are gaining a lot of ground. Suffice it to say that it does, but photosynthesis alone is not enough for healthy corals (or even just live corals).
<a href=showthread.php?s=&postid=12349742#post12349742 target=_blank>Originally posted</a> by Meisen
It seems like you sort of dismiss the idea of doubling days out of hand.
Yup. In order to take a new idea seriously the onus is on the person making the proposal to make a convincing argument as to why it is a legitimate possibility. It has to be consistent with existing data. If that is the case, then there is good reason to test the idea. If, however, the idea is inconsistent with existing data (which this one is) and there is not a convincing argument for why this is even a possibility (I see none here) then I have to reject the idea as a bit of a wild goose chase.
Anybody could propose anything as an explanation, but without good reasons to take some ideas seriously, we’d get nowhere. For example, I could suggest that adding marinara sauce to a tank might increase coral growth, or dropping in a magnet, or using an air filter in the room, or using spring water, etc. ad nauseum. There is an infinite number of things we COULD test. Just because something has not been conclusively ruled out as important, doesn’t mean that it is. Does the price of tea in China affect the growth rate of corals in our tanks? We can’t say it definitely doesn’t because it’s never been tested, but that is a lousy rationale for deciding which ideas to take seriously.
<a href=showthread.php?s=&postid=12349742#post12349742 target=_blank>Originally posted</a> by Meisen
But photosynthesis isnt the way corals grow, at least directly. There is more to it, right? I mean when does physical growth occur?
Growth is always happening, though the rate varies depending on a variety of conditions. For instance, corals in nature typically grow very slowly around the time they spawn, though conditions are otherwise usually good for growth.
<a href=showthread.php?s=&postid=12349742#post12349742 target=_blank>Originally posted</a> by Meisen
Doesnt it involve building of skeleton, capture of food, adding tissue etc?
Building skeleton and tissue yes; capturing food no. Food capture is necessary for building tissue and skeletonsâ€"it is a necessary precursor, not an end product.
<a href=showthread.php?s=&postid=12349742#post12349742 target=_blank>Originally posted</a> by Meisen
When and how do these processes occur?
Phew, that’s a big question. Could you please narrow that a bit? Tissue growth and skeletal growth are constantly ongoing, though the rate of both varies, depending…
<a href=showthread.php?s=&postid=12349742#post12349742 target=_blank>Originally posted</a> by Meisen
Agreed that the corals use the sugar produced by the zooxanthellae to fuel some of their life processes but beyond that I am not clear on what all happens. Perhaps you know more and can share?
Hope this helps
