AUSTRALIAN FRONTIERS OF SCIENCE, 2005

Walter and Eliza Hall Institute of Medical Research, Melbourne, 12-13 April

Session 7: Discussion

Question – My question concerns the mechanism of ‘expelling’ these things. What do you mean by expelling? What is binding them in the first place, and what breaks down? Are they actually physically expelled, or just let loose and wafted away by the current? What is known about this expelling?

Peter Ralph – There are anything up to about six different mechanisms for the expulsion of the zooxanthellae. Some expulsion is a very, very violent and host-controlled response which rips the cell apart. The zooxanthellae are encapsulated within five membranes, so a zooxanthella is not close to the water. It is like china dolls: wrapped, wrapped, wrapped and wrapped. So for the zooxanthellae to be expelled, most times, there is actually a very violent response of the host that just rips the tissue apart.

There are a number of mechanisms that a group in Hawaii are looking at, trying to define how they get out. Some of our data suggested that the more violent the expulsion, the more damage there is to the zooxanthellae. If I can see a single cell and I can see tissue on the side, which is called symbiosome tissue, I can guarantee you that that cell will be dead. If I see a bare, cleanly expelled zooxanthella, however, the chances are that its health is very good.

Under the microscope you can look at a branch of the coral and, literally, see explosions. You see this puff of tissue coming off, at that scale. So it is a violent response, not a passive response.

Question – I was curious, Madeleine, about the D clade. Have you actually tested the performance of the D clade at cooler waters, at 27° in Keppel? I was also curious about whether you can actually partition the effects of the zooxanthellae against the corals, and whether there is a single species where you might be able to compare different populations versus different zooxanthellae, and do a complete comparison between two components of that presumably adaptive shift.

Madeleine van Oppen – To answer your first question: no, we haven’t looked at the performance of clade D at colder temperatures. But there is one published study, I think, from Taiwan where there is a correlation – they find a high abundance of clade D zooxanthellae in cooler waters. Clade D seems to be a bit more of a weedy type, but then there might be multiple species within this D clade, so there is a lot of resolution that we haven’t explored.

The host factors will definitely be very important in the bleaching response, and there might be population differences in the hosts as well. We have been focusing on the zooxanthellae because the techniques are available to do those kinds of experiments, but also we have started to explore host factors, using micro-array experiments, to try to get a handle on which genes are involved in the bleaching response and whether there are expression differences between different populations, maybe allelic variation. So that is a field that is just taking off, but it is a bit lagging behind understanding the role of the algal symbiont to the bleaching response.

Question – How robust are the zooxanthellae when they are outside the coral? If they are wrapped in five layers of membrane, presumably once they are in the coral they are pretty fixed where they are. Where are they breeding, how are they breeding and what’s happening outside?

Peter Ralph – That’s the $64 million question: finding them. There is a lot of work that has been done to try and identify zooxanthellae in the field. We can’t find them. We assume they are in the sediment. They are not in the water column. We know that clams, every day, are spewing out large numbers of very healthy zooxanthellae. Where they go, we don’t know. Where these bleached ones go, the ones with quantum yields of 0.6. They have been collected; I’ve had zooxanthellae that I’ve collected and kept alive just in a petri dish for six or seven days, without any nutrients, without anything useful. We’ve actually got a colleague that we sent some expelled ones to and she has cultured them up. So they leave, but where they go, nobody knows.

Madeleine van Oppen – There is one report of free-living zooxanthellae being found in the sediments. They are fairly benthic, I think. If you culture them, you don’t see them free in suspension; they are always attached to the walls of your culturing dish.

Peter Ralph – They’ll grow there fully in gel and back within three or four days. They return to their native state very quickly.

Question – I have two questions. One is linked to that. Are they actually reproducing inside the coral tissues, as far as you know, or do they need to be…

Madeleine van Oppen – Population genetic studies suggest that no, they are actually haploid inside the coral tissues, and there is very little evidence for sexual reproduction inside the host tissues. But they must have sexual reproduction, because the genetic variation in populations suggests that sexual reproduction does occur – but probably not inside the host tissues.

Question (continued) – The other part of the question is related to what Peter was saying about the expulsion, and the ripping off of parts of the polyp tissue to expel these zooxanthellae. In terms of recovery, are the polyps just basically growing new tentacles? How are those corals actually physically taking up the D clade, for example? Are the individual polyps dying, or is the colony surviving by generating new polyps after the expulsion?

Peter Ralph – No. Maybe I painted it too violently. The tentacles survive; the tissue generally will survive for a month without zooxanthellae. So there is damage to the tissue, yes, but the host isn’t decimated. Provided it is not damaged over the next month, it has the capacity ‘somehow’ of re-establishing its population. So it is not as violent as I described, sorry.

Madeleine van Oppen – You should also realise that even if the coral is bleached totally white, it still has zooxanthellae in its tissues. It is just that you can’t see them externally. I suspect that repopulation is usually from remaining zooxanthellae rather than uptake anew, but that remains to be proven.

Question – Is the host coenocytic, or is a lot of different cells? Why I am asking is that you mentioned that some hosts are mosaic, in terms of the zooxanthellae. I was just wondering whether that represents, perhaps, a mutation or segregation of genetic factors in the host organism, and whether or not you might expect adaptation to involve selection of better partnership – co-evolution, if you like, or co-adaptation of the tissues – after an event.

Madeleine van Oppen – They are not multinucleate cells. They are just cells with one different cell to the host tissues.

There is little evidence of co-evolution of the zooxanthellae and the host, at least at the level of those clades, although when we look at more recently evolved species and strains there seems to be more correlation between the identity of those two symbiotic partners, so there might be some evidence for co-evolution.

I think the problem is that in many corals the association is not heritable, because in every generation the coral will need to acquire new symbionts. So I think it is unlikely that there is very strong evolution in the sense that you are describing.

Question – This is more of an ecological-type question. What is the relationship, if any, between the frequency of disturbance, such as these bleaching events, and the diversity of corals on the reef? Does anyone know that?

Peter Ralph – I think we are speculating that we are going to see structural changes to the community composition on the reef. That is probably the main and most likely scenario we are going to see in a very short period of time, if particular species are more threatened than other species.