Teachers Notes - Dr Nicole Webster

Sponge symbiosis stressors

For your PhD thesis, you looked at the symbiosis between Great Barrier Reef sponges and bacteria, and how that symbiosis changed with stress. Can you explain for us what you mean by ‘symbiosis’?

There are multiple definitions for ‘symbiosis’, in its loosest possible definition symbiosis is a consistent association between two different organisms. That doesn’t have to imply any sort of benefit to either partner. When people think of symbiosis, they mostly think its two things living in association with each other where both of the partners benefit. For example, if there is a microbe living inside a sponge or a coral, the microbe gets some sort of protection from the surrounding environment and the coral gets some sort of nutrition from the microbe. When most people talk about symbiosis, that is the sort of relationship that they’re thinking of.

But the one that you’re talking about is coexisting without necessarily any detriment?

In that sense, pathogenesis or disease is also considered symbiosis, in the loosest possible definition. With the sponges that I work on now, I’m looking at the whole cross-section: all the microbes that are in there, the aspects of disease and the consistency of the relationship. I think that is probably how we define symbiosis: how stable that relationship is. So, if you change the environmental conditions, or look at it over a really broad geographic range, and find that relationship is identical over those sorts of gradients, to me that implies a really consistent relationship and a true symbiosis.

With the sponge microbiology that we do, it has taken a very long time to try to describe what microbes are present in sponges and how consistent the relationships are, and it is only now that we are really starting to look at what function the microbes might have. For example, whether they provide the sponge with nutrition, structural rigidity or maybe even metabolising some of the waste compounds from the sponge. There are many possible functions that could be happening, but we’re really only on the tip of the iceberg of exploring those now.

Is a sponge a plant or an animal?

In fact, I was just asked that question a few minutes ago by one of the scientists downstairs. Right up until the 1700s, sponges were considered plants because you were only an animal if you were sentient and were you capable of muscular response and movement. In about the mid­1700s, a couple of scientists contradicted that and described sponges as animals. They definitely are animals, but they are the lowest of the metazoa, so they’re the most ancient of the multicellular organisms.

What did you discover in your PhD studies? How did the relationship between microbes and sponges change?

When I started my PhD, it was really interesting. Sponges at that time were considered quite sexy research topics because there was a lot of research happening around drug discovery. And sponges produce almost all of the compounds from the marine environment that have made it into clinical trials. They produce a wide range of anticancer compounds, anti-inflammatory compounds and anti-tumour compounds. Part of the reason for this is because they are sessile—they just sit on the bottom; they can’t move and they can’t escape—so, as a natural deterrent, they produce these really nasty compounds. For a sponge researcher, that was fantastic because all of a sudden drug companies were investing large amounts of money into sponge research.

I was not so interested in the application of the chemistry; what I really wanted to know was what happens with the relationship. I was more interested in looking at the symbiosis, but I was able to use some of those funds. I looked at one of the sponges that produce some anticancer compounds and I looked at what microbes were inside it. This research was based on the idea that maybe some of the microbes were producing the interesting drug compounds. When I started looking, I found that the microbial associations were really broad and that they were really consistent over a really broad range. I looked right from the top of the Great Barrier Reef almost towards the bottom of the Great Barrier Reef and I saw the same associations over that massive geographical gradient. That made me think that there was something really specific about the partnership between the sponge and the microbe.

In the latter parts of my PhD, I wanted to look at how environmental stress might affect the relationship. In the last six months of my PhD I started a fairly large experiment that exposed the sponges to heavy metals—something that may happen in the environment from industry—and had a look at what happened to the relationship. What I found then, directed all of my future research. I found from that project that the symbiotic partnership—the relationship between the microbes and the sponges—actually broke down just before we saw signs of stress in the sponge. So that gave me an indication that maybe they’re very sensitive indicators for stress and maybe, if we can detect the stress prior to seeing the signs of stress in the animal itself, we then have a better opportunity to conserve them.

What sorts of experiments did you do?

One of the amazing things about sponges that almost nobody knows is that all of the cells in a sponge are totipotent, which means that they can differentiate into any other cell type. With a skin cell or what we call a ‘pinacoderm cell’ in the sponge—if you cut that sponge, other cells, cells that might be involved in water flow or nutrition, can actually migrate to the site where the sponge has been damaged and can turn into new skin cells. Basically, they are effectively stem cells. That means that we can take one sponge and chop it up into 100 little mini-sponges and leave them out on the reef to heal for a month or so, while they form a new skin surface. Then we can bring them back into the lab and we’ve got a whole heap of replicates that we can use that are genetically identical, and we can form those sorts of experiments there. Then we keep them in aquaria and use a dosing facility; so we have a header tank with the contaminants or whatever and we just drip them into the tanks.

Focus questions

• What is the definition of an animal? What is the definition of a plant? Is a sponge a plant or an animal? Why?
• What is totipotency? For Dr Webster’s experiments, why is it useful that sponge cells are totipotent?