As you know from our former WOW web entries, macroalgae and the question of whether or not they are using chemicals to defend themselves against predators are major parts of our project down here at Palmer Station. But how do we find out if defensive chemicals are active in macroalgae?
First, we have to know which organisms are potential predators of macroalgae and then we have to design laboratory experiments to test if these organisms eat or reject the different algae. We call these experiments bioassays. From our observations during diving and previous work we know that fish and amphipods are major consumers of macroalgae in Antarctic waters and that sea stars have also been observed feeding on algae. Chuck reported about the bioassays we conducted with amphipods, and Andy wrote about the sea star assay.
The fish bioassay is done with an abundant fish species with the scientific name Notothenia coriiceps that lives in the dense macroalgal belt. From my previous studies I know that nearly 40% of the diet of this fish consists of macroalgae.
We caught the fish right off the station using fishing poles, and Andy even caught one by hand during a dive. We let them rest for a few days to get adjusted to their new environment in a large outside tank and to the fact that I would feed them by hand as in the assays. Each day I went there and offered them snails or worms with a pair of tweezers, so they would get used to me and the new way of feeding. Some fish adapted easily to the new situation and came swimming up to the surface for their meals whenever I showed up.
Then I transferred twelve fish into large divided table tanks so each fish would be in a separate compartment. In an assay, I offered each fish a ďtreatmentĒ, which is a piece of the macroalga that we want to test and noted whether the alga was eaten or rejected. After that, each fish got a ďcontrolĒ which is a piece of alginate (see Chuckís Journal on amphipod bioassays ) that contained ground algae as a feeding stimulant. If the fish rejected the treatment but ate the control, I knew that they disliked this particular algal species. If they also didnít eat the control, it might be that the fish was not hungry (or just didnít want a vegetarian meal at that moment).
From the algal species which were not eaten by the fish, we wanted to test if defensive chemicals were responsible for this rejection. Using different organic solvents, we prepared extracts of the algae and incorporated those extracts into alginate. Because of this, the physical structure of the algae wass destroyed and only the chemistry preserved. This extract in alginate was then fed to the fish and compared to an alginate control without alga extract.
Let me give an example: In my last entry (ďAlchemyĒ) I talked about phlorotannins in brown algae, and that they may have an antipredator function. Most of the brown algae fed to the fish were rejected. To test the chemistry, I incorporated phlorotannin extracts of those brown algae into alginate and offered it to the fish. And as it turns out, the fish rejected most of the phlorotannin treatments. These results will guide my future work on the ecological role of phlorotannin in brown algae - and we will go fishing again when we come back to Palmer Station the next time!