Getting to the Chemistry in Chemical Defenses

Photos from UAB in Antarctica group on Flickr, related to this post
By Craig
Posted on 04/19/07

Chemical extractions are another important part of our research here at Palmer Station.  In previous entries, I have discussed feeding assays and methodology for determining the palatability of various algal species to herbivores.  However, just because an alga is unpalatable does not make it chemically defended.

Algae can also be unpalatable because of physical attributes like a tougher texture or calcified exterior.  Some algae are unpalatable because they simply do not provide enough nutrition to make them worth eating…much like celery (a little joke).  So, once we have determined that an alga is unpalatable, we try to figure out why.

One possibility is that the alga is chemically defended.  That is, the alga produces some chemical compound which makes it unpalatable.  How do we figure that out?  Well, it starts by extracting natural products physiologically produced by the alga.  Once we have these natural products, we can add them to food we know is eaten by our taste-testing amphipods and see if it has any effects.

If the answer is no, the algae most likely have another method of defending themselves.  If the amphipods refuse to eat their normal food though, it tells us that the algae produce some chemical to deter grazing.

Obtaining the natural products produced by an alga can be a time consuming but easy process.  First, I had to submerge my algal samples into specific solvent combinations for long periods of time to soak in the compounds.  Currently, I am extracting the natural products from three different red algal species; Acanthococcus antarcticus, Picconiella plumosa, and Pantoneura plocamioides.  I soaked all three species in different flasks with a combination of methanol and dichloromethane to extract all the hydrophobic (literally ‘water hating’ compounds that are soluble in fats within the cell) natural products from the algae.  This includes chemical compounds known as steroids, terpenes, and polyketides, among others.

Next, I placed the algae in a solvent mixture of methanol and water.  This mixture extracts all the hydrophilic (literally ‘water loving’ compounds that are dissolved in water within the cell) products from the algae.  Examples of these would be sugars and biochemical compounds called carboxylic acids and fatty acids.

After initial extraction, I was left with two solutions complete with the chemical products produced by the algae and dissolved in the solvents.  To purify the samples, I had to remove the solvents.  This was done using a rotary evaporator.

A rotary evaporator, or ‘rotovap,’ is an instrument which can lower the pressure, thereby lowering the boiling point of constituents within a solution.  Solvents, like methanol, can then be removed via evaporation without any excess heating which may harm the extracts.  However, even using the rotovap there is still potential to have some water mixed in with all the extracted natural products.

To remove the water, I moved the remnants from the rotovap over to another instrument called a freeze-dryer.  The freeze-dryer works by freezing the material and then reducing the surrounding pressure.   The machine than adds some heat, permitting the frozen water in the material to sublime (like evaporating except from a solid rather than liquid) directly from a solid phase to gas phase.  This will remove all the water from my extracts, leaving me with some pure compounds to experiment with.

As I type this journal entry, my extracts are currently on the freeze-dryer having the water completely removed from them.  Afterward, I look forward to testing my extracts to see what potential chemical defense the three aforementioned algae might posses. 

Hopefully, in future journal entries I will be able to mention the new chemically defended algae we have found here at Palmer Station.  Between this entry and the feeding assays, you should now know all the steps we used to come to those conclusions.

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