In my last post, I explained the underwater surveys and briefly mentioned collecting a reef-depth water sample. In this post, I am going to talk about the method that I am using to extract the very small molecules from the reef seawater, a method called solid phase extraction (or SPE for short). All living things produce and release molecules into the environment. Microbes- like bacteria and archaea- produce, uptake, transform, and even communicate with these molecules. I am interested in how microbial communities influence the chemical composition of reef water and if we can use the chemical diversity and composition on reefs to understand the roles of these microbial communities on reefs.
I collect reef-depth water with a Niskin bottle at the end of my dive and I bring this water back to the Alucia so I can “process the water sample”.
When I use this phrase, I am referring to two major separation steps. First, I separate the microbial cells (bacteria, archaea, and single-celled eukaryotes; “the biomass”) from the seawater by pumping the seawater through a filter with a very small pore size (0.22 micrometers) and I collect this seawater (I call this filtered seawater “filtrate”).
Second, I separate the small molecules that are present in this filtrate by passing it through a special material, called a SPE cartridge. The material in this cartridge has special chemical properties that allow it to bind to chemicals present in the seawater. This binding process removes chemicals from the seawater and retains them on the cartridge. You can also think of this cartridge as a chemical trap. I use vacuum filtration to pull the seawater filtrate through these cartridges. The seawater that has passed through this cartridge should be stripped of most of the chemicals in the seawater and I collect this seawater in a vacuum carboy.
After I have passed all of the seawater through each cartridge (1 seawater sample per cartridge), I freeze each cartridge to prevent the molecules from degrading. I will finish processing these samples when I get back to Woods Hole Oceanographic Institution; I need a some special equipment to complete the final steps of this method!
Special thanks to the people of the Kujawinski Molecular Environmental Science Laboratory at Woods Hole Oceanographic Institution (Dr. Liz Kujawinski, Dr. Krista Longnecker, Dr. Melissa Soule, and Gretchen Swarr) and Dr. Cara Fiore for teaching me how to use this technique to understand the chemical composition of reef seawater
Thanks for reading and let me know if you have questions!