Why Do We Care About Dissolved Organic Matter (and What Is It)?

We have our three types of water samples ready for processing to extract out the dissolved organic matter:

1) "Ambient" or "Inhalent" water, which was collected near the sponges that we sampled

2) "Exhalent"water, which was collected as water was expelled from the same sponge

3) "Off reef" water, which was taken from the water surface away from the sponge reef

We want to see what types of organic nutrients that are dissolved in the water are removed by the sponges and what is added to the water column and surrounding environment by the sponges.

These 'organic nutrients' are part of a large 'pool' of compounds that are dissolved in seawater called dissolved organic matter (DOM). The amount of carbon that is in this complex mixture of dissolved compounds is huge – as much as the total amount of carbon in the atmosphere!

The part of DOM that we are most interested in is the 'small', or low molecular weight (this refers to the mass of a single molecule– more on this in a later lab note!), part of DOM.

This includes compounds like simple sugars or carbohydrates, as well as building blocks of DNA (nucleotides), building blocks of proteins (amino acids), and other compounds that organisms produce or need to survive. For example there are many different compounds used in signaling and communication between cells and in the growth and general metabolism of an organism.

Just like we fuel our metabolism with food, bacteria in the ocean and larger organisms like sponges and corals can get 'food' from DOM to drive their metabolism.

These compounds are important because their availability and concentration can determine what types of organisms can survive in a given habitat – from bacteria to sponges!

Here is one brief example of how these dissolved nutrients (DOM) can have a big impact in the ecosystem we are studying:

Some sponges known as boring, or coral-excavating sponges, bore into hard substrates such as corals or bivalve shells. Typically, the sponge eventually kills whatever it is growing on. These sponges can be detrimental in large numbers but they also are an important part of coral reef ecosystems with a role in reef regeneration.

While most sponges rely on small particles and bacteria from the water for a source of nutrition, recent research indicates that boring sponges get most of their food from dissolved nutrients in the water. This means that changes in dissolved nutrients, which could happen as a result of a change in sponge populations for example, could influence the growth and activity of the boring sponges.

 

Photo by Sean Nash under creative commons license on Flickr: a boring sponge (orange) in a star coral

For our project, in order to find out how sponges are altering DOM, we first need to extract it from our seawater samples.

We do this by drawing the water sample through a cartridge that is specially designed so that the compounds we are interested in stick to the column:

 

Photo: Close up of one cartridge

So the compounds of interest stick to the column in the cartridge, then we elute these compounds using a small amount of solvent (in this case, methanol). Now we have taken the DOM from 1 liter of water and concentrated it down to a few milliliters!

This DOM extract is what we will run on our instruments. Let the extractions begin!