The ‘Missing’ Carbon Conundrum… Part 1

Here’s a little thought experiment for you:

Imagine you have some fish in a tank, and you feed them a mixture of small, easy to digest flakes and larger, more difficult to digest flakes. Assuming you feed enough of each type of food to satisfy their hunger, what would you expect to happen?

You’d expect the fish to take the easy option and eat the smaller, easier to digest flakes, whilst the larger, more difficult to digest flakes are mostly left to sink out and end up on the bottom of the tank, right?

Well, that’s basically what happens in coastal waters when microorganisms have a choice between different kinds of food – they pick the easiest option.  

(Let’s just say that anything which is alive and so small you need a microscope to see it is a microorganism, for anyone who wasn’t sure. And because we are scientists and have to be careful with the meaning of words, when we talk about microorganisms we’ll say that they metabolise (or break down) organic matter rather than saying that they digest food, and we’ll use labile and recalcitrant instead of easy to break down and difficult to break down. Don’t worry, you’ll get the hang of it!)

So, coastal waters tend to be incredibly productive places. The surface waters in these regions are teeming with life, and the phytoplankton and algae which live there turn light and carbon dioxide into lovely, fresh, labile organic matter for other organisms to enjoy.  

At the same time tho, huge amounts of recalcitrant organic matter, mostly produced by land plants, flows out of the rivers that feed into these coastal waters. A lot of this river material is old, having been broken down and recycled by river microorganisms so many times as it travels downstream that it has become nutrient deplete and extremely difficult to metabolise by the time it reaches the sea.

With plenty of fresh food being produced in the surface waters, coastal microorganisms don’t need to spend precious energy breaking down this old river material. So what happens to it? Does it sink out and end up on the bottom, just like the fish food did?

No – and that’s the mystery!

Around 50% of the land-plant-derived organic matter which flows out of rivers and into the ocean goes ‘missing’. It’s not floating around in the water column, it’s not sitting in the sediments, and the scientific literature tells us that microorganisms aren’t eating it. So where does it go? 

This might not seem hugely important at first, but for scientists who study the global carbon cycle, solving this conundrum is vital. You see, a large portion of organic matter is made up of carbon, and we know that carbon is linked to climate. 

So let’s think about it! What could be happening to all that organic matter?  

Some of the biggest scientific breakthroughs have come about because people from different fields of expertise and backgrounds got together and threw around some ideas, so have a go!

In my next blog post, I’ll talk about how we know the organic carbon is going missing. In the mean time, over to you!