Isotopes

We figured out that larvae in the polar night are not quiescent - they are active. When you give them food, they eat it. They're even capable of metamorphosing and settling. Incredible.

When Kharis and I tested whether the larvae could feed, we gave them phytoplankton because we knew it was something they would eat. But there's no phytoplankton in the polar night. It's pitch black outside. We know that larvae did eat phytoplankton in our lab test, but they can't be eating phytoplankton out in the fjord. What are they eating instead?

We ran an experiment to try and figure that out. I had two hypotheses: maybe the larvae are eating bacteria, and maybe the larvae are eating dissolved organic matter. There have been studies from other parts of the world showing that larvae can use both bacteria and dissolved organic matter as food sources, but as far as I know, nobody's tested them in the polar night. 

To test my hypotheses, I turned to isotopes. You might remember learning about isotopes in high school chemistry class - they're different forms of an atom with different numbers of neutrons in the nucleus. The exact chemistry doesn't matter that much, because what's important here is that you can use isotopes to tell what an organism has recently eaten. Every organism has a certain ratio of isotopes for different elements in their body, and if you eat another organism, you assimilate its isotopes. You are what you eat, literally.

Prior to the trip, I bought isotopically-labeled molecules to use in our experiment. For the bacteria test, I bought bicarbonate because I knew it would be taken up by bacteria in the water column, which then might be eaten by the larvae. For the dissolved organic matter test, I bought amino acids. Kharis and I collected seawater from the fjord and filtered it through a very fine mesh, then added our isotopically-labeled food sources. We left larvae in the treatments for 24 hours, then froze them for analysis later. If we found elevated levels of heavy isotopes in our experimental larvae compared to the control, we could conclude that the larvae had eaten our labeled food sources. 

It doesn't look like much, but those petri dishes hold precious
larvae. With any luck, we'll be able to tell what they eat 
during the polar night. 

The experiment sounds super straight-forward, but the story is much more convoluted. I literally did not know until our last day in Svalbard whether the experiment would work. Larvae are so small that you need to pool multiple individuals together to even get a good reading. At the beginning of the trip, we didn't have enough larvae to even start a single trial. Then we started catching more larvae and thought the experiment was a go. But by the time we got the lab prepped and were ready to start the isotope experiments, the larvae had all but disappeared - we were catching only small numbers again. 

I guess I should have known. I mean, last time I spent a winter sampling trip in Svalbard, I wrote an entire paper about how the larval community can change over a few days when there's a wind storm and a new water mass enters the fjord. Larvae are super patchy, so you have to collect them while you can. 

In the end, we had enough for a reduced experiment - 3 replicates each of control, bacteria, and dissolved organic matter treatments. The best part is that another researcher from France offered to help with the analysis. I have only ever done one very small project using stable isotopes, so I was grateful for his help! He was arriving in Ny-Ålesund the day we were leaving, so I left the samples in the marine lab freezer and thanked him as we passed at the airport. Now it's a waiting game - once he completes the analysis, we should get answers!

Man, I hope this works!