14C Madness
Phytoplankton are able to turn dissolved inorganic compounds, like dissolved CO2, into particulate (biomass) or dissolved organic matter while, in turn, releasing O2. This process is commonly known as photosynthesis or primary production. One of my main responsibilities on board is to measure photosynthetic efficiency through Carbon-14 (14C) uptake. Meaning, Dr. Nauzet Hernández-Hernández (my 14C “sensei” and main team-mate on this boat) and I are looking into how phytoplanktonic communities, collected from different depths, use inorganic carbon up when exposed to different light intensity levels (see picture below).
Incubation setup to carry out photosynthesis-Irradiance curves.
How do we measure this? And, better yet, what is it like to do so on a boat? Well, it is intense at times. The idea is to collect water, filled with living phytoplankton adapted to the light and temperature conditions of the depth layer they lived in, provide them with a known amount of an inorganic compound marked with 14C, and expose them to a light intensity gradient (picture above) for a given amount of time. They will then take up part of that inorganic radioactive (traceable) carbon and turn it into biomass or exude it as dissolved organic carbon.
Once a given amount of time has passed, the culture flasks containing the “radioactive” phytoplanktonic community, are filtered using a structure (see picture below) called filtration manifold. The filters collect all the particulates, including the cells with the newly incorporated 14C, and the bottles below, all the dissolved organic and inorganic matter.
Laura Marín-Samper, me, PhD candidate, filtrating 14C marked samples with a filtration manifold (OCEOMIC).
Now that you have seen what filtering looks like in the picture above, can you imagine doing it when the vessel is rocking up and down, side to side, and all over the place? Trust me, it can be tricky. It is in that moment when everything that could itch, itches. Nonetheless, you must stay focused as not to spill any water because the vessel is moving, and so are you. Madness!
Once you have filtered out all the little green capped flasks, you collect your filters that now contain all the particulate matter, and a small volume of water that has been filtrated (filtrate). You then must get rid of whatever inorganic carbon is left in the samples to be able to account only for the amount of 14C that is now part of organic compounds. To do so, you place all the filters in a desiccator with fuming hydrochloric acid and add liquid hydrochloric acid to the collected filtrate.
Now, with a few more steps in between, you will be able to measure using a scintillation counter, in disintegrations per minute, how much of the initial radioactivity that you added to the sample in the form of an inorganic carbonated compound, is now organic.
Therefore, how much your phytoplanktonic communities have photosynthesized in the different light conditions you have exposed them to. It is a measure of their metabolic fitness, which is highly relevant considering they are the basis of all food chains in the ocean and contribute 50% to global primary production.
With Nauzet Hernández-Hernández as 14C sensei and the filtration manifold collecting particulates, one step still goes unmentioned: after fuming hydrochloric acid, the filters need a quench curve and a known 14C standard before the scintillation counter can turn counts into disintegrations per minute. Otherwise chemiluminescence and cocktail volume skew the counts, and even the most useful AI tools for lab notes won’t fix a bad calibration.
Staying focused didn’t keep my filtrate in the flasks; bracing the filtration manifold with a knee did. During a swell, I lost two 14C samples before adding fuming HCl in the desiccator, then counted the rest in disintegrations per minute. A calculator that estimates garden profits had taught me to expect tidy inputs, but here the light gradient and rocking made each photosynthesis-irradiance curve its own mess.
Filtering 14C under a rocking vessel is rough, but my shore-side constraint is different: only two working scintillation vials per depth and no fume hood for the fuming HCl step, so I pool filtrates and lose the dissolved organic carbon signal. Dr. Nauzet Hernández-Hernández’s light-gradient approach would need more sample than my oligotrophic lake gives. I label every flask by hand, sometimes with a cursive worksheet maker, because the DPM corrections have to survive my handwriting.