K-Axis Day 27: Harnessing the sun’s energy

Last night we were treated to a glorious sunset. Its the first time we have seen the sun since the first days of the marine science leg of this voyage, around 2 weeks ago. A group of us were out on the heli deck enjoying the display, taking some photos and basking in the rejuvenating effect the sun seems to have on one’s body and mind.

On the topic of sunshine, I haven’t written about what my job on this voyage is as yet. I’m working on primary and bacterial productivity again, using the same lab and protocols that I used in Davis last year for my PhD work. I can’t recall how well I explained it before so I’ll briefly go over it again.

Primary Productivity
Lets start right at the start. Phytoplankton are single-celled marine plants and just like all other plants they use the sun to create energy, through photosynthesis. This is what we call “primary productivity” and we’re interested in it because all the creatures in the ocean either eat phytoplankton, or eat something that does. Climate scientists are also interested in phytoplankton because they consume CO2, which is super important when we keep trying to fill our atmosphere with it. Lastly, everyone should be grateful for phytoplankton because every second breath of oxygen we consume was created by them. They’re really the unsung heroes of the global environment, at least in my opinion anyway!

While you can find phytoplankton in all the oceans of the world, that doesn’t mean they are evenly distributed through the ocean and just like terrestrial plants, different species occupy their own special regions of the world’s oceans. We’re interested in the phytoplankton that live in the Southern Ocean, but again the Southern Ocean isn’t one uniform water mass. It contains things called “Fronts” and these are areas where warm water from the north comes into contact with cold water from the south. Interestingly, these two water masses don’t really mix and you end up with what is effectively a barrier for some marine species, particularly phytoplankton. Many species are adapted to water temperatures and conditions on their side of the Front and you won’t find them on the other side. Other interesting things happen with nutrients and water mixing, which makes these Front regions very productive so a telling sign of being near one is an increase in birds and mammals and other wildlife feeding.

To bring this back to what I am doing, we’re interested in seeing how much photosynthesis is occurring in the water in the regions this voyage is passing through. We’re also interested in how deep the phytoplankton are growing. They’re only able to grow as deep as light can penetrate, however, too much light can also be damaging to cells so this means that often the optimal growth conditions are not in the top 5m of the ocean. Sometimes it can be as deep as 40-50m. We can also use that data to compare with satellite photos to see how accurately they are able to estimate phytoplankton abundance and to understand how much carbon is being consumed by them, which is important when looking at climate change and food webs and other natural processes of the ocean.

So how do we measure primary productivity? We do it by feeding the phytoplankton a radioactive form of carbon (carbon-14), which is easily detectable with a specialised instrument that is able to measure radioactive decay. Now I’m sure the idea of working with radioactive compounds sends a shiver up many people’s spines, but without going into specifics, its safe to say that not all radioactive compounds are created equal and our carbon-14 is the lesser of those evils. Of course we ensure the necessary training and strict safety precautions are maintained at all times but we’re not working with a lump of enriched uranium. The only thing in our lab that glows are the LED lights in our instruments and incubators. We actually have our lab in a shipping container so that we can separate what we do from everything else. This includes safety clothing and equipment that we don’t remove from that space and constant monitoring. We take what we do very seriously.

Back to the productivity measurements, when we feed the carbon-14 to the phytoplankton we expose them to lights across a range of intensities (from dark to very bright) for 1 hour. This allows us to accurately calculate the amount of carbon consumed by the phytoplankton, using chlorophyll (the part of the cell where photosynthesis occurs) and cell counts generated from other experiments occurring on board. All up this gives us a picture of what is happening at each of the stations where we deploy the CTD and we can use this data with all the other chemistry and biology measurements being taken from the same water to create a picture of whats happening in this region of the ocean and how everything interacts.

And on that note, I have another experiment to attend to.


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