Standing up for the little guys – Phytoplankton and Southern Ocean Ecology

By Laura Bretherton on January 24, 2013 in Antarctic, Ocean Acidification
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If you’ve been keeping up with the blog, you’ll by now be well acquainted with marine plankton. These little guys are the topic of my PhD, though more specifically, I look at the phytoplankton – the photosynthetic microalgae – and how they might be affected by ocean acidification. That means I’ve been particularly focussed on the bioassay experiments we’ve been conducting at sea, and today marked the end of our second one. With that comes an early start and many water samples to process…

My role on this cruise is to look at the physiology of phytoplankton populations, and how that changes over the course of the bioassays. I use an instrument called a fast-rate repetition fluorometer (or FRRF for short), which monitors photosynthesis by detecting fluorescence given off by the chlorophyll inside the microalgae. Because we understand how changes in this chlorophyll fluorescence are linked to photosynthesis, the fluorometer can tell us things like how efficiently the phytoplankton are photosynthesising, or if they’re changing the structures within their cells that catch sunlight.

The FRRF (top left), water bath (middle right) and my dark sample bottles (bottom). @Laura Bretherton

The FRRF (top left), water bath (middle right) and my dark sample bottles (bottom). @Laura Bretherton

My sample bottles are made of opaque dark plastic to keep the phytoplankton out of the light for a period of time, in preparation for being put in the FRRF. This process is called dark-acclimation, and sort of “resets” the algae’s photosynthetic apparatus to ensure all the models the FRRF uses fit correctly. Once they’ve been dark-acclimated, a small volume of sample gets put in the FRRF, surrounded by cold water supplied from a water bath that’s pumped into the instrument to keep the phytoplankton at a constant and appropriate temperature. Finally, I use a laptop to tell the FRRF to start a sequence of flashes of light, and about 20 seconds later, I get some data. The results are available instantaneously (a bit of a luxury on a cruise!), and as phytoplankton can change their physiology quite rapidly, the fluorescence data is able to give us an immediate insight into what’s going on in our experimental bottles.

I’m often asked what’s so important about phytoplankton, and why I bother studying them at all. I think the most easily appreciated example of their significance is in the incredible diversity of the Southern Ocean. Every day of our cruise has been filled with wildlife of some description, whether it’s a majestic albatross gliding effortlessly along the air currents, porpoising penguins bouncing through the water, or a pair of chubby seals hauled out on an ice floe. The mainstay of many of these animals is krill, either eating it directly, or feeding on the fish and assorted invertebrates that prey on it. The krill in turn are sustained by phytoplankton, the marine world’s main primary producers that use the sun’s light to convert carbon from the inorganic form of carbon dioxide into organic compounds that can be used by living things.

They become of particular importance in this region of the world because of the scarceness of terrestrial plants on the Antarctic continent and general surrounding area, causing the ecosystem to be heavily dependent on marine productivity to keep it going. The result is that the Southern Ocean is able to boast a staggering variety of mammals, seabirds and invertebrates, which I have been only too happy to experience myself!

The diverse result of phytoplankton productivity. Left to right: black-browed albatross, chinstrap penguins, fin whale (top row); crabeater seals, snow petrel, Antarctic fur seal (second row); wandering albatross, cape petrels, Antarctic minke whale (bottom row) @Laura Bretherton

The diverse result of phytoplankton productivity. Left to right: black-browed albatross, chinstrap penguins, fin whale (top row); crabeater seals, snow petrel, Antarctic fur seal (second row); wandering albatross, cape petrels, Antarctic minke whale (bottom row) @Laura Bretherton

We are currently in transit to South Georgia, a beautiful example of how much Southern Ocean ecology is dependent on marine productivity. There is currently an algal bloom in the waters around this isolated island, supporting much of the marine life in the area. Almost all the animals found on South Georgia feed offshore, and therefore owe their existence to these regular blooms. Everyone on board is quite excited to have the opportunity to explore this remote sliver of land, and all the wildlife it has to offer. Phytoplankton might certainly be microscopic, but I enjoy reminding myself that I don’t necessarily need a microscope to admire how very important they are.

Laura Bretherton

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