The deal with phytoplankton

I was trying to wait until I had some new artwork to post a new entry, but since I don’t, and it’s been a while now, I’m just going to post this instead. I’ve realised that I haven’t really explained what exactly I’m doing on my PhD, and while I’m sure no one particularly cares, communicating what I’m doing to others is a good way of straightening thoughts out in my own head. So here’s a really brief and lame explanation of what I’m currently working on.

Okay, so here is a little phytoplankton:

Usually when I imagine things in my head, it’s easier to think of phytoplankton with faces. Otherwise they become very abstract things very quickly. It’s difficult to imagine a single cell as an organism all on its own. So here he is. Phytoplankton are photosynthetic organisms, so they tend to stay in the upper part of the ocean where the sunlight is. What the plankton also need is carbon, which comes in the form of CO2. There is about as much CO2 in the ocean as there is in the atmosphere, and this will diffuse into the cells, like so:

Unfortunately, the enzyme that converts the CO2 into the useful compounds the cell actually needs (called RubisCO) evolved at a time when CO2 levels were much higher than they are now. So, at current CO2 levels, RubisCO remains pretty unsaturated, and photosynthesis is pretty slow. Some groups of plankton that evolved more recently have RubisCO enzymes that have a higher affinity to CO2 (are “more attracted” to CO2, if you want to think of it that way), but others have evolved what are called carbon concentrating mechanisms, or CCMs. One common CCM is the use of another enzyme called carbonic anhydrase (CA), which sits on the outside of the cell and converts the plentiful bicarb (HCO3-) into CO2, which the cell can take up. Unfortunately… this is pretty energy intense.

We can tell when plankton invest energy in CCMs because increasing the amount of CO2 available to them doesn’t really increase the rate of photosynthesis (because the CCMs are keeping it saturated). I’m looking at this in the context of ocean acidification, which results in an increase in CO2 in seawater. There are two main groups to consider; those with CCMs and those without. Your first thought might be that plankton without CCMs will have an advantage in that they will suddenly find themselves with a lot of extra carbon at no extra energy cost to themselves (they simply rely on the diffusion of carbon into the cell). While that’s true, we also need to consider that ALL cells will be less limited by carbon at this point. So those investing energy in CCMs may find that they don’t need them anymore as simple diffusion does them just fine, and can divert the energy that’s freed up into things like chloroplasts and cell division. Suddenly, they look like they might be winning after all.

I’m looking at this with the added depth of cell size, since the smaller a cell is, the less limited it is by diffusion (due to the surface area:volume ratio thing). Basically, it’s not straightforward. Understanding how things will change is crucial, because what’s really clear is that some groups of plankton will do well and others might get left behind. The species composition of a plankton community is very important in an ecosystem, as they form the very bottom of the food chain. Different groups of plankton contain different nutrients that might favour different groups of organisms higher up the chain (e.g. for some kinds of fish larvae to successfully make it to adulthood, they require that certain plankton bloom because they make special essential oils). So… that’s why I think my PhD is important, and what I’m studying is important. I’ll try and make my next post a little arty, but it depends on the time I have available. I hope that I’ve explained my ideas clearly, and if you ever have any questions, leave a comment!

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5 thoughts on “The deal with phytoplankton

  1. I like the marshmallow design. I’ll have to say, I’ve never seen anyone personify a phytoplankton, single-celled or not. Nice job.

  2. Hi Laura I am Tullio, a master student in marine biology in Italy. I am about to graduate and my master thesis is very similar to you PhD topic! I am sure that talking a bit wound be useful for both of us. I worked for my thesis at the University of California: Santa Barbara and i focused on the impact of OA on the growth and aggregation of Skeletonema marinoi. How can I contact you? Cheers

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