When I found out that I had the opportunity to undertake a PhD, I was ecstatic. A career in research is what I’ve always wanted - in fact I’ve never imagined doing anything else. Where I have to remind myself of this when things get tough, I’m still incredibly grateful for the chance that I have been given. However, when I told people that I would be researching ’The Role of Diatoms as Global Producers of DMS and DMSP’, I was understandably met with countless expressionless faces. It dawned on me at a recent public event that - bafflingly - some people don’t find the oceans as interesting as I do. I’m often asked why I do what I do. Upon explaining the finer details of my project to a friend recently, he said ”I can see why you find that interesting, but what’s the point of it?”. Well, with a little more literary guile than my good friends response, I’m going to attempt to explain it within the confines of this blog post.
”Without diatoms, life as we know it would not exist”
The first task is to explain what on earth (or in sea) a diatom is. For this, I will revert to the explanation I employed with my sister - a history student. ’Diatoms are a kind of plankton, you know, like the little green guy from Spongebob?’. I’m not sure that the creative genius behind Spongebob ever divulged which species of phytoplankton Plankton is, but for the sake of argument lets say he was a diatom…
There is a huge diversity in the diatoms found in our oceans. Currently, there are 200,000 identified species and counting, some of which are only differentiable under a microscope - and sometimes even that is a struggle. Diatoms are tiny organisms, some as small as 5 micrometers in diameter (thats 0.05 millimetres), and constitute just one branch of the phytoplankton family tree. At such a small size, you’ve probably eaten at least 10 million in your life time from all those times you took a mouthful of seawater at the beach as a child. Fortunately for us, diatoms do not share Plankton-from-Spongebob’s appetite for destruction. In fact without diatoms, and phytoplankton as a whole, life as we know it simply would not exist. These invisible engineers are the drivers of global processes, climate and habitability.
”They survived the mass extinction that wiped out the dinosaurs, making diatoms harder than a T-Rex”
Phytoplankton are autotrophs, which means that they create their own energy from photosynthesis. Amazingly, diatoms are responsible for as much as 20% of the planets photosynthesis - producing as much organic carbon as all of the world’s rainforests combined. So to put that in perspective, every fifth breath you take is oxygen produced by diatoms (in case you don’t remember, photosynthesis uses carbon dioxide and water to produce oxygen and glucose). Diatom photosynthesis has important global consequences, particularly when it comes to global warming and climate change, as well as engineering the atmosphere we take for granted everyday. You see, Earth’s atmosphere today is almost unrecognisable in comparison to say, 100 million years ago, when diatoms began to become prominent in the primitive oceans. Quick-fire fact: coming to prominence 100 million years ago means they survived the mass extinction that wiped out our friends the dinosaurs (65 million years ago), making these minuscule critters harder than a T-Rex! 100 million years ago, CO2 levels were extortionately high, and oxygen levels extremely low in comparison to today’s. The introduction of diatoms and other types of dominant phytoplankton resulted in big reductions in CO2 and sizeable increases in oxygen, paving the way for modern life. Furthermore, these large diatoms had high sinking rates, which means that they were buried in the ocean floor all those years ago, creating the majority of the petrol resources we are so heavily reliant on. I wasn’t kidding when I said life as we know it would not exist without diatoms…
More recently, as we continue to pump out carbon dioxide through the burning of fossil fuels, phytoplankton and diatoms take it away. They absorb it at the oceans surface, take it in to their cells and then they sink. Relative to their size, diatoms are very heavy, and they require upwelling water to reach the oceans surface, where light is at it’s highest. Upwelling is common in coastal regions, which is where diatoms are most abundant. When upwelling subsides, diatoms are able to sink - due to the silica cell walls - taking carbon dioxide with it, moving it away from the atmosphere. Some of the larger species, can control this movement themselves, by manipulating the buoyancy of their cells.
”Diatoms transport carbon dioxide away from the atmosphere, deep into the ocean, limiting further enhancement of the greenhouse affect."
As diatoms sink, they bury carbon dioxide deep in the ocean. Water currents move diatoms away from the continental shelf, and push them below thermoclines, where they become trapped underneath. Thermoclines are gradients where the temperature of the waters above and below are different. Think of the ocean like a huge lasagne. If water is the mince meat, diatoms are the finely chopped onions and thermoclines are the pasta sheets. Diatoms cannot easily cross back across a thermocline, so the carbon dioxide they have absorbed at the surface has no pathway back to the atmosphere. This means that the ocean is what we call a CO2 sink, storing carbon dioxide and preventing it from further enhancing the greenhouse effect.
Transporting carbon is just one of the amazing functions these microscopic organisms are capable of performing. Phytoplankton account for 50% of the global sulphur flux, which is key in cloud formation and weather systems. They’re the base of countless marine food webs, and therefore drivers of the fishery industry. Furthermore, diatoms secrete - for want of a better term - biological goop. This goop forms what scientists refer to as biofilms, which perform important functions in themselves, holding together sediments and providing gel-like houses for marine bacteria and other plankton species. They’re also excellent communicators, releasing nitric oxide to signal to other diatoms when under threat, resulting in the production of compounds known as aldehydes which are harmful to potential predators. Some diatoms reach the very deepest parts of the ocean when they die, where they are sedimented into rocks forming carbon reservoirs and contributing to petroleum reserves.
”What may seem remarkably uninteresting to some, can be truly fascinating with a closer look”
For organisms of minute size, diatoms and plankton have had, and continue to have, huge impacts on the world we live in. It’s the way that such a tiny thing can have such a big effect that fascinates me. I find it amazing how reliant global systems are on something which is so seemingly insignificant at a fleeting glance. The point that I am trying to make is that what may seem remarkably uninteresting to some, can be mind-bogglingly awesome with a closer look. The processes in this post have been incredibly simplified, so in reality, diatoms as well as other plankton species are incredibly complex. Further posts may go into more detail, but I hope I have sparked your intrigue with this very brief overview. At the very least hopefully I come across as less boring. If you are interested in learning more about these amazing creatures, it may be worth checking out two very readable books Mapping the Deep and Ocean of Life. For any burning questions, feel free to contact me using the social media links associated with this article, or drop me a comment at the bottom of the page.
Seth Thomas, Editor.
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References:
Armrest, E. V. (2009). The life of diatoms in the world’s oceans. Nature, 459(7244), 185–192. dos:10.1038/nature08057
Katz, M. E. et al. Biological overprint of the geological carbon cycle. Mar. Geol. 217, 323–338 (2005).
Falkowski, P. G. et al. The rise of oxygen over the past 205 million years and the evolution of large placental mammals. Science 309, 2202–2204 (2005).
