Beautiful image isn’t it.  This is a phytoplankton bloom - basically a truck load of phytoplankton all grouped in one area.  

Consisting of just a single cell, phytoplankton spends their life drifting around the oceans (and indeed freshwater too).  You may have heard some of the names of the most common kinds of phytoplankton – like diatoms, dinoflagellates, and coccolithophores.  Like plants on the land, phytoplankton needs sunlight, nutrients like nitrates and phosphates, and carbon dioxide. Just like land plants, phytoplankton photosynthesise.

Photosynthesis in the terrestrial environment goes a little something like this.  The leaves of plants are green because they contain chlorophyll, which is really good at absorbing sunlight.  This energy is combined with carbon dioxide (also absorbed through their leaves) and water (which primarily comes from the roots) which results in a chemical reaction that gives the plant glucose (sugar) and oxygen.

For our phytoplankton friends, the process is remarkably similar. However, phytoplankton doesn’t have leaves or roots. Instead, they absorb all the bits they need directly through their cell wall.  The phytoplankton uses up the glucose produced by photosynthesis, but the oxygen…well, that’s what is known as a waste product.  That’s good news for you, me, and life on Earth. Oxygen may only make up ~21% of the atmosphere, but if it were to disappear tomorrow, we would be up the proverbial creek, and a paddle wouldn’t help us even if we did have one.

It’s thought that somewhere between 50% and 80% of the oxygen on Earth originates from the ocean, but the jury is still out on just how much ocean oxygen ends up in the atmosphere. It is clear that most ocean oxygen is used up by the myriad marine life that also needs oxygen to survive. 

Let’s dive into a couple of open-access science papers exploring phytoplankton photosynthesis.

Late Cambrian oxygen

Back in 2011, Matthew Saltzman from Ohio State University and colleagues published a paper looking at some huge changes in atmospheric oxygen content during the Cambrian period.  Apparently, at around the time of the mass extinctions, oxygen declined dramatically but picked up again.  The most likely source of this oxygen?  Yup phytoplankton.

Variation is everywhere

The amount of oxygen produced by phytoplankton isn’t constant.  It can change depending on where you are, and what time of year it is.  Dr Sarma from Nagoya University and colleagues studied oxygen production in Sagami Bay in Japan between May and October 2002.  They found that August was peak oxygen time, whilst October saw the least amount.

More carbon dioxide = more oxygen? Not quite

With us lot pumping more and more carbon dioxide into the atmosphere, you might think that for phytoplankton this isn’t so bad.  After all carbon dioxide is vital for photosynthesis – and we like photosynthesis because it gives us oxygen.  Well… t’s not quite that simple.  In 2013 Dr Kim from Chonnam National University and colleagues took a look at how predicted future climates might impact coastal phytoplankton photosynthesis.