The Secret Life of Phytoplankton

Hello everyone! My name is Diana, and during this cruise, I am responsible for measuring the chlorophyll concentrations in the water at each of the stations along our transect. As today’s guest blogger, I will tell you a little bit about phytoplankton and how these fascinating microbes are closer to us and do more for us than we think!

Phytoplankton are microscopic plants that live in both fresh water and marine ecosystems. Diatoms are one type of phytoplankton. This makes most people think of them as these mysterious living beings that have nothing to do with us. But the truth is that these little guys are constantly working hard at photosynthesizing and, through this process, marine phytoplankton produce about 50% of the oxygen in the atmosphere. So, make sure you thank marine phytoplankton for every other breath you take!

Also through photosynthesis, they can remove up to one third of the carbon dioxide we humans put into the atmosphere by burning fossil fuels. So, not only do they provide us with oxygen to breathe, they also clean our atmosphere and make the effects of human emissions a bit less severe.

As if they weren’t doing enough, marine phytoplankton are also vital for those charismatic, bigger marine animals that we all know and love such as whales, as they are the base of marine food webs.

Photosynthesis is a process that most of us learned about in primary school and perhaps even later on, but I know we tend to forget things we may not have thought about for a while. So, let’s backtrack a little before I go on. Plants, algae, and some bacteria carry out photosynthesis to convert light energy (generally from the sun) into chemical energy that they can use to fuel their cellular processes. This makes them photoautotrophs and primary producers.

When we eat plants, we use the energy in their cells to fuel our own activity. In the simplest terms, during photosynthesis, primary producers utilize light energy, carbon dioxide in the atmosphere, and water to generate carbohydrates (stored energy) via a process called the Calvin Cycle and oxygen via the light reactions. For those who are visual learners, the schematic below does a great job of depicting the two parts of photosynthesis.

http://wikipedia/wikipedia_en_all_2016-05/A/Photosynthesis.html

Chlorophyll is a pigment found inside compartments called chloroplasts in the cells of plants and algae. It is key for photosynthesis because it absorbs the light energy that the sun provides and propels this process. Chlorophyll absorbs wavelengths within the visible light spectrum, most strongly in the blue portion, and less so in the red portion. We see chlorophyll as green because it does not absorb the green portion of the visible light spectrum. Instead, it reflects it and that is what is perceived by our eyes.

Since all phytoplankton cells have chlorophyll, its concentration is used to calculate an estimate of total phytoplankton biomass, usually expressed as µg/L (micrograms/liter). When phytoplankton cells have played out their life and are beginning to die, the chlorophyll in their cells dies with them and degrades to phaeophytin (a degradation product of chlorophyll). So, if there is a lot of phaeophytin compared to chlorophyll in a sample, this tells us that the phytoplankton bloom is unhealthy. Knowing how much phytoplankton is in the water and how healthy they are can give us an idea of how productive a certain ocean region is.

By now, you’re probably wondering how we’re able to isolate and measure chlorophyll, right? Well, here’s my routine for each station:

Collect water with a rosette, which consists of 24 12-L Niskin bottles arranged in a circle. This arrangement is lowered to 12 different depths of interest with the CTD. Two bottles are closed at each depth, thus collecting water from only that corresponding depth. For my chlorophyll measurements, I collect 355mL samples from a range of depths (from 5 to 250 meters). This encompasses the euphotic zone*—the depth of water where there’s enough light for photosynthesis—and a little below that to be sure we get any adventurous phytoplankton cells as well!

*The euphotic zone is defined by its lower limit, which is the depth to which 1% of the light that entered the ocean at the surface penetrates, and this depth varies depending on the quantity of cells and other particles in the water. Since phytoplankton need light to survive, they are restrained to the euphotic zone.

2. Filter the samples through nitrocellulose filters with pores 0.45µm in diameter. These filters let the

seawater through but catch phytoplankton cells.

3. Keep these filters in glass vials and freeze them for about 6 hours to break open the cell walls for

easier access to the chlorophyll.

4. Add about 12mL of 90% acetone, which extracts the chlorophyll from the phytoplankton cells and

dissolves it over a period of 36 hours. The nitrocellulose filters are also dissolved in the acetone,

so I don’t need to worry about them anymore!

5. The chlorophyll is now ready to be read on the fluorometer:

a. Pour about 3mL of the sample into a glass tube and place it inside the fluorometer. The

device then shoots blue light (430nm wavelength) at the chlorophyll solution, and this

energy input excites the chlorophyll’s electrons to the next outer electron shell. Because

this configuration is not stable, the chlorophyll emits red light (663nm wavelength) in order

to go back to its ground state; this is called fluorescence. The fluorometer reads the

intensity of this fluorescing red light and outputs a number in raw units.

b. Add two drops of 10% hydrochloric acid to the samples in order to degrade all the

chlorophyll into phaeophytin and then measure its fluorescence. Phaeophytin is

chlorophyll without the magnesium in the center of the ring structure that anchors the

chlorophyll molecule.

c. These results are then used to calculate chlorophyll and phaeophytin concentrations in

µg/L.

The following depth profile depicts our latest chlorophyll and phaeophytin concentration results. The highest chlorophyll concentration of 1.03 µg/L was found at 50 meters. The phaeophytin concentrations are much lower than the chlorophyll concentrations, telling us that this phytoplankton bloom is still healthy and photosynthetically active.

So, in a nutshell, or should we say in a frustule, chlorophyll gives us an estimate of how much biomass is in the water column. The more phytoplankton there are, the more that have the potential to end up sinking to the bottom of the ocean. This not only provides “history records” for paleoceanographers, but it also keeps the carbon that was removed from the atmosphere by these hard workers at the bottom of the ocean and away from the atmosphere.

Alright, I will leave you all with that! This is just a tiny part of the amazingly vast field of oceanography. Imagine what else lies out there for us to discover and learn about!