In January of this year and in 2014, I invaded the NOAA NMFS SWFSC (our glorious branch of the federal National Marine Fisheries Service), where they let me use their glorious Loligo respirometer. The respirometer is perhaps the most awesome animal physiology tool ever. A respirometer is essentially a sealed chamber in which an animal is placed. Inside the sealed chamber is something to make them exercise–for a lizard or a wolf this might be a treadmill; for a fish, this is a steady current of moving water. While the animal exercises inside, an oxygen probe measures the decrease of oxygen as the animal breathes, consumes oxygen, and respires CO2. You can see this in action in this video featuring the amazing Caleb Bryce.
Since, however, I don’t work with animals (please, so mainstream), I used the respirometer BACKWARDS. Instead of swimming a fish in the Loligo respirometer, I swam our favorite kelp, Macrocystis pyrifera. And instead of measuring the decrease in oxygen (inevitable for an exercising fish), I measured the increase in oxygen as a measure of photosynthesis. The energy from each photon that strikes photosynthetic pigments is used to split a water molecule into a proton and a molecule of O2, liberating the electron that sets the electron transport chain in motion. Thus measuring the increase of O2 as photons drive photosynthesis is a great proxy for measuring photosynthesis itself.
My physiology questions for my kelp were the following:
- Can I demonstrate that M. pyrifera uses bicarbonate, in addition to CO2, as a carbon source during photosynthesis?
- What proportion of photosynthesis is driven by bicarbonate in M. pyrifera?
- Does flow velocity alter their carbon use? For example, I hypothesized that higher velocities would reduce the thickness of the frictional boundary layer on the kelp surface, facilitating gas and molecule exchange.
- Does light intensity alter carbon use? For example, high light intensity should drive high rates of photosynthesis that exhaust the supply of CO2 in seawater and require the supplemental use of bicarbonate.
What I found was very interesting. On average, it seems that M. pyrifera in the Monterey Bay uses 37.3% bicarbonate. However, this number varies a lot–bicarbonate use is definitely influenced by the light and flow environments that the kelp experiences! You can see how this works in the figure above. The blue bars are the rate of photosynthesis supported by CO2 only*; this rate tops out at 12 µmoles O2/g of kelp tissue/hour, but is limited to 7 µmol O2 in low flow conditions. In general, low flow limits the maximum photosynthetic rate of both CO2 and bicarbonate uptake, even when kelp is experiencing super high light levels. On the other hand, kelp in the dark actually seem to be satisfied with only or mostly using CO2. When kelp are exposed to high light, however, the tiny concentrations of CO2 in seawater are not sufficient. High light and high flow drive the kelp to take up a ton of bicarbonate in order to satisfy photosynthetic demand.
When you think about terrestrial plants, who only use CO2, all the time, this is pretty fascinating. Kelp have two (or more) carbon uptake strategies, and can use them differently in different conditions. Pretty awesome.
* When I write “CO2 only”, I am simplifying. This photosynthesis could also be supported by active uptake of bicarbonate, since my inhibitor (acetazolamide) only stops extracellular mechanisms of bicarbonate uptake. However, active bicarbonate uptake seems to be a pretty tiny proportion of photosynthesis in our California populations of M. pyrifera.