Biology

Phytoplankton are experts at adjusting themselves to changing temperatures

Reviewing: Kunze, C., Gerhard, M., Jacob, M., Franke, N. A., Schröder, M., & Striebel, M. Phytoplankton community performance depends on the frequency of temperature fluctuationsFrontiers in Marine Science, 2140.

When I was little, I used to think that the straw mats wrapped around the trunks of pine trees in our apartment complex every winter were for keeping the trees warm. I learned much later that it was actually to protect them against pests. But even to this day, as I wrap myself in a warm coat every winter, it seems pretty remarkable to me that the pine trees could survive the extreme cold without any protection.

Straw mats wrapped around trees during the winter to protect trees from insects (Flickr)

Just as pine trees – and other land plants – experience and grow across a wide range of temperatures, so do plants in the ocean (phytoplankton). Phytoplankton are single-celled algae that take up carbon dioxide and light energy to grow and produce oxygen, like any other land plants. In fact, phytoplankton are responsible for half the oxygen you breath every day! Because phytoplankton is so essential to sustaining life, many scientists are very deeply interested in understanding how phytoplankton adapt to changes in ocean temperature – especially as climate change is bringing more frequent and intense extreme weather events into our life.

A “Planktotron” (image from Gall et al. 2017)

A team of scientists decided to specifically look into the effect of changing temperature on phytoplankton growth. They put seawater in twelve 600 L (~150 gallons) stainless steel tanks and added plankton collected from the North Sea to each of the tanks (These tanks are called “Planktotron“, and they are kept in an indoor facility in Wilhelmshaven, Germany). During 36 days of the experiment, the scientists fluctuated water temperature between 15 and 21 °C, which is the natural North Sea temperature range during the summer, at different frequencies of 6, 12, 24, 36 and 48 hours. (In other words, they gradually increased water temperature from 15°C to 21°C, then gradually decreased it back to 15°C, after every 6, 12, 24, 36 and 48 hours).

 

The scientists found that there was more phytoplankton in tanks that experienced quick temperature changes (every 6 and 12 hours) than tanks with slow temperature changes (every 24, 36 and 48 hours). However, the types of phytoplankton in the water didn’t change much over 36 days. This means that changes in the amount of total phytoplankton in tanks are not related to individual types of plankton – instead, the same type of phytoplankton were just more efficiently reproducing and growing over time. It seems that plankton do not prefer staying in stressful temperatures for longer periods; they dealt better with shorter but more frequent exposure to stressful temperatures.

Another possible explanation for this result may be related to the short lifetime of an individual phytoplankton, which is about a day. Let’s say that the “parent” phytoplankton stays at a cold temperature (15°C) for over 24 hours. They start thinking that their “child” needs to adapt in this cold temperature. So when the parent phytoplankton produce child phytoplankton, they pass on the genetic traits that help their child survive in a cold environment (a process called “epigenetic plasticity”). You can think of these child phytoplankton as being born wearing fur hats and coats to keep them warm and safe from the cold. But under temperature fluctuations over every 36 or 48 hours, these born-to-be-cold-adapted child phytoplankton are exposed to a warm temperature (21°C) that they were not prepared for at all. Some child phytoplankton may not survive at all. Others may survive, but since they remember that they had to suffer from warm temperatures, they will produce grandchild that can survive best at warm temperature. This is basically a never-ending cycle of adaptations that are not ideal for the new environment.

Diatoms are a very common type of phytoplankton. These diatoms were collected and arranged on a microscopic slide by A. L. Brigger (Flickr). The scale bar on the bottom left is 100um long (about as wide as a strand of your hair).

Overall, the results of this study may suggest that phytoplankton may be very capable of adjusting themselves very quickly to temperature changes. However, the scientists point out that they only exposed these phytoplankton to a natural temperature range. As climate change will result in more extreme and unpredictable temperatures, we will still need to find out how phytoplankton will respond to these new environments.

 

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