A grinch named CO2 - How phytoplankton is affected by multiple drivers

Miriam Seifert is a PhD candidate at the Alfred-Wegener-Institute. She participated the ESM Summer School in 2019 in Bad Aibling. In this article, the explains her work about the statistical analysis for common patterns in driver interactions and the influence of pCO2 on phytoplankton.

Marine phytoplankton constitutes about half of the world’s biomass production. Similar to trees it needs CO2, water, and light as major energy source for a process called photosynthesis, and nutrients for the general cell functioning. However, while trees grow at fixed locations and can become hundreds of years old, these unicellular algae are freely floating in the water and have high turnover rates. Changes in the environmental conditions are therefore quickly mirrored in the phytoplankton’s abundances, locations, and species compositions. The most important factors determining the growth of phytoplankton - the environmental drivers - are water temperature, light intensity, and the availability of nutrients and inorganic carbon.

Climate change is massively altering each of these drivers, pushing them outside their natural fluctuation ranges. Phytoplankton therefore has to face completely new conditions in its habitats. As it plays a significant role in the fixation of atmospheric CO2, the transport of carbon to the deep ocean, and in the oceanic food web, it is of urgent need to make robust projections on its future.

Since many years, laboratory studies have analyzed the effects of one driver on the phytoplankton growth, by applying different driver levels to a phytoplankton culture and monitoring the rate of change in cell numbers. Under natural conditions, however, it hardly occurs that one driver is changing alone, but it is rather a set of drivers that is about to vary. In recent years, a number of studies have therefore investigated the growth response to two, three, or more jointly changing drivers. The majority of them revealed that the single driver effects do seldomly add up to the final growth response, but rather have interacting effects on the phytoplankton cells. For example, growth-enhancing higher partial pressure of CO2 (pCO2) make a damage of the cell from high light more likely and, thus, decrease the benefit it would otherwise have from increasing light intensities alone.

To summarize the advances in multiple driver research, we reviewed the available literature on dual driver interactions and conducted a statistical analysis for common patterns in driver interactions. Indeed, we could show that the positive effects of increasing temperature and light are mutually dampened if pCO2 is increased at the same time. By contrast, the interaction between decreasing nutrient availability and increasing pCO2 is ambiguous. The responses of one group of phytoplankton, the calcium carbonate-producing coccolithophores, clearly differs from other groups, making it a candidate that should be considered more closely in future studies. In addition to dual driver interactions our results also revealed that polar species are suffering under increasing pCO2, while temperate and tropical species are growing even better, highlighting the fragility of polar ecosystems.

Our meta-analysis does not only add to the orientation of future multiple driver research. Numerical projections of biomass development, being in the ocean or on land, are computed by biogeochemical models. The effects of interacting drivers on organism’s growth rates are barely considered in current biogeochemical models. By statistically evaluating dual driver interactions based on the literature of the past years, we hope to facilitate their implementation into ocean models and, hence, increase the accuracy of projections of marine phytoplankton’s future.

 

We also had some questions for Miriam to answer:

How did the experience in the ESM summer school help you with your work and career?

I'm a biologist by training and just started with modelling from scratch with the beginning of my PhD. That's why I couldn't follow every detail in the talks during the summer school as parts of them were quite too advanced for me. Nonetheless, next to some talks that were much closer to my topic and very interesting, I could receive a broad understanding of the possibilities and diversity of models, the ideas behind them, and the current research topics. The hands-on session, especially the one on the assembly of computing nodes, were pretty much supporting this. By these means I could get a better understanding of our model, too. In addition, it was really nice to get to know the community, also in terms of future job plans.

What are you currently working on?

After I could publish the literature-based meta-study of multiple driver effects on marine phytoplankton, I'm now working on the model development of REcoM. After I've already implemented another phytoplankton group next to the two existing ones, I currently add a growth dependency on CO2 to the parameterization. Subsequently, I will implement the results of our meta-analysis into the model, thus, adding interactive effects between environmental drivers (temperature, light, CO2, nutrients) on the growth of our phytoplankton groups in the model.

How has COVID-19 changed your daily life/working situation?

I'm working now a lot more home-based. From time to time I'm also working in the office, however, as we are only allowed to be alone in the office, I barely meet my colleagues and the "by-the-way" exchange of information does not exist anymore, which is quite unfortunate. Nonetheless, we try to account for this in regular video meetings.

How would you describe your job or research to an eight year old?

Our climate is changing, and this is affecting our nature - the land, the oceans, all plants and animals. To understand in which world we will live in in the next fifty, hundred years, we want to make forecasts, just as the weather forecast for the next day. The way we do it is that we try to copy the real world into our computer, including the major processes and parts of our earth. With this we can then look first whether what we have is realistic to the present world, and if it is, we can then see what our computer tells us about the situation of our nature in a couple of years. In my work, I specifically look at the ocean and the very small algae therein. They can live, just as the plants on the balcony, only from sun, air, and some nutrition, and they are the food for many other animals in the sea, like some crabs and fishes. Because they are everywhere in the ocean, and the ocean is huge, they are super important for our nature. This is why we put a lot of effort into being able to forecast the future of these algae.

What are your plans after you finished your PhD?

Short answer: I still need to figure that out ;-)! There are many options, and I'm confident that I will find my way, but I'm not there yet.