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Checking for silica uptake

What do diatoms need to grow?

 Andrea Timoncini Sep 30, 2019 3 min read

Diatoms need nutrients to grow. These are silicate, nitrate and phosphate for the most. They are usually limiting in the ocean and they become abundant only during an upwelling event, in this time phytoplankton can grow developing a bloom. Diatoms are in particular limited by the concentration of dissolved orthosilicic acid, Si(OH)4 because it is used to build their highly ornate cell wall (frustule). They transport it immediately before silica polymerization and cell division. In water with equal concentration of nitrate, diatoms tend to grow more with higher concentration of silicic acid. Therefore, their abundance and distribution is mostly regulated by the availability of this inorganic nutrient.

Why do we care about diatoms? Diatoms account for 40% of the total primary production in the world’s oceans. Therefore, it is of great importance to study at which rate and proportion they need silicic acid to grow. They are essential for carbon export and for trophic transfer. The former is due to their capacity to sink, so their carbon content is exported to depth, the latter is because they are the primary food for copepods which represent the link between primary producers and fish.

So why do we need to measure silica uptake? From the silica uptake rate we can estimate the growth rate of diatoms and thus compare this for the different silicic acid treatments in our mesocosms. The treatments cover a wide range in the supply of silicic acid, while all other essential nutrients are provided in equal amounts across all mesocosms.

I measure silica production by adding a fluorescent dye (PDMPO) to water samples from each mesocosm to analyse the amount of silica taken up by the diatoms during a given time. After spiking two bottles with the dye, we incubate them for 24h at the fountain of Parque Tecnológico, which kind of mimics natural conditions.

Jacqueline at fountain in Techno Parque Jacqueline, my lab-mate, at the fountain of Parque Tecnológico. The blue device is the incubator.

The next day we filter the water to collect all the phytoplankton in it and we store them in methanol. The last week of the experiment we will measure all the samples using a fluorometer and hopefully we will see in which mesocosms the diatoms were taking up more silicate. We will also try to see the silica uptake in the individual diatom cells under the microscope using a UV lamp with the help of the really nice technicians of the Aristegui lab. However, since the dye is light sensitive it might be challenging to see something.

We expect to see high overall silica uptake rates in the mesocosms with higher silicic acid concentrations. It will interesting, however, to find out whether the cell specific silica uptake rate also differs between mesocosms or whether the uptake per cell is similar across treatments and the difference is mainly driven by the higher abundance of diatom cells in the high silica treatments. Actually the other day I passed by the miscrocopy lab and my colleague Greta showed me the samples that she was analysing under the microscope. You can already see different amounts of diatoms in the highest and the lowest silica treatments. So let’s see what the silica uptake rate measurements will tell us.

View in the microscope A view through the microscope: the phytoplankton community in the high (left) and low (right) silicate treatment.

Also when talking to my colleague Ole, who is in charge of measuring the biogenic silica (a proxy for diatom biomass), he told me that his data shows that the high silica treatments have higher biogenic silica than the lower ones. But the difference doesn´t seem to match the silica treatments, given that we give 20 times more silicic acid to the highest compared to the lowest silica treatment. I wonder what is going on, since all the nutrients that we supply into the mesocosms with our deep water addition every second day are completely taken up. I am very excited to see what the results will show in the end.

To finalize, I would like to express my happiness to be here and to have the opportunity to learn a lot about marine research, and at the same time grow up in terms of autonomy and developing different points of view. I would like to thank all the people that help me, in particular Dr. Mar Fernández-Méndez for supervising me, Jacqueline for being the best lab-mate ever, she smiles every day, Andrea Ludwig for her assistance and Professor Ulf Riebesell, who gave me this wonderful opportunity.

Andrea at the clean bench In the end one photo of me while preparing 0.5 molar HCl solution. I usually switch from a serious face to a happy one, keeping up the desire to do and to learn new things