The Effects of Varying CO₂ Concentrations on the Marine Diatom Thalassiosira  weissflogii

Kurt D. MacDonald, Bukky Okusanya,

Abstract

The effects of 100, 370, and 750ppm CO₂ on the biological, chemical, and enzyme activity of the marine diatom Thalassiosira weissflogii were investigated for seven days in sterilized seawater culture supplemented with nutrients, vitamins, and trace metals under laboratory conditions. 

There was a steady increase in the growth of these diatoms with time, with populations cultured in 370ppm CO₂ experiencing significantly more growth than the other cultures.  The pH of the culture medium was more basic with increased CO₂ concentration in response to the amount of carbonic acid formed in the culture medium.  However, this pH difference did not directly affect the growth of the diatoms maintained under different CO₂ concentrations.  While the fluorescence of chlorophyll also increased with elevated CO₂ concentrations, the total pigment content (Chlorophylls a, c, and carotinoid) generally decreased with increased CO₂ concentration.  This result indicates that fluorescence may not be a reliable indicator of photopigment content. 

The carbonic anhydrase (CA) activity was highest at the lowest CO₂ concentration and at more basic culture pH.  These results indicate that more CA was produced to utilize the bicarbonate ions that are more plentiful in media with low CO₂.  Over time, there was a decrease in the concentration of inorganic nutrients (nitrate, phosphate, and silicate) with increased CO₂, and this was correlated with increased diatom growth.  The decrease in phosphate and silicate was rather dramatic and their concentrations did not recover after being depleted, unlike nitrate.  The sigmoid shape of the growth curves suggest that one or more of these nutrients may be limiting population growth.

These results indicate that populations of marine diatom T. weissflogii did not respond positively to increased CO₂ concentration in terms of growth as a likely result of inorganic nutrient limitations.   There was, however, increased chlorophyll fluorescence, decreased photopigments and CA activity as CO₂ concentrations increased.  The possible roles of diatoms and other photosynthetic marine algae in the flow of carbon between the atmosphere and the ocean, and their involvement in global warming were discussed.