Discussion

The significantly larger populations cultured in 370ppm CO₂ than at 750ppm CO₂ demonstrates that T. weissflogii populations did not respond to concentrations of atmospheric CO₂ above those currently present in the atmosphere.  At the conclusion of the experiment, the nutrients appear to be limiting population growth (Figs.7, 8 & 9). Data collected over the last two days of the experiment seem to suggest that all populations, regardless of culture condition, appear to be leveling off.  That noted, it would be interesting to prolong the experiment to determine more precisely when and where the different cultures reach their carrying capacity.  This is particularly relevant because if all populations ultimately plateau at the present level, with the 370ppm culture producing more cells  than either the 750ppm and 100ppm cultures, then marine diatoms may not serve as a long-term sink for atmospheric CO_2.  These results did not support our hypothesis that there is a direct relationship between the concentration of atmospheric CO₂ and the growth of T. weissflogii.

The pH of the culture media was directly correlated with the amount of CO₂ introduced into the medium.  This result occurred because CO₂ forms carbonic acid in the presence of water.   As a result, the medium exposed to 750ppm CO₂ was the least basic (pH≈8.0) whereas the most basic medium was that which was incubated with 100ppm CO₂ (pH≈9.1).  Despite these differences in pH, they did not appear to affect the growth rates of the experimental populations.

The CO₂ concentrations of 370ppm and 750ppm produced diatom cultures whose chlorophyll yielded the greatest fluorescence (Fig. 3), but subsequent spectrophotometer-based determination of the chlorophyll concentration did not support this finding (Figs. 4, 5, and 6).  This disparity was also noted by Lee (1997) and could have resulted from the different sampling techniques.  Fluorescence indirectly measures chlorophyll concentration by recording the fluorescence generated when light-activated electrons fall back to their original orbital, but direct measurements of chlorophyll concentration rely on spectrophotometer readings that record the amount of light absorbed by the sample at specified wavelengths.  Fluorescence, then, may not be a reliable indicator of chlorophyll concentration.  As a result of this inconsistency, more research needs to be done to either justify or refute a link between atmospheric CO₂ concentrations and photopigment quantities. 

The highest rate of carbonic anhydrase (CA) activity in the diatom cultures maintained at 100ppm CO₂ agrees with principles of aquatic chemistry (Fig. 10).  In particular, it is known that in more basic media, there are a higher percentage of bicarbonate ions than carbon dioxide.  Diatoms use carbonic anhydrase to convert bicarbonate into useable carbon dioxide.  Therefore our results show a direct relationship between low CO₂ concentrations, more basic pH (Fig. 2), and increased CA activity (Fig. 10).  Similar research performed in 1999 yielded similar findings, but in 2000 different results were obtained.  The results of this study support our hypothesis that increased pH should yield more active CA enzymes because of the low CO₂ concentration present in this environment.

The drastic reduction in nitrate, phosphate, and silicate experienced in the first four days of the experiment in the 370ppm and 750ppm CO₂ cultures was largely caused by the exponential growth of these diatom populations.  Those cultures reared in 100ppm CO₂ experienced less population growth and, as a result, consumed less inorganic nutrients (Figs. 7, 8, and 9).  Additionally, it was discovered that phosphate and silicate, unlike nitrate, did not recover after being depleted.  This may be explained by the relative insolubility of phosphate and silicate; once living diatoms have absorbed these nutrients they are not readily released even following death of the diatom.  The continued growth of the experimental populations after inorganic nutrient levels approached zero may be explained by metabolic inertia of the diatoms (Latasa, 1995).  Overall, there seems to be a strong relationship between diatom growth rate and available nutrients.

In all, the data from this short duration experiment suggest that T. weissflogii may play a role in the global carbon cycle, but may not have the potential to increase their growth rates in conditions of elevated atmospheric CO₂ concentrations.  Consequently, marine diatoms may not help to lessen the impact of global warming by increasing carbon storage during periods of increased atmospheric CO₂.  Longer duration studies should be performed to quantify the long-term impact of diatoms on atmospheric CO₂ given the finite availability of inorganic nutrients especially in light of previous research, which has indicated that inorganic nutrients can limit T. weissflogii growth Lee (1997).