29Si{1H} CP-MAS NMR comparison and ATR-FTIR spectroscopic analysis of the diatoms Chaetoceros muelleri and Thalassiosira pseudonana grown at different salinities

Diatoms are key indicators of marine environmental health. To further understand how diatoms respond to varying degrees of salinity, either due to climate change or brine waste discharge into marine environments, two different diatom species were studied. Thalassiosira pseudonana and Chaetoceros muelleri were cultured at three different salinities namely, 26 practical salinity units (PSU or parts per thousand), 36 PSU (standard salinity for culturing of seawater species) and 46 PSU. Changes in silica and organic content within the cultured diatoms were analysed using solid-state ²⁹Si{¹H} cross-polarization-magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopies coupled with analysis of variance. ²⁹Si CP-MAS NMR showed that qualitatively the Q4:Q3 area ratios of C. muelleri, grown away from standard salinities, increased in response to the formation of more condensed (2 ≡SiOH → ≡Si-O-Si≡ + H₂O) and/or an increase in closely associated organic matter to the Q4 component of the diatoms. This was not observed for T. pseudonana. However, both species showed the appearance of a new peak centered at 1575-1580 cm^-1 in the ATR-FTIR spectra, designated as the CâN band of nitrogenous purine-type compounds. Further, the C. muelleri species was shown to produce more extracellular polymeric substances at non-standard salinities. On this basis, results suggest that there is a strong relationship between diatom composition and salinity and that C. muelleri is more sensitive to its environment than T. pseudonana.