2020 Volume 67 Issue 1 Pages 1-18
The centric diatoms Chaetoceros debilis and Thalassiosira nordenskioeldii were isolated from the waters of the Oyashio region during the spring of 2006. Photosynthetic responses to temperature, light, and iron availability of the two diatom species grown in a batch culture medium were determined in terms of cellular pigment concentration, the relative electron transfer rate in photosystem II (ETRPSII), oxygen evolution rate and carbon fixation rate. Temperature had an influence on these photosynthetic parameters in both species. Values of the maximum photochemical quantum efficiency (Fv/Fm) of photosystem II during the exponential growth phase were consistently high at temperatures between 5–15°C in C. debilis (0.55–0.70) and between 5–10°C in T. nordenskioeldii (0.65–0.70). Light intensity also affected ETRPSII, the oxygen evolution, and carbon fixation rates in the diatom species. Batch culture experiments with a strong iron chelator desferrioxamine B (DFB) were carried out at 10°C. Iron starvation induced by the addition of DFB decreased the Fv/Fm values (0.23±0.06 in C. debilis and 0.41±0.02 in T. nordenskioeldii). The addition of DFB also decreased the concentration of cellular chlorophyll a (Chl. a), ETRPSII, the oxygen evolution rate, and carbon fixation rate in both species. Interestingly, values of the oxygen evolution rate relative to ETRPSII (O2/ETRPSII) at 10°C were higher in the DFB-amended cultures (0.217±0.050 in C. debilis and 0.171±0.040 in T. nordenskioeldii) than in the iron-sufficient cultures (0.143±0.074 in C. debilis and 0.128±0.037 in T. nordenskioeldii). However, in both species, values of the carbon fixation rate relative to ETRPSII (C/ETRPSII) were similar between the two iron conditions. In the exponential growth phase, values of the photosynthetic quotient (O2/C) were higher in the DFB-amended strains at 10°C (2.72±1.00 in C. debilis and 4.10±0.39 in T. nordenskioeldii) than in the iron-sufficient strains (1.12–1.37 between 5–15°C in C. debilis and 1.63–1.87 between 5–10°C in T. nordenskioeldii). We concluded that rather than having a significant effect on oxygen evolution, iron deficiency significantly affected ETRPSII and carbon fixation rate in the investigated diatom species.
