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.
Using plankton sampled from the Southern Ocean and archived at the National Institute of Polar Research (NIPR, Tokyo), we prepared resin-embedded plankton specimens as a marine education material. This form of preservation allows one to observe the specimens’ appearance in multiple aspects with the naked eye. In addition, resin-embedded specimens are solid, robust and lightweight, and so can be easily transported and handled. Moreover, such specimens can be used for observations of fine structure, which has various advantages as a learning material for graduate students and young scientists. As of October 2019, we produced 210 individual pieces, comprising 52 species of marine plankton in 12 major groups. The specimens have so far been used at various locations and events, such as the “Open House” and the “Polar Science Museum” of the NIPR, for display at the GRAntarctic booth of the JpGU, at junior high school classes, and in science cafes. Plankton are important organisms for understanding marine ecosystems, yet social awareness of this assemblage of organisms is often poor. Therefore, we intend to improve and increase our production of resin-embedded marine plankton specimens. Resin-embedded specimens are now registered in the ‘Database of Animal Specimens from Polar Regions’ of the NIPR. We expect that this type of preserved material will make a valuable contribution to marine education, at the primary- to graduate-school levels as well as for the general public.