Cellular changes during boron‐deficient culture of the diatom Cylindrotheca fusiformis

The effects of boron‐deficient culture were studied on the unicellular diatom. Cylindrotheca fusiformis Reimann and Lewin. After 24 to 30 h, cell division was almost completely inhibited in boron‐deficient cultures. By 48 h of culture, boron‐deficient diatoms had approximately twice the modal cell volume of control cells, and at least twice the amount of organic constituents such as protein (2.0x), insoluble carbohydrate (2.4x), total phenols (2.6x), and chlorophyll at (2.1x). Boron deficiency led to irreversible damage after this time. Dark respiration was 1 nmol O 2 /min × 10 6 cells for both control and boron‐deficient diatoms prior to 40 h of culture. By 48 h, the respiratory rate of boron‐deficient diatoms was double that of controls. The proportion of 14 C‐glucose metabolized by the pentose phosphate pathway was similar in both control and boron‐deficient diatoms after 24 and 48 h of culture. After 24 h, the in vitro activity of glucose‐6‐phosphate dehydrogenase was similar in both control and boron‐deficient cells, although the pool size of its substrate, glucose‐6‐phosphate, was 26% greater in boron‐deficient cells. The cellular amount of glucose‐6‐phosphate dehydrogenase continued to increase once mitosis was arrested in boron‐deficient diatoms. Boric acid (1 mM) inhibited 6‐phosphogluconate dehydrogenase by 18% in diatom homogenates. During the early stages of boron deficiency, the uptake of silicate, nitrate, and phosphate, and the in vitro activity of β‐glucosidase were similar to control diatoms. After cell division was inhibited, boron‐deficient diatoms accumulated more nitrate and phosphate, and retained a higher level of β‐glucosidase than control cells.