Estimation of the nutrient consumption by various cell sizes of the diatom Eucampia zodiacus: A representative organism causing bleaching of aquacultured nori

Abstract

The diatom Eucampia zodiacus is a harmful species that indirectly causes bleaching to nori (Pyropia) cultivation through competitive utilization of nutrients during its bloom, however cellular storage and changes in physiology by asexual reproduction remains unclear. In the present study, we experimentally investigated the nitrate (N), phosphate (P) and silicic acid (Si) consumption by various cell sizes of E. zodiacus strains, the apical axis length of which ranged from 10.2 to 77.3μm. Nutrient cell quotas of E. zodiacus ranged from 2.7 to 8.4pMcell−1 for N, 0.34–0.76pMcell−1 for P and 1.7–7.3pMcell−1 for Si, and they increased with cell size, in which there is a significant correlation between these two elements. The N and P quotas were estimated to be several times higher than the minimum cell quotas. In contrast, the Si cell quotas were approximately equal to those of the minimum values. Based on the present cell quotas, total nitrate consumption by E. zodiacus population when the blooms reached maximum cell density (=1000cellsml−1) were estimated to be 6.5μM. Monthly mean concentrations of dissolved inorganic nitrogen (DIN) range from 3.5 to 8.2μM during the period of late nori harvest season when E. zodiacus blooms occur, and nori bleaching is reported at the condition of DIN concentration of less than 3μM in Harima-Nada, eastern Seto Inland Sea, Japan. Therefore, the present results suggest that E. zodiacus causes serious damage to nori cultivation due to high levels of nutrient consumption.