Effect of KNO3 on Lipid Synthesis and CaCO3 Accumulation in Pleurochrysis dentata Coccoliths with a Special Focus on Morphological Characters of Coccolithophores

Abstract

Pleurochrysis genus algae are widely distributed in ocean waters. Pleurochrysis sp. algae are popularly known for its coccolithophores. Calcium carbonate (CaCO₃) shells are major components of the coccolithophore, and they are key absorbers of carbondioxide. In this study, we have reported the effects of potassium nitrate (KNO₃) concentration on calcium accumulation and total lipid, carbohydrate and protein contents of Pleurochrysis dentata. Results obtained from complexometric titration and scanning electron microscopy analysis showed higher rates of CaCO₃ accumulation on Pleurochrysis dentata cell surface. We have also observed that overall cell size of Pleurochrysis dentata reached maximum when it was cultured at 0.75 mmol L^-1 of KNO₃. During 10 days of Pleurochrysis dentata culture total lipids and carbohydrate contents decreased, with slightly increased protein content. Results obtained from Fourier-Transform Infrared Spectroscopy (FTIR) also reported an increase in protein and decrease in lipids and carbohydrate contents, respectively. Similarly, Pleurochrysis dentata cultured at 1 mmol L^-1 concentration of KNO₃ exhibited the lowest carbohydrate (21.08%) and highest protein (32.87%) contents. Interestingly, Pleurochrysis dentata cultured without KNO₃ exhibited 33.61% of total lipid content which reduced to a total lipid content of 13.67% when cultured at 1 mmol L^-1 concentration of KNO₃. Thus, culture medium containing higher than 1 mmol L^-1 of KNO₃ could inhibit the cell size of Pleurochrysis dentata and CaCO₃ accumulation in shells but it could promote its cell growth. For the first time we have reported a relatively complete coccolith structure devoid of its protoplast. In this study, we have also described about the special planar structure of Pleurochrysis dentata CaCO₃ shells present on its inner tube of the R unit and parallel to the outer tube of the V unit which we named it as "doornail structure". We believe that this doornail structure provides structural stability and support to the developing coccoliths in Pleurochrysis dentata. Also, we have discussed about the "double-disc" structure of coccoliths which are closely arranged and interlocked with each other. The double-disc structure ensures fixation of each coccolith and objecting its free horizontal movement and helps in attaining a complementary coccolith structure.

Keywords: Calcium; Calcium carbonate (CaCO3); Carbohydrate; Coccolithophore; Lipid; Pleurochrysis dentata; Potassium nitrate (KNO3)..