Primary production, calcification and macromolecular synthesis in a bloom of the coccolithophore Emiliania huxleyi in the North Sea

Abstract

Photosynthesis, calcification and the patterns of carbon (C) incorporation into different biomolecules were investigated during a bloom of the coccolithophore Emiliania huxleyi in the North Sea during June and July 1994. The bloom was confined to an area of ca 3000 km2 centred at 59'50' N. 00°42'E and characterized by enhanced thermal stability and low nitrate concentrations in the upper mlxed layer. Surface E. huxleyi densities within the bloom area ranged between 1 and 6 X 106 cells I-' The mesoscale distribution of E. huxleyi abundance suggested that the bloom formation was related to the presence of low concentrations of nitrate rather than phosphate. The bloom was sampled during an early stage of its development, as indicated by the low calcite-C levels (c50 my C m-3], the low calciteC to particulate organlc carbon (POC) ratlo ( ~ 0 . 2 5 ) and the low density of detached coccoliths (2 to 3 X 10' m1 l ) . Reduced levels of chlorophyll a (<45 mg m-2) and productivity (<1.2 g C m-L d-') were measured in the coccolithophore-rich waters as compared to stations outside the bloom area. Typical calcification rates within the bloom were 135 mg C m-' d-', representing up to 20% of the total C incorporation. Relative C incorporation into lipids in coccolithophore-rich waters was 1.5 times higher than outside the bloom area. Most of the recently synthesized lipid (70 to 90%) belonged to the neutral lipid fraction. The enhanced lipid synthesis resulted in a higher lipid content of the particulate matter. The relationships between irradiance and photosynthate partitioning consistently showed that C is preferentially channeled into the protein fraction at low light levels, whereas the relative synthesis of lipids increases only at high irradiances. These metabolic responses are discussed in relation to the development of E. huxleyi-rich assemblages. It is concluded that the observed patterns of C incorporation represent a general strategy of energy utilization that is also attributable to other groups of marine phytoplankton.