CHEMICAL-COMPOSITION AND PHOTOSYNTHETIC CHARACTERISTICS OF ETHMODISCUS-REX (BACILLARIOPHYCEAE) - EVIDENCE FOR VERTICAL MIGRATION

Ethmodiscus rex (Rattray) Wiseman and Hendey cells from near surface n et tows in the Southwest Atlantic Ocean and Caribbean Sea were examine d for chemical composition, internal nutrient pool concentrations, and oxygen evolution characteristics. Elemental ratios indicated nitrogen limitation with C:N:P ratios of 125:9:1 (atoms), and carbon: chloroph yll (chl) ratios of 129:1 (weight). However, internal nitrate pools (1 . 4-27. 1 mM) suggested that cells were not N-limited. Intracellular N O3- accounted for up to 54% (range = 3-54%) of the total N quota in so me samples. Photosynthetic parameters were consistent with a high-ligh t-adapted population and suggested an instantaneous maximum chl-specif ic photosynthetic rate (P(B)max of 4.8-12.4 nmol O2.mug chl-1.h-1. Res piration rates varied ten-fold and were inversely related to P(B)max. Ethmodiscus chemical composition and buoyancy characteristics are simi lar to vertically migrating Rhizosolenia mats and the non-motile dinof lagellate Pyrocystis noctiluca Murray (Schuett). The presence of inter nal NO3- pools in Ethmodiscus suggests that this genus is also vertica lly migrating to exploit sub-surface nitrogen pools. Such behavior may be widespread in large, non-motile oceanic phytoplankton. Based on as cent rate data, chemical composition, and photosynthetic rates, we est imate that the entire division-migration cycle for Ethmodiscus require s at least 7-12 days.