PHYTOPLANKTON PIGMENTS IN THE DEEP CHLOROPHYLL MAXIMUM OF THE CARIBBEAN SEA AND THE WESTERN TROPICAL ATLANTIC-OCEAN

We report on the distribution of chlorophylls and carotenoids in the d eep chlorophyll maximum (DCM) of the western tropical Atlantic and the Caribbean Sea, focusing on pigments of picophytoplankton. Prochloroco ccus-like cells consistently were the dominant component of the DCM ph ytoplankton community, with divinyl chlorophyll a being a nearly-const ant 38% of total chlorophyll a. Coccoid cyanobacteria were 1 to 2 orde rs of magnitude less abundant at all stations and depths. Prochlorococ cus-like cells in the deeper parts of the DCM contained large amounts of chlorophyll b, often more than twice as much as divinyl chlorophyll a, suggesting the possibility of chromatic adaptation to increasing p roportions of blue-green light at the bottom of the euphotic zone. Bas ed on flow cytometry estimates of cell abundance, divinyl chlorophyll a per cell also increased dramatically through the DCM, with values ra nging from 0.09 to 1.45 fg cell-1. The carotenoids 19'-hexanoyl-oxy-fu coxanthin and 19'-butanoyl-oxy-fucoxanthin increased with depth in the DCM in relation to both total chlorophyll a and non-divinyl chlorophy ll a, supporting the idea that eukaryotic nano- and picoplankters comp rise a higher portion of the phytoplankton community in the deeper por tions of the DCM in the tropics.