Molecular analysis of photosynthetic picoeukaryote community structure along an Arabian Sea transect

Abstract

We developed oligonucleotide probes, based on plastid 16S ribosomal DNA (rDNA) sequences, to target 10 different algal classes (Chlorarachniophyceae, Chrysophyceae, Cryptophyceae, Eustigmatophyceae, Pavlovophyceae, Pelagophyceae, Pinguiophyceae, Prasinophyceae [clade VI], Prymnesiophyceae, and Trebouxiophyceae), for use with dot blot hybridization technology. These class‐specific probes were then used to investigate the community structure of photosynthetic picoeukaryotes (≪3 µm) along a transect in the Arabian Sea during September 2001, using hybridization to plastid 16S rDNA polymerase chain reaction amplicons. The transect ranged from oligotrophic open‐ocean waters, just south of the Equator, through mesotrophic and eutrophic waters toward the north coast of Oman, crossing a region of monsoonal upwelling off the Omani northeast coast. Photosynthetic picoeukaryotes in the surface‐mixed layer (SML) and deep chlorophyll maximum ranged from 1.0 x 103 cells mL‐1 in southern oligotrophic waters to almost 30 x 103 cells mL‐1 at the region of upwelling. Chrysophytes were abundant along most of the transect throughout much of the euphotic zone. Prymnesiophytes were abundant in surface waters along much of the transect. By contrast, trebouxiophytes were confined to deeper waters, below the SML. Pelagophytes were found in surface waters that were more mesotrophic, whereas cryptophytes were only detected in the more nutrient‐rich waters at the northern end of the transect, between depths of 20‐30 m. Pinguiophytes were also detected, but only in the warm surface waters off the north coast of Oman. Chlorarachniophytes, eustigmatophytes, pavlovophytes, and clade VI prasinophytes were essentially below detection limits for the entire transect.