Composition and seasonal variability of picoeukaryote communities at two subtropical coastal sites with contrasting trophic conditions

Abstract

Phylogenetic diversity (18S rRNA gene) of picoeukaryotes in Hong Kong coastalwaters is dominated by heterotrophic forms and influenced by the seasonallydriven combination of freshwater discharge from the Pearl River, oceanic water ofthe South China Sea and coastal water from the China Coastal Current.KEYWORDS: picoeukaryote; subtropical coastal waters; 18S-rDNA; DGGE;seasonal variationPicoeukaryotes, small eukaryotic plankton with diam-eter ,3 mm, have ubiquitous distribution and areimportant contributors to global primary productionand carbon and mineral cycles (Li, 1994). They arephylogenetically, morphologically and metabolicallydiverse and are capable of phototrophy, phagotrophy,osmotrophy and mixotrophy (Moreira and Lo´pez-Garci´a, 2002).Traditionally, morphological features have been usedfor picoeukaryote identification and classification.However, those features alone are not sufficient to dis-tinguish clearly among species or groups of picoeukar-yotes due to their tiny sizes and cryptic morphologicalcharacteristics (Sims et al., 2002), leading to underesti-mation of the diversity at phylum level (Massana et al.,2002). In contrast, molecular approaches based on the18S rRNA gene provide new insights into picoeukaryotecommunity structure in various ecosystems. Over thepast several years, studies have demonstrated tremen-dous molecular phylogenetic diversity among theseorganisms in open marine systems, including the equa-torial Pacific Ocean (Moon-van der Staay et al., 2001),the Antarctic Polar Front (Lo´pez-Garci´a et al., 2001),the Mediterranean and Scotia Sea as well as the NorthAtlantic Ocean (Di´ez et al., 2001b). A large number ofnovel picoeukaryote phylogenetic groups, some without