Photosynthetic picoeukaryote assemblages in the South China Sea from the Pearl River estuary to the SEATS station

Abstract

Photosynthetic picoeukaryotes (PPEs) can be important primary producers in the oligotrophic ocean and coastal waters at certain times of the year. In this study, we investigated the abundance and biodiversity of picoplankton, focusing on PPEs in the South China Sea (SCS) from the Pearl River estuary to SCS basin in January 2010, when the northeast monsoon prevailed. PPE abundance was quantified using fluorescent in situ hybridization associated tyramide signal amplification, and the biodiversity at 5 selected stations was determined using small subunit ribosomal RNA gene (18S rDNA) clone libraries. Our results showed that PPEs were most abundant in the Pearl River estuary (up to 8500 cells ml −1), and that mamiellophycean picoplanktonic green algae, such as Micromonas, Ostreococcus and Bathycoccus, showed peaks of abundance in slope or coastal waters. The 18S rDNA phylogeny revealed that most of the PPEs belonged to prasinophytes, affiliating to 4 clades (Clade IX, Clade V, Clade VII, and Mamiellophyceae). Phytoplankton pigment analysis clearly showed the difference in picophytoplankton community structure along the environmental gradient provided by the selected stations. Among the PPEs, prasinophytes and prymnesiophytes accounted for 18.7 and 41.5%, respectively, of the chlorophyll a biomass. Putting all the data together, we describe a complete picture of PPE assemblages along the coast-offshore gradient, showing that the prasinophytes and prymnesiophytes appear to be the key PPE components in this subtropical-tropical marginal sea.

Revision of the Genus Micromonas Manton et Parke (Chlorophyta, Mamiellophyceae), of the Type Species M. pusilla (Butcher) Manton & Parke and of the Species M. commoda van Baren, Bachy and Worden and Description of Two New Species Based on the Genetic and Phenotypic Characterization of Cultured Isolates

“…All previously published phylogenies (among which the multigene analyses by Slapeta et al 2006) and genomic analyses of Micromonas strongly suggest that this genus comprises a genetically diverse complex of cryptic species or clades that have been attributed different codes (Guillou et al 2004;Lovejoy et al 2007;Slapeta et al 2006;van Baren et al 2016;Worden et al 2009;Wu et al 2014; Table 1). …”

“…None of our new isolates fell into clade B._.4 identified by Worden (2006) and composed solely of environmental sequences. An additional rather deeply diverging clade, already distinguished as an "unknown clade" in Wu et al (2014) , included environmental sequences retrieved from coastal surface waters (<10m) of the South China Sea (Wu et al 2014) and Red Sea (Acosta et al 2013), as well as a sequence retrieved from strain RCC1109, isolated from a Mediterranean lagoon.…”

“…Strains that lacked this pigment were isolated from surface waters (RCC570, RCC676) or unknown depths. Chl cCS-170 was not detected in strain RCC1109 from the "unknow clade" of Wu et al (2014) and arctic strain CCMP2099 (Lovejoy et al 2007). It was detected in only 1 isolate (RCC806) of clade B.E.3 and 1 isolate (RCC833) of clade C.D.5 (out of 10 isolates analysed for each of these clades).…”

“…Compared to other Micromonas clades, information available for clades B._.4 and for the "unknown clade" discovered by Wu et al (2014) is scarce. Both clades exhibit high SSU rDNA genetic divergences with the Micromonas species described.…”

“…Several studies based on phylogenetic analyses of different genetic markers from culture isolates collected worldwide have distinguished three to seven genetic clades and suggested the existence of cryptic species within Micromonas (Guillou et al 2004;Lovejoy et al 2007;Slapeta et al 2006;van Baren et al 2016;Worden 2006;Worden et al 2009;Wu et al 2014, Table 1). Marin and Melkonian (2010) even suggested that some of these clades should be raised to genus status as they are genetically as different from the neotype culture of Micromonas pusilla as they are from Mantoniella.…”