Ecological niche partitioning in the picoplanktonic green alga Micromonas pusilla: evidence from environmental surveys using phylogenetic probes

Abstract

Very few studies have analysed the niches of pelagic protist in details. This is because for most protists, both an accurate species definition and methods for routine detection and quantification of cells are lacking. The morphospecies Micromonas pusilla, a marine unicellular green alga, is the most ubiquitous and cosmopolitan picoeukaryote described to date. This species comprises several independent genetic lineages or clades, which are not currently distinguishable based on comparison of their morphology or biogeographical distribution. Molecular probes were used to detect and quantify the genetic clades of M. pusilla in samples from temperate, polar and tropical environments in order to assess potential ecological niche partitioning. The three clades were detected in all biogeographical regions studied and were commonly found in sympatry. Cell abundances recorded for clades A and B were high, especially at coastal stations. Clade C, when detected, was always at low abundances and is suggested to be a low-light clade. Shifts in the contribution of clades to total M. pusilla abundance were observed along environmental gradients, both at local and basin-wide scales. This suggests that the phylogenetic clades occupy specific niches and confirms the existence of cryptic species within the morphospecies M. pusilla. Parameters which can precisely explain the distribution of these cryptic species remain to be elucidated.

Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

“…Picoplanktonic chlorophytes cells (o3 mm) were represented almost exclusively by the Prasinophyceae Micromonas pusilla. The great abundance of Micromonas is consistent with previous estimation of picoplankton composition in the Mediterranean Sea including Blanes Bay (Foulon et al, 2008) but also in other oceanic regions (for example, Not et al, 2007;2004). Our results also revealed the existence of marked changes in the abundance of some nano-PFs (3-20 mm) along the year (cryptophytes and chlorophytes dominated at the end of winter, pigmented dinoflagellates increased in summer, and at the beginning of winter all groups increased significantly), whereas haptophytes were present all year-round always at a significant number without major fluctuations (Figure 2).…”

Significant CO2 fixation by small prymnesiophytes in the subtropical and tropical northeast Atlantic Ocean

“…Filters were incubated twice at 55 1C for 20 min in TNT buffer and then dried at room temperature. Cells were counterstained with propidium iodide (final concentration 10 mg ml À1 ) (Foulon et al, 2008) to visualize the nuclei. This allowed verification of labelling eukaryotic cells.…”

Phylogeny and Molecular Evolution of the Green Algae

“…Phylogenies based on the 18S rDNA showed that Micromonas and Ostreococcus are divided in several clades corresponding to different ecotypes and probably different species (see below) Šlapeta et al, 2006). This has been shown on the basis of physiological properties Cardol et al, 2008), geographical origin (Foulon et al, 2008), and niche partitioning (Demir-Hilton et al, 2011) of the strains/ecotypes. For example, "low light" and "high light" ecotypes have been described in Ostreococcus, corresponding to different clades .…”