Structure and seasonal dynamics of the eukaryotic picophytoplankton community in a wind-driven coastal upwelling ecosystem

We studied the structure and seasonal dynamics of the photosynthetic picoeukaryote community at a coastal site off central Chile that is strongly affected by seasonal wind-driven upwelling. We determined the picoeukaryote phylogenetic diversity by cloning and sequencing the 18S ribosomal ribonucleic acid (rRNA) gene amplified with Chloroplastida-biased primers. Chloroplastida made up 100% (autumn), 61% (spring), and 20% (summer) of the clone libraries. Most sequences belonged to Mamiellophyceae and clustered with Micromonas (clades A.BC.1, B.E.3, C.D.5), Ostreococcus (clade A), and Crustomastix (clade A). Fluorescent in situ hybridization coupled with tyramide signal amplification (FISH-TSA) was used to determine the time variability in abundance of the total picoeukaryotic community, the Chloroplastida, and the genera Micromonas, Ostreococcus, and Bathycoccus over a period of 2 yr with nearly monthly sampling. The abundance of photosynthetic picoeukaryotes was low during the upwelling season (spring and summer) and high during the nonupwelling season (late fall and winter). Chloroplastida made up, on average, 74% of the total picoeukaryotic community, and the sum of the three genera probed accounted for 100% of the Chloroplastida abundances. Ostreococcus dominated the picophytoplanktonic community numerically throughout the year and, thus, appears to be a key component of the upwelling picoplanktonic community in the eastern South Pacific.