Abundance and distribution of Ostreococcus sp. in the San Pedro Channel, California, as revealed by quantitative PCR

Abstract

Ostreococcus is a genus of widely distributed marine phytoplankton which are picoplanktonic in size (<2 mum) and capable of rapid growth. Although Ostreococcus has been detected around the world, little quantitative information exists on its contribution to planktonic communities. We designed and implemented a genus-specific TaqMan-based quantitative PCR (qPCR) assay to investigate the dynamics and ecology of Ostreococcus at the USC Microbial Observatory (eastern North Pacific). Samples were collected from 5 m and the deep chlorophyll maximum (DCM) between September 2000 and August 2002. Ostreococcus abundance at 5 m was generally <5.0 x 10(3) cells ml(-1), with a maximum of 8.2 x 10(4) cells ml(-1). Ostreococcus abundance was typically higher at the DCM, with a maximum of 3.2 x 10(5) cells ml(-1). The vertical distribution of Ostreococcus was examined in March 2005 and compared to the distribution of phototrophic picoeukaryotes (PPE) measured by flow cytometry. The largest contribution to PPE abundance by Ostreococcus was approximately 70% and occurred at 30 m, near the DCM. Despite its relatively low abundance, the depth-integrated standing stock of Ostreococcus in March 2005 was approximately 30 mg C m(-2). Our work provides a new technique for quantifying the abundance of Ostreococcus and demonstrates the seasonal dynamics of this genus and its contribution to picoeukaryote biomass at our coastal sampling station.

FIG. 1.

Station location (33°33′N, 118°24′W), site of sample collection for the USC Microbial Observatory and San Pedro Ocean Time Series (SPOTS) projects, San Pedro Channel, Pacific Ocean. The map was drawn with Online Map Creation (http://www.aquarius.geomar.de/omc/).

FIG. 2.

Calibration of Ostreococcus qPCR method with a dilution series of plasmid DNA (A) and a dilution series of an Ostreococcus culture, processed as cell lysates (B).

FIG. 3.

CTD profile at the USC Microbial Observatory during March 2005 showing in situ chlorophyll fluorescence, oxygen content, and temperature through the upper 100 m of the water column. Relatively invariant properties in the upper 20 m indicated a well-mixed surface layer.

FIG. 4.

Vertical distribution of total picoeukaryotes (open squares) and Ostreococcus sp. in the upper water column at the USC Microbial Observatory site during March 2005. Ostreococcus abundances were determined for two size classes (<80 μm and 80 to 200 μm) by qPCR (open and closed circles, respectively) and by FCM (open diamonds). Error bars represent the standard errors of the estimates.

FIG. 5.

Flow cytometric signatures of Ostreococcus sp. (left panel) and Micromonas pusilla (right panel) indicating overlap in detection regions (polygons) that could lead to an overestimate of Ostreococcus abundance of ∼30% when both genera are present.

FIG. 6.

Depth variability of the DCM over the period of sample collection (A) and depth-integrated (1 to 40 m) nitrate (shaded circles) and depth-integrated phosphate (open circles) contents for the same period (B). Nutrient data for the last two sampling dates were not available.

FIG. 7.

Ostreococcus and total diatom abundances at 5 m (A) and the deep chlorophyll a maxima (B) during a 2-year period at the USC Microbial Observatory sampling site.