Protist control of phytoplankton growth in the subtropical north-east Atlantic

Abstract

The grazing impact of protists on phytoplankton primary production in a highly oligotrophic area of the subtropical north-east Atlantic, south-east of the Azores, was studied using the dilution method. The phytoplankton community in the experiments, analysed by flow-cytometry, was dominated by Prochlorococcus and Synechococcus although larger phototrophic groups, beyond flow-cytometry sensitivity, were also present. The microzooplankton community was dominated by small gymnodinoid dinoflagellates and aloricate ciliates, and its biomass was low, averaging 0.7 mg C m -3. We measured average phytoplankton growth rates of 0.76 and 0.22 d -1 in surface waters and in deep chlorophyll maximum (DCM) waters respectively; mortality rates due to protists grazing were 0.58 and 0.22 d -1 , respectively. We did not find significant differences in growth and mortality rates of the different phytoplankton groups, i.e. Prochlorococcus, Synechococcus or nanoeukaryotes; thus no prey preference was apparent from the results. Protists consumed on average 79 and 109% of primary production in surface and DCM waters respectively, controlling phytoplankton growth in the area. The net primary production closely oscillated around zero in the chlorophyll a analyses and was positive in the flow-cytometry analyses, suggesting a balanced or slightly autotrophic phytoplanktonprotist compartment.

Seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in the southern South China Sea under the influence of the East Asian monsoon

“…The high µ/µ n (approximate to or higher than one) indicated that phytoplankton growth was only slightly or even not nutrient-limited during the summer cruise (Table 1). Similar results in the oligotrophic subtropical Northeast Atlantic have also been reported (Cáceres et al, 2013;Quevedo and Anadón, 2001). In contrast, severe nutrient limitation indicated by the lower µ/µ n (0.54 ± 0.30) was observed during the winter, which may account for part of the low µ (Table 5).…”

“…Non-linear responses were usually observed in eutrophic waters with high prey abundance (Elser and Frees, 1995;Gallegos, 1989;Teixeira and Figueiras, 2009). Nonlinear feeding responses in dilution experiments conducted in the oligotrophic subtropical Northeast Atlantic in summer have also been reported by Quevedo and Anadón (2001) and Cáceres et al (2013). However, the authors did not explain the underlying reasons for these phenomena.…”

Population dynamics and diversity of Synechococcus on the New England Shelf

“…This broad term encompasses the heterotrophic nanoflagellates, ciliates, and dinoflagellates, all which have been reported to feed on Synechococcus [21,15,85,20,44] and may be important classes of grazers at MVCO. These organisms can quickly react to and take advantage of an increase in prey cells as their own cell division rates can match or exceed those of Synechococcus [52].…”

Growth and specific P-uptake rates of bacterial and phytoplanktonic communities in the Southeast Pacific (BIOSOPE cruise)

“…Production can be expressed as the rate of synthesis of cells or cell mass: production=µ×biomass, where µ is the specific growth rate of the population expressed in units of inverse time t −1 (Ducklow, 2000). Phytoplankton µ estimates vary widely from values of around 0.1-0.3 d −1 (Letelier et al, 1996;Marañon et al, 2000Marañon et al, , 2005 to 1-2 d −1 (Laws et al, 1987;Quevedo and Anadon, 2001). Bacterial µ estimates also vary widely, from very low values 0.004-0.25 d −1 (Sherr et al, 2001;Van Wambeke, 2007b) to higher values of around 2-10 d −1 (Ducklow, 1983;Jones et al, 1996).…”

“…heterotrophic bacterial µ. The two most common direct methods, applicable to both heterotrophic bacteria and phytoplankton, are (1) to observe the frequency of dividing cells (Hagstrom et al, 1979) and (2) the dilution technique (Landry and Hassett, 1982;Quevedo and Anadon, 2001). Direct methods are difficult to set up on board so microbial growth rates are commonly calculated from production and standing stock data (Ducklow, 2000).…”