Influence of irradiance and iron on the growth of colonial Phaeocystis antarctica: implications for seasonal bloom dynamics in the Ross Sea, Antarctica

Abstract

Laboratory culture experiments were used to investigate the growth rate of colonial Phaeocystis antarctica as a function of irradiance and dissolved iron concentration. The experiments were conducted with a P. antarctica strain isolated from the southern Ross Sea, Antarctica, and made use of natural, low-iron (< 0.2 nM dissolved Fe) filtered seawater as a growth medium, thereby avoiding the addition of synthetic organic ligands to regulate dissolved iron concentrations. Under ironand nutrient-replete conditions, colonial P. antarctica attained an average maximum cell-specific growth rate of 0.37 d -1 at an irradiance of 68 µE m -2 s -1, above which growth rates decreased to 0.27 d -1 at an irradiance of 314 µE m -2 s -1. The dependence of growth rate on ambient dissolved iron concentration was examined in dose-response type bioassay experiments using realistic subnanomolar additions of dissolved iron. The experimental results indicate significant changes in the iron requirements for growth of colonial P. antarctica as a function of irradiance, with our estimates of the half-saturation constant for growth with respect to dissolved iron (K μ ) ranging from 0.26 nM at 20 µE m -2 s -1 , to 0.045 nM at ~40 µE m -2 s -1 and to 0.19 nM at ~90 µE m -2 s -1. We interpret these variations in K μ as reflecting an increase in the cellular iron requirements of colonial P. antarctica at suboptimal and supraoptimal irradiance, such that the cells require higher ambient dissolved iron concentrations to attain maximum growth rates under such irradiance conditions. The experiments also provide evidence of a relationship between iron availability and the relative proportion of colonial versus solitary P. antarctica cells, whereby the colonial form appears to be favored by higher dissolved iron concentrations. Our experimental results suggest that the initiation and termination of colonial P. antarctica blooms in the Ross Sea are determined by the combined effects of irradiancedriven changes in cellular iron requirements and a seasonal decrease in dissolved iron availability. KEY WORDS: Phaeocystis antarctica · Growth · Iron · Light · Ross Sea · Bloom dynamicsResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 57: 203-220, 2009 DiTullio & Smith 1995, Arrigo et al. 1999, Sweeney et al. 2000, Schoemann et al. 2005. The factors that control the spatial and temporal distribution of P. antarctica blooms in the southern Ross Sea are not well understood. Bottom-up controls by irradiance, iron and vitamin B 12 have been suggested (Arrigo et al. 1998b, Fitzwater et al. 2000, Sedwick et al. 2000, Smith et al. 2003a, Bertrand et al. 2007), whereas top-down control by grazing pressure is not thought to be significant (Caron et al. 2000, Tagliabue & Arrigo 2003. Field observations document a general decrease in the depth of the surface mixed layer, which allows an increase in mean irradiance during the growing season (Arrigo et al. 1998b, Smith et al. 2003a, Sweeney 2003. Over the same p...

Early season depletion of dissolved iron in the Ross Sea polynya: Implications for iron dynamics on the Antarctic continental shelf

“…In comparison, the apparent biological drawdown in N + N was largely confined to the upper 100 m of the water column (Figure 5f). This apparent decoupling between the drawdown of dFe and nitrogen might reflect the elevated iron requirements of phytoplankton growing under low irradiance [ Raven , 1990; Sunda and Huntsman , 1997; Garcia et al , 2009], such that algal growth had depleted dFe relative to nitrate (i.e., high Fe/N assimilation ratio) in the deeper surface mixed layer of the early spring, prior to our sampling. An alternative hypothesis is that dFe, which is strongly particle reactive, had been scavenged by sinking particles (e.g., Phaeocystis aggregates) over the 100–300 m depth range [ Johnson et al , 1997; DiTullio et al , 2000; Moore et al , 2004; Boyd and Ellwood , 2010].…”

“…This hypothesis is supported by the results of shipboard iron‐addition experiments, which indicate that phytoplankton growth was limited by iron deficiency during both the summer 2005–2006 and spring 2006 cruises [ Bertrand et al , 2007, 2011; Feng et al , 2010; Rose et al , 2009]. Considered together, these findings imply that the typical seasonal succession of phytoplankton taxa in the southern Ross Sea does not reflect taxon‐specific differences in cellular iron requirements, given that ambient dFe concentrations of less than ∼0.1 nM are likely to limit the growth of both P. antarctica and diatoms [ Timmermans et al , 2001, 2004; Garcia et al , 2009].…”

Colony formation in &lt;i&gt;Phaeocystis antarctica&lt;/i&gt;: connecting molecular mechanisms with iron biogeochemistry

“…The Ross Sea is one of the most productive regions of the Southern Ocean (Arrigo et al, 1999(Arrigo et al, , 1998Feng et al, 2010;Garcia et al, 2009;Sedwick and DiTullio, 1997), and the lat- ter is an important contributor to the cycling of carbon in the oceans (Lovenduski et al, 2008;Sarmiento et al, 1998). In the early spring when the sea ice retreats and polynyas form, phytoplankton blooms and regional phytoplankton productivity are fed by the residual winter iron inventory and perhaps iron-rich sea ice melt (Noble et al, 2013;Sedwick and DiTullio, 1997); blooms have also been linked to changes in irradiance and mixed layer depth (Arrigo et al, 1999;Coale et al, 2003;Martin et al, 1990;Sedwick and DiTullio, 1997;Sedwick et al, 2000).…”

Climate change impacts on southern Ross Sea phytoplankton composition, productivity, and export

“…For example, an investigation using structural equation modeling and data from January to February 2012 found summer phytoplankton growth rates in the Ross Sea to be most affected by levels of iron [ Mosby and Smith , ]. Irradiance levels likewise have been found to differentially affect phytoplankton growth in the Ross Sea [ Garcia et al ., ; Mills et al ., ; Feng et al ., ].…”