Growth of <i>C. watsonii</i> WH8501 batch cultures over a 6-day period under three CO₂ treatments.

<p>Shown are concentrations of PN (a), PC (b), and cells (c), molar C:N ratios (d), and <i>p</i>CO₂ in <i>µ</i>atm (e) within each treatment. For (a–c), the concentrations for each time point (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110660#pone.0110660.s001" target="_blank">Table S1</a>) were first normalized to the concentration at the day 1 L6 time point, then ln-transformed. The derived slopes between day 1 L6 and day 3 L6 time points correspond to the exponential growth rates (<i>µ</i>) as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110660#pone-0110660-t002" target="_blank">Table 2</a>. The lines in (a) represent linear regressions through the day 1, day 2, and day 3 L6 time points for high-CO₂ (dashed line) and low-CO₂ (dotted line) treatments. The regression lines have been extended to the full time period (day 0–5) for visualization of exponential growth (day 0–3 L6 time points) transitioning to early stationary growth (L6 time points after day 3). The dotted line in (d) represents the 6.6 C:N ratio expected from Redfield stoichiometry. Shaded areas represent the dark periods. Error bars represent standard deviations from three replicates.</p

Biomass-specific growth rates of <i>Crocosphaera watsonii</i> WH8501 cultures grown under three CO₂ treatments.

<p>Biomass-specific growth rates of <i>Crocosphaera watsonii</i> WH8501 cultures grown under three CO₂ treatments.</p

Carbon-normalized PN (a) and PC production rates (b) of <i>Crocosphaera</i> WH8501 cultures grown under three CO₂ treatments during periods of exponential (day 1–day 3) and early stationary (day 3–day 5) growth phases.

<p>Production rates are calculated as increases in PC and PN concentrations (data provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110660#pone-0110660-t001" target="_blank">Table 1</a>) per time normalized to initial PC concentrations within the time interval. Error bars represent standard deviations from three replicates.</p

Physiological Response of <i>Crocosphaera watsonii</i> to Enhanced and Fluctuating Carbon Dioxide Conditions

<div><p>We investigated the effects of elevated <i>p</i>CO₂ on cultures of the unicellular N₂-fixing cyanobacterium <i>Crocosphaera watsonii</i> WH8501. Using CO₂-enriched air, cultures grown in batch mode under high light intensity were exposed to initial conditions approximating current atmospheric CO₂ concentrations (∼400 ppm) as well as CO₂ levels corresponding to low- and high-end predictions for the year 2100 (∼750 and 1000 ppm). Following acclimation to CO₂ levels, the concentrations of particulate carbon (PC), particulate nitrogen (PN), and cells were measured over the diurnal cycle for a six-day period spanning exponential and early stationary growth phases. High rates of photosynthesis and respiration resulted in biologically induced <i>p</i>CO₂ fluctuations in all treatments. Despite this observed <i>p</i>CO₂ variability, and consistent with previous experiments conducted under stable <i>p</i>CO₂ conditions, we observed that elevated mean <i>p</i>CO₂ enhanced rates of PC production, PN production, and growth. During exponential growth phase, rates of PC and PN production increased by ∼1.2- and ∼1.5-fold in the mid- and high-CO₂ treatments, respectively, when compared to the low-CO₂ treatment. Elevated <i>p</i>CO₂ also enhanced PC and PN production rates during early stationary growth phase. In all treatments, PC and PN cellular content displayed a strong diurnal rhythm, with particulate C:N molar ratios reaching a high of 22∶1 in the light and a low of 5.5∶1 in the dark. The <i>p</i>CO₂ enhancement of metabolic rates persisted despite <i>p</i>CO₂ variability, suggesting a consistent positive response of <i>Crocosphaera</i> to elevated and fluctuating <i>p</i>CO₂ conditions.</p></div

Time series biomass measurements for cultures of <i>Crocosphaera watsonii</i> WH8501 grown under three CO₂ treatments.

<p>Time series biomass measurements for cultures of <i>Crocosphaera watsonii</i> WH8501 grown under three CO₂ treatments.</p

Percent differences in ¹⁴C-PP between controls and elevated <i>p</i>CO₂ treatments.

<p>Shown are percent differences ([treatments (750 μatm) - controls (387 μatm)] / controls) for rates of ¹⁴C-PP from depth-resolved experiments during cruises in August 2010 and March 2011 for >10 μm size class (panel A), 2–10 μm size class (panel B), and the 0.2–2 μm size class (panel C). Dashed line indicates zero. Bold symbols indicate significant differences between controls and treatments.</p

The influence of abrupt increases in seawater <i>p</i>CO₂ on plankton productivity in the subtropical North Pacific Ocean

<div><p>We conducted a series of experiments to examine short-term (2–5 days) effects of abrupt increases in the partial pressure of carbon dioxide (<i>p</i>CO₂) in seawater on rates of primary and bacterial production at Station ALOHA (22°45’ N, 158° W) in the North Pacific Subtropical Gyre (NPSG). The majority of experiments (8 of 10 total) displayed no response in rates of primary production (measured by ¹⁴C-bicarbonate assimilation; ¹⁴C-PP) under elevated <i>p</i>CO₂ (~1100 μatm) compared to ambient <i>p</i>CO₂ (~387 μatm). In 2 of 10 experiments, rates of ¹⁴C-PP decreased significantly (~43%) under elevated <i>p</i>CO₂ treatments relative to controls. Similarly, no significant differences between treatments were observed in 6 of 7 experiments where bacterial production was measured via incorporation of ³H-leucine (³H-Leu), while in 1 experiment, rates of ³H-Leu incorporation measured in the dark (³H-Leu_Dark) increased more than 2-fold under high <i>p</i>CO₂ conditions. We also examined photoperiod-length, depth-dependent (0–125 m) responses in rates of ¹⁴C-PP and ³H-Leu incorporation to abrupt <i>p</i>CO₂ increases (to ~750 μatm). In the majority of these depth-resolved experiments (4 of 5 total), rates of ¹⁴C-PP demonstrated no consistent response to elevated <i>p</i>CO₂. In 2 of 5 depth-resolved experiments, rates of ³H-Leu_Dark incorporation were lower (10% to 15%) under elevated <i>p</i>CO₂ compared to controls. Our results revealed that rates of ¹⁴C-PP and bacterial production in this persistently oligotrophic habitat generally demonstrated no or weak responses to abrupt changes in <i>p</i>CO₂. We postulate that any effects caused by changes in <i>p</i>CO₂ may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.</p></div

Chlorophyll concentrations and station locations.

<p>Satellite ocean color image depicting distributions and concentrations of near-surface ocean chlorophyll <i>a</i> (μg/ L^-1) in the proximity of the Hawaiian Islands on August 21, 2010 (panel A). This image was derived from MODIS Aqua data using the color index (CI) algorithm [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193405#pone.0193405.ref038" target="_blank">38</a>] with no flags applied. The locations of the three stations occupied in August 2010 are also indicated; Station ALOHA is depicted as a circle, while stations S1 and S2 are depicted by triangles. Depth profiles of chlorophyll <i>a</i> during the depth-dependent experiments are also shown (panel B).</p

Depth-integrated rates of ³H-Leu incorporation.

<p>Depth-integrated rates of ³H-Leu incorporation.</p

Depth-resolved rates of ³H-Leu incorporation in August 2010 and March 2011.

<p>Rates of ³H-Leu incorporation (pmol Leu L^-1 h^-1) in the dark for both ambient and elevated seawater <i>p</i>CO₂ (~390 and 750 μatm, respectively) are shown (panels A and C, respectively), as are rates in the light for both ambient and elevated <i>p</i>CO₂ (panels B and D, respectively). Also shown are percent differences between treatments ([treatments—controls] / controls) for ³H-Leu_Dark incorporation (panel E) and ³H-Leu_Light incorporation (panel F).</p