Functional response of log chlorophyll concentration (mg m⁻³) to 4 sets of predictors: (a) mean irradiance and climatological surface nitrate concentration, (b) sea surface temperature, (c) location in basin, and (d) month of year.

<p>Panels (a) and (c) are contour maps of two variable response functions. Dashed lines on panels (b) and (d) indicate point estimates of the standard error of the response function.</p

Log chlorophyll concentration, March and August 1999–2006 averages, predicted using Eq. (1) and observed satellite data.

<p>Log chlorophyll concentration, March and August 1999–2006 averages, predicted using Eq. (1) and observed satellite data.</p

Nitrogen starvation induces distinct photosynthetic responses and recovery dynamics in diatoms and prasinophytes

<div><p>Nitrogen stress is an important control on the growth of phytoplankton and varying responses to this common condition among taxa may affect their relative success within phytoplankton communities. We analyzed photosynthetic responses to nitrogen (N) stress in two classes of phytoplankton that often dominate their respective size ranges, diatoms and prasinophytes, selecting species of distinct niches within each class. Changes in photosynthetic structures appeared similar within each class during N stress, but photophysiological and growth responses were more species- or niche-specific. In the coastal diatom <i>Thalassiosira pseudonana</i> and the oceanic diatom <i>T</i>. <i>weissflogii</i>, N starvation induced large declines in photosynthetic pigments and Photosystem II (PSII) quantity and activity as well as increases in the effective absorption cross-section of PSII photochemistry (<i>σʹ</i>_PSII). These diatoms also increased photoprotection through energy-dependent non-photochemical quenching (NPQ) during N starvation. Resupply of N in diatoms caused rapid recovery of growth and relaxation of NPQ, while recovery of PSII photochemistry was slower. In contrast, the prasinophytes <i>Micromonas</i> sp., an Arctic Ocean species, and <i>Ostreococcus tauri</i>, a temperate coastal eutrophile, showed little change in photosynthetic pigments and structures and a decline or no change, respectively, in <i>σʹ</i>_PSII with N starvation. Growth and PSII function recovered quickly in <i>Micromonas</i> sp. after resupply of N while <i>O</i>. <i>tauri</i> failed to recover N-replete levels of electron transfer from PSII and growth, possibly due to their distinct photoprotective strategies. <i>O</i>. <i>tauri</i> induced energy-dependent NPQ for photoprotection that may suit its variable and nutrient-rich habitat. <i>Micromonas</i> sp. relies upon both energy-dependent NPQ and a sustained, energy-independent NPQ mechanism. A strategy in <i>Micromonas</i> sp. that permits photoprotection with little change in photosynthetic structures is consistent with its Arctic niche, where low temperatures and thus low biosynthetic rates create higher opportunity costs to rebuild photosynthetic structures.</p></div

Nitrogen starvation induces distinct photosynthetic responses and recovery dynamics in diatoms and prasinophytes - Fig 6

<p>The relative distribution of excitation energy among PSII photochemistry (<i>Φ</i>_PSII, green shading), dissipation as heat via energy-dependent non-photochemical quenching (<i>Φ</i>_NPQ, blue shading), and dissipation as heat and fluorescence as constitutive, energy-independent non-photochemical quenching (<i>Φ</i>_NO, red shading) in (A) <i>T</i>. <i>pseudonana</i>, (B) <i>T</i>. <i>weissflogii</i>, (C) <i>O</i>. <i>tauri</i>, and (D) <i>Micromonas</i> sp. with the onset of N-starvation and during recovery following the resupply of N. <i>Fs and Fm’</i> values used in to calculate these quantum yields were determined at growth irradiance (85 <i>μ</i>mol photons m^-2 s^-1). Only recovery from late stationary phase N starvation is shown for simplicity.</p

Nitrogen starvation induces distinct photosynthetic responses and recovery dynamics in diatoms and prasinophytes - Fig 3

<p>Changes in (A,B,C,D) the effective absorption cross-section of PSII photochemistry (<i>σʹ</i>_PSII), (E,F,G,H) electron transfer rate from PSII (ETR), and (I,J,K,L) the rate constant for the reopening of PSII reaction centers (1/<i>τ</i>) with N starvation and during recovery following the resupply of N. Symbols for recovery response are the same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0195705#pone.0195705.g001" target="_blank">Fig 1</a>. The values shown for <i>σʹ</i>_PSII at each sampling point were determined under low actinic light (8 and 21 <i>μ</i>mol photons m^-2 s^-1 for prasinophytes and diatoms respectively) as explained in the text. Error bars represent propagated standard error based on the calculated error of curve fitting by the FRRf software and the standard error among triplicate cultures.</p

Parameters and definitions.

<p>Parameters and definitions.</p

Nitrogen starvation induces distinct photosynthetic responses and recovery dynamics in diatoms and prasinophytes - Fig 1

<p>Growth rate (<i>μ</i>) from N-replete balanced growth to N starvation and in N-starved sub-cultures following the addition of nitrate in (A) <i>T</i>. <i>pseudonana</i>, (B) <i>T</i>. <i>weissflogii</i>, (C) <i>O</i>. <i>tauri</i>, and (D) <i>Micromonas</i> sp. (X) symbols indicate sampling points for cell composition and photochemistry. Recovery after the resupply of N is shown for subcultures collected at early (ES, white triangles), mid (MS, gray triangles), and late stationary (LS, black triangles) phases. The dashed line indicates the <i>μ</i>_<i>max</i> for a species determined during N-replete, balanced growth. Error bars indicate one standard deviation among triplicate cultures.</p

Growth rates and recovery scores.

<p>Growth rates and recovery scores.</p

Description of study species.

<p>Description of study species.</p

Nitrogen starvation induces distinct photosynthetic responses and recovery dynamics in diatoms and prasinophytes - Fig 4

<p>The change from N-replete growth to N starvation in molar ratios of (A) chl <i>a</i> content to cellular carbon, (B) chl <i>a</i> content to cellular nitrogen, (C) the cellular content of active PSII reaction centers to chl <i>a</i>, and (D) pigment content associated with light harvesting antenna complexes to chl <i>a</i>. Error bars indicate one standard deviation among triplicate cultures.</p