Nitrate limitation and ocean acidification interact with UV-B to reduce photosynthetic performance in the diatom &amp;lt;i&amp;gt;Phaeodactylum tricornutum&amp;lt;/i&amp;gt;

W Li,J Beardall,K Gao

doi:10.5194/bg-12-2383-2015

Nitrate limitation and ocean acidification interact with UV-B to reduce photosynthetic performance in the diatom &lt;i&gt;Phaeodactylum tricornutum&lt;/i&gt;

W Li, J Beardall + Show 1 more

Open Access

https://doi.org/10.5194/bg-12-2383-2015

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Abstract

Abstract. It has been proposed that ocean acidification (OA) will interact with other environmental factors to influence the overall impact of global change on biological systems. Accordingly we investigated the influence of nitrogen limitation and OA on the physiology of diatoms by growing the diatom Phaeodactylum tricornutum Bohlin under elevated (1000 μatm; high CO2 – HC) or ambient (390 μatm; low CO2 – LC) levels of CO2 with replete (110 μmol L−1; high nitrate – HN) or reduced (10 μmol L−1; low nitrate – LN) levels of NO3- and subjecting the cells to solar radiation with or without UV irradiance to determine their susceptibility to UV radiation (UVR, 280–400 nm). Our results indicate that OA and UVB induced significantly higher inhibition of both the photosynthetic rate and quantum yield under LN than under HN conditions. UVA or/and UVB increased the cells' non-photochemical quenching (NPQ) regardless of the CO2 levels. Under LN and OA conditions, activity of superoxide dismutase and catalase activities were enhanced, along with the highest sensitivity to UVB and the lowest ratio of repair to damage of PSII. HC-grown cells showed a faster recovery rate of yield under HN but not under LN conditions. We conclude therefore that nutrient limitation makes cells more prone to the deleterious effects of UV radiation and that HC conditions (ocean acidification) exacerbate this effect. The finding that nitrate limitation and ocean acidification interact with UV-B to reduce photosynthetic performance of the diatom P. tricornutum implies that ocean primary production and the marine biological C pump will be affected by OA under multiple stressors.

Highlights

Increasing atmospheric levels of CO2 and the associated dissolution of CO2 into the oceans has resulted in ocean acidification (OA), with increased levels of pCO2, HCO−3 and H+ and decreased CO23− concentration
The carbon fixation per cell in the LC-grown cells was 10.0 % (P = 0.0058) higher in those exposed to presence of UVA (PA), and fixation based on chlorophyll a (Chl a) was higher under the PAR alone or PA treatments, by about 8.4 (P = 0.0253) and 17.9 % (P = 0.005) compared to that of the HC-grown cells
For PAB treatments, there were no significant differences between the HC- and LC-grown cells (Fig. 1a and c)

Summary

Introduction

Increasing atmospheric levels of CO2 and the associated dissolution of CO2 into the oceans has resulted in ocean acidification (OA), with increased levels of pCO2, HCO−3 and H+ and decreased CO23− concentration. At the same time, increased sea surface temperatures are predicted to cause a shoaling of the surface mixed layer, which in turn will lead to enhanced exposure to sunlight (both as photosynthetically active radiation (PAR) and as UV radiation (UVR)). This enhanced stratification will decrease upward transport of nutrients from deeper, nutrientrich layers, leading to more frequent/marked nutrient limitation (Cermeño et al, 2008). Global change is likely to cause changes in a multiplicity of factors that influence phytoplankton growth and it is critical to examine OA in the context of interactive effects with these other environmental drivers (Boyd, 2011).

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