Solar ultraviolet radiation (UVR, 280-400 nm) is a crucial environmental factor to influence marine primary productivity and consequently the marine ecosystems (Häder 2011). UVR can decrease phytoplankton growth and photosynthesis as well as nutrients uptake (Sobrino et al. 2004, Gao et al. 2007
Availability of nutrients is known to affect the photosynthetic responses of algae to UVR (Beardall et al. 2001, 2009). Nutrient limitation reduced the sensitivity of the diatom
The diatom
The diatom
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Irradiance treatments and measurements
In the early morning (7:00 am) of August 4 and 8 of 2010, both the diluted cultures (LN or HN) were dispensed into 500 mL UV-transparent quartz tubes that were incubated in a flow-through water tank to control temperature (20 ± 0.5°C) and exposed to 3 irradiation treatments (triplicate tubes for each nutrient level): a) uncovered quartz tubes, the cells received full sunlight (PAR + UV-A +UV-B [PAB], irradiances above 280 nm); b) quartz tubes wrapped in Folex 320 (Montagefolie, No. 10155099; Folex, Dreieich, Germany), the cells received PAR + UV-A (PA, irradiances above 320 nm); and c) quartz tubes covered with Ultraphan film 395 (UV Opak; Digefra, Munich, Germany), the cells received PAR alone (P, irradiances above 395 nm). The transmission spectra of the tubes and filters are available elsewhere (Sobrino et al. 2004). A radiometer (Eldonet XP; Real Time Computers Inc., Möhrendorf, Germany) was used to monitor the incident solar radiation; it measures every second of UV-B (280-315 nm), UV-A (315-400 nm), and PAR irradiance (400-700 nm) and records the minute-averaged values (Häder et al. 1999). This device has been regularly calibrated with a certified calibration lamp (DH 2000; Oceanic Optics Inc., Dunedin, FL, USA). The PAR irradiance was converted from W m-2 to photon flux (μmol photons m-2 s-1) by multiplying by 4.60 according to Neale et al. (2001).
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Photophysiological parameter measurements
During the incubations (7:00 am to 18:00 pm), 5 mL samples were taken every hour from each tube to determine the photosynthetic performance of
UV-A or UV-B induced inhibition of Y was calculated as:
UV-BInh = (YPA - YPAB) / YP × 100%;UV-AInh = (YP - YPA) / YP × 100%
, where UV-BInh and UV-AInh indicate UV-B and UV-A induced inhibition; YPAB, YPA, and YP indicate Y values of the cells under PAB, PA, and P treatments, respectively.
To determine the significant differences (p < 0.05) among three light treatments and between two nutrient treatments, paired-t test was used for the whole day’s comparisons and one way-ANOVA was used for the each time-point comparisons. Non-linear curve fit was used to obtain the relationships between Y (or NPQ) and PAR irradiance, whereas Kendall’s τ test was used to establish the correlations of Y and UV inhibition between the HN and LN treatments.
During a diurnal cycle of solar radiation (Fig. 2A), the effective quantum yield (Y) decreased with increasing solar radiation regardless of the radiation treatments with or without UVR, to a minimum value at noon, and then increased with decreasing solar radiation (Fig. 2B). The cells grown under nitrate (N)-limited condition had a relatively lower Y value than those under N-repletion e.g., 0.44 in the early morning, that decreased to a minimum of 0.29 at noon and almost completely recovered in the late afternoon (Fig. 2B). The diurnal changes of NPQ displayed an opposite pattern to Y (Fig. 2C), with the higher values in the presence of UVR than that in PAR alone (p < 0.01). In view of the NPQ ratios of HN to LN grown cells, higher NPQ were found in the LN-grown cells (Fig. 1C), indicating a higher heat dissipation. UVR significantly increased the NPQ (p < 0.05), by approximately 57% in LN and 30% in HN-grown cells at noon (Fig. 2C).
When PAR intensity increased over 1,500 μmol photons m-2 s-1 (326 W m-2), the Y values decreased by approximately 60% as compared to the initials in LN grown cells (Fig. 3A & B) with the presence of UV-A reducing the yield by 2.2-21% and addition of UV-B further decreasing it by 6.0-24%, the total inhibition caused by UVR being 12 to 30%. The NPQ value reached 0.89 in LN-grown cells as the PAR was over 1,500 μmol photons m-2 s-1 (326 W m-2), being elevated by 46 and 31% respectively by solar UV-A and UV-B. The N limitation enhanced the NPQ by 180% under PAR alone, by 204% under PA and by 76% under PAB, compared to that in N repletion (Fig. 3C & D). Moreover, a clear threshold of NPQ of LN-grown cells (Fig. 3C & D) occurred when the PAR irradiance was ~230 μmol photons m-2 s-1 (50 W m-2) ‒ one fourth of that in HN-grown cells, providing evidence that the lower light energy is needed to spike the NPQ under N-limited conditions.
Fig. 4 showed the relationships of the Y values, UV-A and UV-B caused inhibition between LN- and HN-grown cells. The LN-grown cells showed about 13% lower Y values than that of HN-grown cells (Fig. 4A), and 24.4 and 21.4% higher inhibition caused by UV-A and UV-B, respectively (Fig. 4B & C), indicating the N limitation exacerbated the UVR effects on the diatom photosynthetic performance.
Grown under nitrate limited condition, the diatom
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The threshold of light intensity that triggers the NPQ in LN-grown cells was one-fourth of that of HN-grown cells (Fig. 3C & D). The NPQ, an important strategy for phytoplankton to rapidly (seconds to minutes) regulate photochemistry, is one of the first lines of defense that diatoms use to attenuate the photoinhibitory oxidative damage caused by light stress (Lavaud et al. 2007, Korbee et al. 2010). The LN-grown cells had significantly (p < 0.01) higher NPQ and lower light to trigger NPQ, comparable to the HN-grown ones (Fig. 3C & D); they could have redissipated the excessive energy more effectively under stressful light condition, thus protecting the cells from photoinhibition and maintaining their photosynthetic activity. The field measurements of NPQ by Kashino et al. (2002) and Fujiki et al. (2003) also indicated that the NPQ process is of importance to maintain the photosynthetic activity of phytoplankton. On the other hand, the substances, that need N for their synthesis, e.g., UV-screening compounds like mycosporine-like amino acids were recorded to increase with increasing nitrogen levels (Litchman et al. 2002, Korbee et al. 2010, Barufi et al. 2011) and might also attribute to the higher UVR sensitivity in LN- than in HN-grown cells.
The diatom grown under N-limited condition exhibited higher sensitivity to UVR than that grown under N-replete condition, based on the changes in the photochemical quantum yield and NPQ, which indicates that the N limitation exacerbates the effects of UVR on its photosynthetic performance and stimulate its NPQ. Presently, the increased global temperature has directly and indirectly altered the natural conditions of aquatic bodies, e.g., increasing the stratification of surface ocean and making it more oligotrophic (Boyd et al. 2010). Taking into account the worldwide oligotrophic oceans wherein the growth of phytoplankton is limited and the limitation could be exacerbated by the decreased nutrient levels within the upper mixed layer; the negative effects caused by solar UVR would be exacerbated, making phytoplankton cells more sensitive to ambient UVR stress.