Abstract. Ocean acidification (OA), which is a major environmental change caused by
increasing atmospheric CO2, has considerable influences on marine
phytoplankton. But few studies have investigated interactions of OA and
seasonal changes in temperature and photoperiod on marine diatoms. In the
present study, a marine diatom Skeletonema costatum was cultured
under two different CO2 levels (LC, 400 µatm; HC,
1000 µatm) and three different combinations of temperature and
photoperiod length (8:16 L:D with 5 ∘C, 12:12
L:D with 15 ∘C, 16:8 L:D with
25 ∘C), simulating different seasons in typical temperate oceans, to
investigate the combined effects of these factors. The results showed that
specific growth rate of S. costatum increased with increasing
temperature and day length. However, OA showed contrasting effects on growth
and photosynthesis under different combinations of temperature and day length:
while positive effects of OA were observed under spring and autumn conditions,
it significantly decreased growth (11 %) and photosynthesis (21 %) in
winter. In addition, OA alleviated the negative effect of low temperature and
short day length on the abundance of RbcL and key photosystem II (PSII)
proteins (D1 and D2). These data indicated that future ocean acidification may
show differential effects on diatoms in different clusters of other factors.
To understand how Ulva species might respond to salinity stress during future ocean acidification we cultured a green tide alga Ulva linza at various salinities (control salinity, 30 PSU; medium salinity, 20 PSU; low salinity, 10 PSU) and CO2 concentrations (400 and 1000 ppmv) for over 30 days. The results showed that, under the low salinity conditions, the thalli could not complete its whole life cycle. The specific growth rate (SGR) of juvenile thalli decreased significantly with reduced salinity but increased with a rise in CO2. Compared to the control, medium salinity also decreased the SGR of adult thalli at low CO2 but did not affect it at high CO2. Similar patterns were also found in relative electron transport rate (rETR), non-photochemical quenching, saturating irradiance, and Chl b content. Although medium salinity reduced net photosynthetic rate and maximum rETR at each CO2 level, these negative effects were significantly alleviated at high CO2 levels. In addition, nitrate reductase activity was reduced by medium salinity but enhanced by high CO2. These findings indicate that future ocean acidification would enhance U. linza’s tolerance to low salinity stress and may thus facilitate the occurrence of green tides dominated by U. linza.
Climate changes such as seawater acidification caused by rising atmospheric CO2 and increased ultraviolet radiation (UVR) intensity resulting from shoaling of the upper mixed layer may interact to influence the physiological performance of marine primary producers. But few studies have investigated long-term (>30 days) effects of UVR under seawater acidification conditions, along with less attention on the differential effects of long- and short-wavelength UVA. In the present study, four spectral treatments (>280, >320, >360, and >400 nm) under two pCO2 levels (400 and 1,000 μatm) were set to investigate the interactive effects of seawater acidification and UVR on the bloom-forming diatom Skeletonema costatum. The results showed that UVR decreased growth and effective quantum yield of Photosystem II (PSII) by 9%–16% and 11%–24%, respectively, but it enhanced cell sizes significantly. Long- and short-wavelength UVA showed differential effects on cell volume and the effective quantum yield of PSII, especially at the elevated CO2 level. Generally, seawater acidification depressed the effective quantum yield of PSII and cell volume by 6%–18% and 8%–39%, respectively. Additionally, the contents of key PSII proteins (D1 and D2) decreased at the elevated CO2 level. Elevated CO2 significantly increased the inhibition of UVR on growth in the >280 nm spectral treatment when compared with ambient CO2, while it showed no effects in other spectral treatments. Overall, the results indicate that the effects of seawater acidification on the ubiquitous diatom are light wavelength-dependent.
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