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<p>Ocean acidification (OA), which is a major environmental change caused by increasing atmospheric <span class="inline-formula">CO₂</span>, 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 <i>Skeletonema costatum</i> was cultured under two different <span class="inline-formula">CO₂</span> levels (LC, 400 <span class="inline-formula">µatm</span>; HC, 1000 <span class="inline-formula">µatm</span>) and three different combinations of temperature and photoperiod length (<span class="inline-formula">8:16</span> <span class="inline-formula">L:D</span> with 5 <span class="inline-formula">^∘C</span>, <span class="inline-formula">12:12</span> <span class="inline-formula">L:D</span> with 15 <span class="inline-formula">^∘C</span>, <span class="inline-formula">16:8</span> <span class="inline-formula">L:D</span> with 25 <span class="inline-formula">^∘C</span>), simulating different seasons in typical temperate oceans, to investigate the combined effects of these factors. The results showed that specific growth rate of <i>S. costatum</i> 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.</p>