Responses of Different Temperature-Acclimated Diatom Species, Smaller Thalassiosira pseudonana and Larger Thalassiosira rotula, to Increased Ambient Temperature

The acute rise in temperature due to marine heatwaves has a strong impact on marine phytoplankton. To determine whether these effects depend on ambient temperature and cell size, we acclimated two diatom species, smaller <i>Thalassiosira pseudonana</i> (Hasle and Heimdal, 1970) and larger <i>Thalassiosira rotula</i> (Meunier, 1910), at low (LAT), medium (MAT) and high ambient temperatures (HAT) and examined their physiochemical and transcriptional responses to temperature rise (AT + 6 °C). The specific growth rate (µ) of smaller cells was increased by 32% due to temperature rise at LAT, but decreased by 13% at HAT, with the stimulatory and inhibitory extent being ~50% less than that of larger cells. At LAT, chlorophyll <i>a</i> (Chl <i>a</i>), carotenoid (Car) and carbon (POC) contents were increased in smaller cells due to temperature rise, but were decreased in larger cells; at HAT, Chl <i>a</i> and Car were increased in both smaller and larger cells and POC was increased in only smaller cells. At LAT, temperature rise led to a disproportionate increase in photosynthesis and dark respiration, resulting in an increase in carbon utilization efficiency (CUE) in smaller cells and a decrease in CUE in larger cells; at HAT, there was a decrease in CUE in both the smaller and larger cells, but to a lesser extent in the former than in the latter. Our results also show that smaller cells cope with the acute temperature rise mainly by strengthening their enzyme activity (e.g., the antioxidant system) and conservatively regulating their metabolism, while larger cells mainly regulate their photosynthetic and central carbon metabolism. Moreover, larger cells can outperform their smaller counterparts when the temperature rise occurs at lower ambient temperature.