AUTHOR=Mai Guangming , Liu Jihua , Xia Xiaomin , Pang Xinyue , Li Binkai , Yu Linghui , Tan Yehui , Song Xingyu , Li Gang 

TITLE=Acutely Rising Temperature Reduces Photosynthetic Capacity of Phytoplankton Assemblages in Tropical Oceans: A Large-Scale Investigation

JOURNAL=Frontiers in Marine Science

VOLUME=8

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.710697

DOI=10.3389/fmars.2021.710697

ISSN=2296-7745

ABSTRACT=<p>Climate changes interacting with human activities are raising the temperature in global oceans. To explore physiological responses of <italic>in situ</italic> phytoplankton assemblages to increasing temperatures, we conducted a shipboard experiment in tropical regions of the eastern Indian Ocean, Java Sea, and southern South China Sea. Throughout the surveyed areas, phytoplankton biomass (Chl<italic>a</italic>) ranged from 0.09 to 0.86 μg L<sup>−1</sup> (median, 0.22 μg L<sup>−1</sup>) in the surface and from 0.30 to 0.99 μg L<sup>−1</sup> (median, 0.50 μg L<sup>−1</sup>) in maximal chlorophyll layer (DCM), respectively. Picophytoplankton that occupied 27–89% (79%) and 83–92% (88%) of total Chl<italic>a</italic> in the surface and DCM layers, ranged from 0.32 × 10<sup>4</sup> to 23.10 × 10<sup>4</sup> cells mL<sup>−1</sup> (3.69 × 10<sup>4</sup> cells mL<sup>−1</sup>) and from 7.44 × 10<sup>4</sup> to 25.70 × 10<sup>4</sup> cells mL<sup>−1</sup> (12.60 × 10<sup>4</sup> cells mL<sup>−1</sup>), respectively. <italic>Synechococcus</italic> took up 30–97% (78%) of pico-cells compositions in the surface layer, while, in the DCM layer, <italic>Prochlorococcus</italic> took up 42–98% (91%). Moreover, the maximal photochemical quantum yield (F<sub>V</sub>/F<sub>M</sub>) of photosystem II (PS II) and the rapid light curve (RLC)-derived light utilization efficiency (<italic>α</italic>) were lower in the surface layer than that in the DCM layer, but the saturation irradiance (E<sub>K</sub>) was higher. In particular, we found that acutely rising temperature decreased the F<sub>V</sub>/F<sub>M</sub> and <italic>α</italic> in both the surface and the DCM layers but increased the absorption cross-section (σ<sub>PSII</sub>) of PSII photochemistry. Our results clearly indicate that the presently rising temperature adversely affects the photophysiology of natural phytoplankton assemblages in tropical oceans.</p>