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

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

doi:10.3389/fmars.2021.710697

Abstract

Climate changes interacting with human activities are raising the temperature in global oceans. To explore physiological responses of in situ 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 (Chla) ranged from 0.09 to 0.86 μg L−1 (median, 0.22 μg L−1) in the surface and from 0.30 to 0.99 μg L−1 (median, 0.50 μg L−1) in maximal chlorophyll layer (DCM), respectively. Picophytoplankton that occupied 27–89% (79%) and 83–92% (88%) of total Chla in the surface and DCM layers, ranged from 0.32 × 104 to 23.10 × 104 cells mL−1 (3.69 × 104 cells mL−1) and from 7.44 × 104 to 25.70 × 104 cells mL−1 (12.60 × 104 cells mL−1), respectively. Synechococcus took up 30–97% (78%) of pico-cells compositions in the surface layer, while, in the DCM layer, Prochlorococcus took up 42–98% (91%). Moreover, the maximal photochemical quantum yield (FV/FM) of photosystem II (PS II) and the rapid light curve (RLC)-derived light utilization efficiency (α) were lower in the surface layer than that in the DCM layer, but the saturation irradiance (EK) was higher. In particular, we found that acutely rising temperature decreased the FV/FM and α in both the surface and the DCM layers but increased the absorption cross-section (σPSII) of PSII photochemistry. Our results clearly indicate that the presently rising temperature adversely affects the photophysiology of natural phytoplankton assemblages in tropical oceans.

Highlights

Marine phytoplankton, a group of single-cell organisms (Pachiappan et al, 2019), are the major primary producers in aquatic ecosystems
We showed that the acutely increased temperature reduced the maximum photochemical efficiency (FV/FM) and light utilization efficiency (α) of photosystem II (PSII) of natural phytoplankton assemblages from both the surface and DCM layers in tropical oceans, providing physiological evidence for the adverse effect of rising temperature on the primary productivity
We found both the FV/FM and σPSII showed similar spatial variations throughout the surveyed regions, and the larger σPSII, the higher α, and the lower EK presented in the DCM than the surface layer, suggesting the lowlight acclimation of phytoplankton assemblages (Jin et al, 2016; Xie et al, 2018)

Summary

Introduction

A group of single-cell organisms (Pachiappan et al, 2019), are the major primary producers in aquatic ecosystems. Rising temperature is often detected to enhance the activities of photosynthetic-involved enzymes of phytoplankton, like the Ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO), leading to the promotion of photosynthetic oxygen production and carbon fixation (Li et al, 1984; Young et al, 2015). Such an increased temperature was observed to compensate for the negative effects of other stressors like high light (Bouterfas et al, 2002) or UV-B radiation (Jin et al, 2019) upon photosynthesis. The phytoplankton growth is often stimulated by increased temperature, such as for diatoms Chaetoceros tenuissimus and Synedra sp. (Jin and Agustí, 2018) or coccolithophore Emiliania huxleyi (Schlüter et al, 2014)

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Conclusion