Effects of Temperature on Transparent Exopolymer Particle Production and Organic Carbon Allocation of Four Marine Phytoplankton Species

SIMPLE SUMMARY: TEP is the bridge mediating DOC and POC conversion in the ocean, which is a key component of the marine carbon cycle. TEP accelerates the accumulation or deposition rate of organic carbon at the sea surface and promotes the short- and long-term sequestration of carbon, counteracting atmospheric CO(2) increase and global warming. Temperature, as the main driving factor of carbon fixation during phytoplankton photosynthesis, is closely related to TEP production. However, little is known about the effects of temperature on TEP the production, DOC secretion, and carbon pool allocation of phytoplankton. This study analyzed the effect of temperature on the carbon pool allocation of phytoplankton and the significance of TEP in the marine carbon pool. Our results suggest that increased temperature affects carbon pool allocation in phytoplankton cells by promoting DOC exudation and extracellular TEP formation. This study provides an important basis for understanding the contribution of TEP to the allocation of POC and DOC and will benefit the prediction of phytoplankton TEP production and the marine carbon cycle under the background of global warming. ABSTRACT: Transparent exopolymer particles (TEP) are sticky polymeric substances that are commonly found in the periphery of microbial cells or colonies. They can naturally flocculate smaller suspended particles into larger aggregates and thus play a crucial role in the biological pump and the global carbon cycle. Phytoplankton are the major contributors to marine TEP production, whereas the way TEP production interacts with abiotic factors at the species level is generally unknown but critical for estimating carbon fluxes. In this study, the effects of temperature on TEP production and carbon allocation were studied in two representative diatom species (Nitzschia closterium and Chaetoceros affinis) and two model dinoflagellate species (Prorocentrum micans and Scrippisella trichoidea). The results showed that temperature had a significant impact on TEP production in all species. First, increased temperature promoted the TEP production of all four species. Second, elevated temperature affected the carbon pool allocation, with enhanced dissolved organic carbon (DOC) exudation in the form of TEP in all species. The TEP-C/DOC percentages of N. closterium and P. micans were 93.42 ± 5.88% and 82.03 ± 21.36% at the highest temperature (24 °C), respectively, which was approximately two to five times higher than those percentages at 16 °C. In contrast, TEP’s contribution to the POC pool is lower than that to the DOC pool, ranging from 6.74 ± 0.79% to 28.31 ± 1.79% for all species. Moreover, phytoplankton TEP production may be related to cellular size and physiology. The TEP content produced by the smallest N. closterium (218.96 ± 15.04 fg Xeq./μm(3)) was ~5 times higher compared to P. micans, S. trichoidea, or C. affinis. In conclusion, TEP production is temperature sensitive and species specific, which should be taken into consideration the regarding TEP-mediated oceanic carbon cycle, particularly in the context of global warming.