The photosynthetic conversion of dissolved inorganic carbon (DIC) into organic carbon (OC) by using aquatic phototrophs in rivers may serve as a potential carbon sink, especially in the carbonate rock areas, thereby offering a clue for finding the missing carbon sink. However, primary-produced autochthonous OC is erroneously considered as terrestrial-derived allochthonous OC. Therefore, distinguishing sources of autochthonous and allochthonous OC is vital in the assessment of carbon sink. Traditional geochemical methods (δ13C, C/N) are highly variable and influenced by remineralization and decomposition processes, and the stable isotopic signatures usually overlapped, therefore leading a vague and overlapping result. By contrast, lipid biomarkers comprise a group of natural molecules that store energy and act as structural components of cell membranes. Different lipid compositions are associated with diverse biosynthesis pathways. Hence, lipid biomarkers with distinct structural features can provide source-specific information.

Recently, a research team led by Prof. LIU Zaihua from the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) used biomarkers to distinguish the autochthonous and allochthonous organic carbon in the Pearl River and investigated the contribution of autochthonous organic carbon to carbon sink.

The researchers found that lipid biomarkers can be a useful tool in tracing organic sources. The calculated average autochthonous OC was approximately 65% of the total OC, indicating intense in-river primary productivity in the Pearl River. Autochthonous organic sources, as products of primary productivity, were positively correlated with DIC concentrations, which could be a DIC fertilization effect. This phenomenon was also shown by the growth of phytoplankton, which demonstrated a coupled process that consumed DIC source for their bloom and produced specific lipid organic sources.

They also found that high total suspended solid (TSS) on the water surface can blocked the sunlight and then reduced phytoplankton production. However, in situ photosynthesis of phytoplankton could also produce autochthonous OC, even larger than the allochthonous source at sites with high DIC, and even with higher TSS concentrations.
The study comprehensively elucidated the formation of autochthonous OC based on the coupling action of rock weathering and photosynthetic activity in the riverine system, suggesting a potential direction for finding the missing carbon sink.

This work was supported by the National Basic Research Program of China (973 Program 2013CB956703), the National Natural Science Foundation of China (41430753) and the China Postdoctoral Science Foundation (2015M582577). The study entitled "Organic carbon source tracing and DIC fertilization effect in the Pearl River: Insights from lipid biomarker and geochemical analysis" has been published in Applied Geochemistry.

Contact:
LIU Zaihua
Institute of Geochemistry, Chinese Academy of Sciences
E-mail: liuzaihua@vip.gyig.ac.cn

(By Prof. LIU Zaihua's group)