Data Sheet 1_Biogeochemistry of phytoplankton RuBisCO in the ocean.docx

Form I Ribulose-1,5-bisphosphate oxygenase/carboxylase (RuBisCO) is the most abundant enzyme on Earth, playing a key role in carbon fixation during oxygenic photosynthesis. Using published sequence data, I show that there are significant differences in the amount of elemental resources (C, N and S) and energy required to synthesize the different Types of Form I RuBisCO. The shorter amino acid lengths of cyanobacterial RuBisCO had lower resource requirements to build the holoenzyme compared with eukaryotes. Consequently, the rise to dominance of eukaryote phytoplankton during the Neoproterozoic (1000–541 Ma) led to a shift to more expensive eukaryote RuBisCO. There are also significant differences in the elemental composition of RuBisCO between eukaryotes in different supergroups. Estimates of resource allocation were used to estimate how much C, N and S is associated with RuBisCO in the modern ocean. The marine cyanobacterium Prochlorococcus is the most numerically abundant photosynthetic organism on Earth and accounts for 7.3 – 8.9% of net ocean primary productivity. There are 2.11- 2.69 x 106 mol RuBisCO in Prochlorococcus, which amounts to 4 to 5% of the total RuBisCO pool in the ocean. The relatively low RuBisCO content compared with productivity indicates highly efficient photosynthesis in Prochlorococcus. The total marine RuBisCO reservoir is equivalent to 0.016 Pg C, 5.1 Tg N, and 0.4 Tg S. The estimated annual productivity of RuBisCO is equivalent to 0.725 - 0.890 Pg C yr-1, 228–283 Tg N yr-1, and 16.5 - 22.5 Tg S yr-1. In the context of the marine nitrogen cycle, the amount of nitrogen fluxing through the pool of RuBisCO each year is equivalent to, or even higher, than the rate of biological nitrogen fixation (223 ± 30 Tg N yr−1). Turnover of RuBisCO is rapid, occurring every 6.6 to 8.2 days. In conclusion, RuBisCO is not only significant as the primary carbon fixation enzyme in the ocean, but also as a pool of chemical elements, particularly nitrogen.

I型核酮糖-1,5-二磷酸羧化酶/加氧酶（Form I Ribulose-1,5-bisphosphate oxygenase/carboxylase，RuBisCO）是地球上含量最为丰富的酶类，在产氧光合作用的碳固定过程中发挥关键作用。本研究借助已发表的序列数据发现，不同亚型的I型RuBisCO在合成所需的元素资源（碳C、氮N与硫S）及能量总量上存在显著差异。与真核生物相比，蓝细菌RuBisCO的氨基酸序列更短，组装成全酶（holoenzyme）所需的资源成本更低。因此，新元古代（1000–541 Ma）真核浮游植物逐渐占据生态主导地位，这一转变使得海洋RuBisCO库转向资源成本更高的真核生物型RuBisCO。不同超群（supergroup）的真核生物，其RuBisCO的元素组成也存在显著差异。本研究通过资源投入量的估算值，推算出现代海洋中与RuBisCO结合的碳、氮、硫总量。海洋蓝细菌原绿球藻（Prochlorococcus）是地球上数量最多的光合生物，其贡献的海洋净初级生产力占比达7.3%–8.9%。每株原绿球藻中含有2.11×10^6至2.69×10^6 mol的RuBisCO，占全球海洋RuBisCO总库的4%–5%。相较于其生产力水平，原绿球藻的RuBisCO含量相对较低，这表明其光合作用效率极高。全球海洋RuBisCO总库的元素储量约为0.016拍克碳（Pg C）、5.1太克氮（Tg N）以及0.4太克硫（Tg S）。RuBisCO的年周转生产力约为0.725–0.890 Pg C·yr^-1、228–283 Tg N·yr^-1以及16.5–22.5 Tg S·yr^-1。从海洋氮循环的视角来看，每年流经RuBisCO库的氮通量与生物固氮速率（223±30 Tg N·yr^-1）相当，甚至更高。RuBisCO的周转速率极快，平均每6.6至8.2天即可完成一次周转。综上，RuBisCO不仅是海洋中主要的碳固定酶类，同时也是一类重要的化学元素储库，尤以氮元素储库最为突出。