Differential CO2-fixation potentials and supporting roles of phagotrophy and proton pump among plankton lineages in a subtropical marginal sea

Lineage-wise physiological activities of plankton communities in the ocean are important but challenging to characterize. Here we conducted whole-assemblage metatranscriptomic profiling at continental shelf and slope sites of South China Sea to investigate carbon fixation in different lineages. We catalogued 4.4 million unique genes, ~37% being annotatable and mainly involved in microbial metabolism, photosynthesis, amino acid synthesis, oxidative phosphorylation, and two-component systems. With RuBisCO expression as proxy, Calvin carbon fixation (CCF) was mainly contributed by Bacillariophyta, Chlorophyta, Cyanobacteria, Haptophyta andnon-diatom Stramenopiles, which was differentially affected by environmental factors among lineages. CCF exhibited positive or negative correlations with phagotrophy gene expression depending on lineages, suggesting phagotrophy enhances (Bacillariophyta, Haptophyta, and Chlorophyta) or complements (Dinophyta) CCF. Our data reveal significant potential of non-Calvin carbon fixation (NCF), mainly contributed by Flavobacteriales, Alteromonadales, Oceanospirillales and Rhodobacterales. Furthermore, in Flavobacteriales, Alteromonadales, Pelagibacterales and Rhodobacterales, NCF potential was positively correlated with proteorhodopsin expression, suggesting that NCF is energetically supported by proteorhodopsin. The novel insights into lineage-dependent potential of carbon fixation, widespread mixotrophy, and proteorhodopsin as energy source for NCF lay a methodological and informational foundation for further research to understand the carbon fixation and trophic landscape in the ocean. ### Competing Interest Statement The authors have declared no competing interest.