Unlike the complex processes of chlorophyll and bacteriochlorophyll-based photoenergy conversion, the rhodopsin-based process of light energy conversion only requires an opsin harboring a molecule of all-trans retinal as a chromophore. Microbial rhodopsins are widely distributed in marine ecosystem, but the distribution characteristics and ecological role of the microbial rhodopsins in the China sea are still unclear. Here, to address the gap, we report the abundance and distribution of microbial rhodopsins along a transect from the continental shelf to the ocean basin of the South China Sea (SCS) through conducting metatranscriptomic, DNA barcoding analyses and retinal concentration. Through metatranscriptomic data, both prokaryotes and eukaryotes are important microbial rhodopsin contributors. However, expression level of prokaryotic and eukaryotic rhodopsin correlated with different nutrients and nutrients-related pathways. The metatranscriptomic data also indicated that proton pumping rhodopsins were the most abundant type rhodopsin in the SCS. DNA barcoding data indicated, in the top 20 abundance microbes, most orders of prokaryotes and a small portion class of eukaryotes expressed microbial rhodopsin. Pigments concentration results showed that retinal abundances were on average 20 times higher than those of chlorophyll a and 40 times higher than those of bacteriochlorophyll throughout the transect. Among the three photosystems, the expression of rhodopsin showed a significant positive correlation with the expression of anoxygenic photosynthesis-related genes, while exhibiting a negative correlation with the expression of genes associated with photosystem II. These findings suggest microbial rhodopsins are widely distributed and potentially an important solar-energy capturing mechanism and an important adaptation strategy of microorganismsin the South China Sea ecosystem.
 

the South China Sea,microbial rhodopsin,retinal abundance,nutrients,adaptation