Diversity and distribution of isiA-containing Cyanobacteria in the global ocean

The expression of the isiA (iron stress inducible) gene, which encodes chlorophyll a binding proteins, proved to be a novel photosynthetic strategy for a number of cyanobacteria responding to iron deficiency. Our study investigated the isiA-containing cyanobacteria community of 15 water samples collected from Pacific Ocean, Atlantic Ocean, Indian Ocean, South China Sea and Bering Sea, using clone libraries and quantitative PCR (qPCR). A new generated primer pair suitable for oceanic cyanobacterial isiA was designed and tested with our samples. Out of 401 isiA sequences retrieved from our samples, 368 were from Synechococcus and only 33 were from Prochlorococcus (isiA-like pcb C/D). Abundance of isiA (gene copies/mL) in the surface ocean varied from 2.45E+03 in the Bering Sea to 3.23E+04 in the eastern equatorial Pacific Ocean. The vast majority (71%) of these sequences did not cluster with any known isiA references on the phylogenetic tree, which indicated a large amount of isiA-containing cyanobacteria remain unstudied in the ocean. While the abundance of isiA genes was significantly related to the iron concentrations in the surface ocean (P < 0.05), their biodiversity was not necessarily correlated to iron. Furthermore, cluster analysis of isiA genes showed some grouping of the stations, especially of the warm waters from low latitudes. It is noteworthy that the South China Sea was the only surface station that revealed PSI-pcb gene type (PSI-pcbC), but the RNA amplification results suggest they were not being expressed. Both the high diversity and abundance of isiA genes at typical high-chlorophyll low-nutrient, iron deplete stations (e.g. EEP1 and EEP2) suggested a significant influence of iron deficiency on the photosynthetic antenna genes of cyanobacteria. In conclusion, our work shed lights on how iron deficiency over geologic time has imprinted on the survival strategies used by cyanobacteria.