Metatranscriptomics of the bacterial community in response to atmospheric deposition in the Western North Pacific Ocean.

Atmospheric deposition represents a major vector of both macro- and micro-nutrients to the oligotrophic open oceans, potentially imposing a profound impact on the functioning of the microbial community. Whereas bacterial responses to atmospheric deposition are being studied at the community level, corresponding functional changes are essentially unknown. Here we conducted a microcosm experiment coupled with metatranscriptomic analyses to elucidate taxonomic and functional profiles of the bacterial community in response to East Asian aerosols in the Western North Pacific Ocean (WNPO). While the abundance of heterotrophic bacteria showed a minor change, cyanobacterial cell count number decreased dramatically, with Prochlorococcus and Synechococcus counts reduced by 83.2% and 21.5% in the aerosol treatment in relation to the control. Expression of transcripts related with Prochlorococcus, Synechococcus, Trichodesmium and Crocosphaera both were lower in the treatment (5.7%, 2.3%, 0.5% and 0.02%, respectively) than in the control (18.6%, 2.7%, 9.8% and 0.14%, respectively). Aerosol addition led to an increase in transcripts involved in iron metabolism (tonB, feoB, irr, exbB), indicating Fe limitation. Heavy metal toxicity was evidenced by an elevated expression of resistance genes, such as czcC, czcB, czcA and a probable Co/Zn/Cd efflux protein, and a range of genes functioning against oxidative stress. Our findings provide insights into an inhibitory effect of high-flux East Asian aerosols on cyanobacteria in the WNPO likely due to Fe scavenging and heavy metal toxicity.