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Abstract Iron is an essential nutrient for all microorganisms of the marine environment. Iron limitation of primary production has been well documented across a significant portion of the global surface ocean, but much less is known regarding the potential for iron limitation of the marine heterotrophic microbial community. In this work, we characterize the transcriptomic response of the heterotrophic bacterial community to iron additions in the California Current System, an eastern boundary upwelling system, to detect in situ iron stress of heterotrophic bacteria. Changes in gene expression in response to iron availability by heterotrophic bacteria were detected under conditions of high productivity when carbon limitation was relieved but when iron availability remained low. The ratio of particulate organic carbon to dissolved iron emerged as a biogeochemical proxy for iron limitation of heterotrophic bacteria in this system. Iron stress was characterized by high expression levels of iron transport pathways and decreased expression of iron-containing enzymes involved in carbon metabolism, where a majority of the heterotrophic bacterial iron requirement resides. Expression of iron stress biomarkers, as identified in the iron-addition experiments, was also detected in  situ. These results suggest iron availability will impact the processing of organic matter by heterotrophic bacteria with potential consequences for the marine biological carbon pump. 

ABSTRACT Dissolved organic matter (DOM) comprises diverse compounds with variable bioavailability across aquatic ecosystems. The sources and quantities of DOM can influence microbial growth and community structure with effects on biogeochemical processes. To investigate the chemodiversity of labile DOM in tropical reef waters, we tracked microbial utilisation of over 3000 untargeted mass spectrometry ion features exuded from two coral and three algal species. Roughly half of these features clustered into over 500 biologically labile spectral subnetworks annotated to diverse structural superclasses, including benzenoids, lipids, organic acids, heterocyclics and phenylpropanoids, comprising on average one‐third of the ion richness and abundance within each chemical class. Distinct subsets of these labile compounds were exuded by algae and corals during the day and night, driving differential microbial growth and substrate utilisation. This study expands the chemical diversity of labile marine DOM with implications for carbon cycling in coastal environments.