A Spectrum of Nutrient Limitation in the Heterotrophic Prokaryotic Community of the California Current Ecosystem

Marine dissolved organic matter consists of one of the largest pools of carbon on Earth, and it is now recognized that marine heterotrophic bacteria are the key determinant in the turnover of this carbon and its associated macro- and micronutrients. The rate of this turnover has significant impacts on global biogeochemical processes, ultimately determining the efficiency of carbon export and storage in the marine environment. However, the factors controlling this heterotrophic activity, including potential nutrient limitations, are not well understood. Members of the genus Alteromonas are widespread marine copiotrophs with the ability to disproportionately affect the processing of organic matter. Recent transcriptomic work has shown that Alteromonas has distinct cellular responses to carbon versus iron limitation in culture. Here we utilize these patterns of gene expression in the field as indicators for potential nutrient limitation of the in situ Alteromonas population. Samples collected in August 2014 from the California Current Ecosystem at both the surface and deep chlorophyll maximum exhibit gene expression patterns reflecting carbon limitation but replete iron conditions for Alteromonas in offshore waters. In contrast, the gene expression patterns of nearshore samples are not diagnostic of exclusively iron or carbon limitation but are dominated by the expression of an array of cellular transporters. However, iron addition incubations with nearshore waters show that copiotrophic groups, including Alteromonas, responded to added iron. This response was also accompanied by an overall increase in bacterial productivity and abundance, indicating an iron-limited heterotrophic community. These results illustrate that heterotrophic bacteria can experience a range of nutrient limitations across varying oceanic regimes and support the need for further characterization of the molecular responses to nutrient limitations in the heterotrophic prokaryotic community.