Award: OCE-0961900

The ocean’s microbial realm is the largest and arguably most complex ecosystem on Earth.  It plays a central role in controlling global element cycles, such as carbon, and is tightly linked to climate-feedback processes.  Key to the behavior of climate and marine ecosystems is the fate of nitrogen (N), which has been greatly altered by human activities over the past century.  Nitrogen (N) is an essential element for all life on Earth.  Organisms need N to form proteins and DNA and will compete vigorously for N to survive and grow.  The pool sizes of different portions of the marine N cycle are often fairly well understood.  What is not well known are the identities of microbes that are responsible for the inter-conversion of different forms of N and the rates at which they perform these transformations.  A failure to capture the ‘who’ and the ‘how much’ dimensions of the N cycle can have large consequences for our understanding of global geochemical fluxes.  Hence, this grant focused on utilizing ¹⁵N-DNA stable isotope probing (SIP) techniques in combination with ¹⁵N-uptake rate measurements to further our understanding of microbial N transformation in marine systems.  The goal was to specifically link N uptake from different sources to individual microbial populations.

 DNA-SIP has been used as a powerful tool for the investigation of carbon metabolism in microbial communities.  Substantially less work has, however, focused using ¹⁵N as a tracer, mainly because of the greater technical challenge.  We applied ¹⁵N-based SIP in marine systems in order to investigate N uptake by phyto- and bacterioplankton populations.  Historically, it has been perceived that inorganic forms of N are utilized by phytoplankton, while organic N is primarily a source of nutrients for bacteria.  Evidence suggests that this distinction may be too simplistic and that there can be considerable plasticity within the range of a given organism’s use of N substrates.  It is also unclear how phytoplankton and heterotrophic bacteria interact in the environment when forced to compete for limited N resources, and what the N substrate ranges of bacterial and phytoplankton populations are under natural conditions.  A series of experiments on subtropical and arctic, coastal communities was conducted using a range of ¹⁵N-labeled organic and inorganic substrates. 

In subtropical, coastal environments we observed that multiple organic and inorganic sources of N were fueling cyanobacterial and diatom primary production simultaneously.  This is contrary to the traditional new-versus-recycled production paradigm, which is frequently invoked for coastal environments, and our study is among only a few in the literature that directly demonstrate the uptake of a specific form of N into individual phytoplankton species in the environment.  In a subsequent, related study, we focused on heterotrophic bacterial nitrate utilization.  Molecular evidence had suggested that nitrate-utilizing bacteria are abundant, wide-spread, and potentially active in marine systems, but direct evidence of incorporation of N from nitrate into individual bacterial species had not been reported.  We combined uptake rate measurements with ¹⁵N-SIP and RNA-based functional microarray analysis of whole community gene expression.  The aim was to gain a more comprehensive perspective of N cycling by considering whole community transcription of N cycling genes, while targeting nitrate utilization more specifically.  Most importantly, the study allowed us for the first time to demonstrate nitrate uptake into several specific marine bacterioplankton taxa.  We also conducted work in the Artic, which has focused on ...