Deborah A. Bronk

Nitrogen Uptake, Dissolved Organic Nitrogen Release, and New Production

In oceanic, coastal, and estuarine environments, an average of 25 to 41 percent of the dissolved inorganic nitrogen (NH4 + and NO3 –) taken up by phytoplankton is released as dissolved organic nitrogen (DON). Release rates for DON in oceanic systems range from 4 to 26 nanogram-atoms of nitrogen per liter per hour. Failure to account for the production of DON during nitrogen-15 uptake experiments results in an underestimate of gross nitrogen uptake rates and thus an underestimate of new and regenerated production. In these studies, traditional nitrogen-15 techniques were found to underestimate new and regenerated production by up to 74 and 50 percent, respectively. Total DON turnover times, estimated from DON release resulting from both NH4 + and NO3 – uptake, were 10 � 1, 18 � 14, and 4 days for oceanic, coastal, and estuarine sites, respectively.

Chapter 5 – Dynamics of DON

This chapter describes the dynamics of dissolved organic nitrogen (DON). DON is that subset of the dissolved organic matter (DOM) pool that contains nitrogen (N). From the perspective of a microorganism, this is where the action is—one-stop shopping for N, carbon (C), and energy. Research in DON, however, has lagged far behind that of the larger dissolved organic carbon (DOC) pool. This situation is primarily the result of the substantial analytical challenges inherent in DON research: (1) DON exists in substantially lower concentrations than DOC, (2) multiple chemical analyses are required for a single DON measurement, (3) inorganic N removal is a nightmarish undertaking, and (4) unless one has an easy access to a nuclear reactor manufacturing short-lived 13 N, he or she must be content with labor-intensive stable isotopes rather than the quicker and more sensitive radiotracers. Measurements of DON concentrations have become a routine component of many studies. Nonetheless, because of space limitations DON concentrations in lakes, streams, or groundwater—with some exceptions—are not included in studies.

Total dissolved nitrogen analysis : comparisons between the persulfate, UV and high temperature oxidation methods

We compared the persulfate (PO), ultraviolet (UV), and high temperature oxidation (HTO) methods used to analyze total dissolved nitrogen (TDN) concentrations in aquatic samples to determine whether the three methods differed in terms of standard parameters (blanks, limits of detection and linearity, and precision) or in oxidation efficiency of standard compounds and field samples of varying salinity. The TDN concentrations of several N-containing standard compounds, as well as a humic mixture and a suite of field samples collected from the Sargasso Sea, Chesapeake Bay and an aquaculture pond were determined with the three methods. The PO method had the highest percent recoveries for the range of labile and refractory standard compounds tested (93±13). The HTO method yielded recoveries of 87±14; recoveries increased to 91±10 under optimized conditions. The standard UV method, with 30% H2O2 as the oxidant, was found to be highly variable, producing the lowest percent recoveries (71±21); the oxidation efficiency of the UV method increased substantially in subsequent trials (91±12), when the PO reagent was used in place of H2O2. In the field sample comparison, the PO, UV with PO reagent, and HTO method produced similar results (slopes of the Model II regression lines comparing them ranged from 1.00 to 1.05 with r2≥0.99). The standard UV method, however, produced concentrations 5% to 40% lower than the other methods. Analysis of the spectra emitted by the UV lamp used in this study suggests that variations in the UV spectra reaching the sample may have caused the reduced efficiencies. The poor recovery of some standard compounds with each of the methods suggests that concentrations of TDN, and subsequently DON, measured in the field with any of the methods will likely be underestimated to some degree depending on the composition of the TDN pool at that time. With careful attention to detail, however, the PO and HTO methods can provide reproducible results consistent with each other. The standard UV method, however, was found to be highly unpredictable in practice.

Jellyfish blooms result in a major microbial respiratory sink of carbon in marine systems.

Jellyfish blooms occur in many estuarine and coastal regions and may be increasing in their magnitude and extent worldwide. Voracious jellyfish predation impacts food webs by converting large quantities of carbon (C), fixed by primary producers and consumed by secondary producers, into gelatinous biomass, which restricts C transfer to higher trophic levels because jellyfish are not readily consumed by other predators. In addition, jellyfish release colloidal and dissolved organic matter (jelly-DOM), and could further influence the functioning of coastal systems by altering microbial nutrient and DOM pathways, yet the links between jellyfish and bacterioplankton metabolism and community structure are unknown. Here we report that jellyfish released substantial quantities of extremely labile C-rich DOM, relative to nitrogen (25.6 ± 31.6 C:1N), which was quickly metabolized by bacterioplankton at uptake rates two to six times that of bulk DOM pools. When jelly-DOM was consumed it was shunted toward bacterial respiration rather than production, significantly reducing bacterial growth efficiencies by 10% to 15%. Jelly-DOM also favored the rapid growth and dominance of specific bacterial phylogenetic groups (primarily γ-proteobacteria) that were rare in ambient waters, implying that jelly-DOM was channeled through a small component of the in situ microbial assemblage and thus induced large changes in community composition. Our findings suggest major shifts in microbial structure and function associated with jellyfish blooms, and a large detour of C toward bacterial CO2 production and away from higher trophic levels. These results further suggest fundamental transformations in the biogeochemical functioning and biological structure of food webs associated with jellyfish blooms.

A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater

Abstract Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods. This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses. This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology.

Phytoplankton–bacterial interactions mediate micronutrient colimitation at the coastal Antarctic sea ice edge

Significance The coastal Southern Ocean is a critical climate system component and home to high rates of photosynthesis. Here we show that cobalamin (vitamin B12) and iron availability can simultaneously limit phytoplankton growth in late Austral summer coastal Antarctic sea ice edge communities. Unlike other growth-limiting nutrients, the sole cobalamin source is production by bacteria and archaea. By identifying microbial gene expression changes in response to altered micronutrient availability, we describe the molecular underpinnings of limitation by both cobalamin and iron and offer evidence that this limitation is driven by multiple delicately balanced phytoplankton–bacterial interactions. These results support a growing body of research suggesting that relationships between bacteria and phytoplankton are key to understanding controls on marine primary productivity. Southern Ocean primary productivity plays a key role in global ocean biogeochemistry and climate. At the Southern Ocean sea ice edge in coastal McMurdo Sound, we observed simultaneous cobalamin and iron limitation of surface water phytoplankton communities in late Austral summer. Cobalamin is produced only by bacteria and archaea, suggesting phytoplankton–bacterial interactions must play a role in this limitation. To characterize these interactions and investigate the molecular basis of multiple nutrient limitation, we examined transitions in global gene expression over short time scales, induced by shifts in micronutrient availability. Diatoms, the dominant primary producers, exhibited transcriptional patterns indicative of co-occurring iron and cobalamin deprivation. The major contributor to cobalamin biosynthesis gene expression was a gammaproteobacterial population, Oceanospirillaceae ASP10-02a. This group also contributed significantly to metagenomic cobalamin biosynthesis gene abundance throughout Southern Ocean surface waters. Oceanospirillaceae ASP10-02a displayed elevated expression of organic matter acquisition and cell surface attachment-related genes, consistent with a mutualistic relationship in which they are dependent on phytoplankton growth to fuel cobalamin production. Separate bacterial groups, including Methylophaga, appeared to rely on phytoplankton for carbon and energy sources, but displayed gene expression patterns consistent with iron and cobalamin deprivation. This suggests they also compete with phytoplankton and are important cobalamin consumers. Expression patterns of siderophore- related genes offer evidence for bacterial influences on iron availability as well. The nature and degree of this episodic colimitation appear to be mediated by a series of phytoplankton–bacterial interactions in both positive and negative feedback loops.

Importance of heterotrophic bacterial assimilation of ammonium and nitrate in the Barents Sea during summer

Abstract In a transect across the Barents Sea into the marginal ice zone (MIZ), five 24-h experimental stations were visited, and uptake rates of NH4+ and NO3− by bacteria were measured along with their contribution to total dissolved inorganic nitrogen (DIN) assimilation. The percent bacterial DIN uptake of total DIN uptake increased substantially from 10% in open Atlantic waters to 40% in the MIZ. The percentage of DIN that accounted for total bacterial nitrogen production also increased from south to north across the transect. On average, at each of the five 24-h stations, bacteria accounted for 16–40% of the total NO3− uptake and 12–40% of the total NH4+ uptake. As a function of depth, bacteria accounted for 17%, 23%, and 26% of the total NH4+ assimilation and 17%, 37%, and 36% of the total NO3− assimilation at 5, 30, and 80 m, respectively. Bacteria accounted for a higher percentage of total NO3− uptake compared to total NH4+ uptake in 12 out of 15 samples. Bacterial productivity explains a substantial amount of the variability associated with bacterial DIN uptake, but the relationship between bacterial production and bacterial DIN uptake is best explained when the data from the open Atlantic water stations are grouped separately from the MIZ stations. The percentage of DIN that accounts for bacterial N production is approximately four-fold higher in 24 h MIZ stations compared with open Atlantic stations. This suggests that bacteria play a larger role in NO3− utilization, particularly in the MIZ, than previously hypothesized and that bacterial uptake of NO3− should not be ignored in estimates of new production. Understanding processes that affect autotrophic based new production, such as heterotrophic bacterial utilization of NO3−, in polar oceans is of particular significance because of the role these regions may play in sequestering CO2.

Temporal and Spatial Dynamics of Urea Uptake and Regeneration Rates and Concentrations in Chesapeake Bay

We examined the temporal and spatial variability of urea concentrations and urea uptake and regeneration rates collected on cruises along the longitudinal axis of the Chesapeake Bay between 1972 and 1998. Interannually, mean Bay-wide surface urea concentrations ranged between 0.49 and 0.91 μg-at N l−1 with a nearly 50% decrease in surface concentrations observed between 1988 and 1998. Concentrations of urea from samples collected within ∼1 m of the bottom were generally higher and much more varable than surface samples. Seasonally, two different patterns were observed in mean Bay-wide surface urea concentrations. Urea concentrations from near surface waters exhibited a clear summer peak for 1988 through 1994, while for 1973 and 1996 to 1998 a distinct winter-spring peak in concentration was observed. Urea concentrations from deeper waters showed a similar seasonal trend each year with peak concentrations measured in spring. Spatially, urea concentrations in the surface waters decreased in a conservative-type pattern from 0.91 μg-at N I−1 at the freshwater end member to 0.46 μg-at N I−1 at the ocean end member. Mean Bay-wide surface urea uptake rates displayed a seasonal pattern throughout the data set with maximum uptake rates (up to 0.33 μg-at N I−1 h−1) consistently observed during summer. Mean Bay-wide surface regeneration rates were highest but most variable during fall (1.63±0.82 μg-at N I−1 h−1). Mean urea uptake and regeneration rates displayed opposing spatial trends along the axis of the Bay with uptake rates being lowest in the North Bay where regeneration rates were highest. The average temporal and spatial patterns of urea concentration in Chesapeake Bay appear to reflect a balance between external inputs and internal biological recycling.

Ocean urea fertilization for carbon credits poses high ecological risks

The proposed plan for enrichment of the Sulu Sea, Philippines, a region of rich marine biodiversity, with thousands of tonnes of urea in order to stimulate algal blooms and sequester carbon is flawed for multiple reasons. Urea is preferentially used as a nitrogen source by some cyanobacteria and dinoflagellates, many of which are neutrally or positively buoyant. Biological pumps to the deep sea are classically leaky, and the inefficient burial of new biomass makes the estimation of a net loss of carbon from the atmosphere questionable at best. The potential for growth of toxic dinoflagellates is also high, as many grow well on urea and some even increase their toxicity when grown on urea. Many toxic dinoflagellates form cysts which can settle to the sediment and germinate in subsequent years, forming new blooms even without further fertilization. If large-scale blooms do occur, it is likely that they will contribute to hypoxia in the bottom waters upon decomposition. Lastly, urea production requires fossil fuel usage, further limiting the potential for net carbon sequestration. The environmental and economic impacts are potentially great and need to be rigorously assessed.

Chapter 4 – Dynamics of Dissolved Organic Nitrogen

Dissolved organic nitrogen (DON) is that subset of the dissolved organic carbon (DOC) pool that also contains N. This chapter reviews four major areas of DON research. First, DON concentrations and distributions in aquatic environments are reviewed with respect to the methods used to analyze DON concentrations, global distributions, comparison of DON concentration across a range of aquatic systems and seasonal variations. Second, the composition of the characterizable and uncharacterized DON is reviewed with respect to pools of specific DON forms and the methods used to study and chemical characteristics of the fraction of the DON pool that is not defined. Third, sources of DON to the water column are reviewed with respect to autochthonous sources, allochthonous sources, methods used to measure rates of DON release and release rates of bulk DON and individual DON forms in the literature. The final section reviews sinks for DON with respect to DON bioavailability, methods used to estimate DON uptake rates, mechanisms that contribute to DON bioavailability, and DON uptake rates published in the literature. Recommendations for future research are presented for each research area. This chapter focuses on work published largely after 2001 and topics not included in the earlier review (Bronk, D.A., 2002. Dynamics of DON. In: Hansell, D.A., Carlson, C.A. (eds.) Biogeochemistry of Marine Dissolved Organic Matter. Academic Press, San Diego).