In both marine and freshwaters, the concentration of dissolved organic nitrogen (DON) frequently exceeds that of dissolved inorganic nitrogen (DIN), including ammonium, nitrate, and nitrite. Recent evidence indicates that many organic N compounds are released into the DON pool and taken up from this pool by planktonic microbiota on timescales of hours to days. This observation suggests that many components of the DON pool can play an active role in supplying N nutrition directly or indirectly to phytoplankton and bacteria and, in so doing, may affect the species composition of the ambient microbial assemblage. Here we present an overview of the state of knowledge of DON pools in aquatic environments, focused mainly on data gathered in the last decade. We review information on DON concentrations in freshwater and marine systems, analytical methods for the determination of DON, and the biotic and abiotic sources and sinks of DON. More detailed discussion addresses specific components of the DON pool: urea, dissolved combined and free amino acids, proteins, nucleic acids, amino sugars, and humic substances. The DON pool in natural waters is not inert and can be an important sink and source for N. There is a need for greater appreciation and understanding of the potential role of DON as a dynamic participant in the nitrogen cycle within aquatic ecosystems, particularly in freshwater environments.KEY WORDS: Dissolved organic nitrogen · Phytoplankton · Bacteria · N nutrition · N cycling · Marine/freshwater ecosystems Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 31: 2003 ranged from 3.5 to 10.4 µM N. Such observations are not consistent with the perception of a DON pool composed entirely of recalcitrant compounds.Subsequent to the comprehensive and detailed review of Antia et al. (1991), some salient new features of N flux into and out of DON pools have become evident. These imply that many DON compounds are cycled more rapidly in aquatic environments than previously recognized. We now know that DON may be released from actively growing phytoplankton, sometimes in appreciable quantities (Bronk et al. 1994, Bronk & Ward 1999, Diaz & Raimbault 2000, possibly via viral lysis or autolysis of bacteria (Fuhrman 1999) and algae (Gobler et al. 1997, Agusti et al. 1998, that elevated concentrations of DON are common in blooms of Trichodesmium, the primary N 2 fixer in the ocean (Capone et al. 1994, Glibert & Bronk 1994, Vidal et al. 1999, and that atmospheric inputs of DON to the oceans can be substantial (Cornell et al. 1995.Sinks for DON have also been studied in more detail over the last decade. Algae (Lewitus et al. 2000), cyanobacteria (Berman 2001), bacteria (Antia et al. 1991, archaebacteria (Ouverney & Fuhrman 2000) and perhaps even protists (Tranvik et al. 1993) have been shown to exploit various components of the DON pool either directly or after bacterial degradation. The potential of photochemical modification or degradation of DON constituents ha...

No abstract

No abstract

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.colimitation | Southern Ocean primary productivity | metatranscriptomics | phytoplankton-bacterial interactions | cobalamin P rimary productivity and community composition in the Southern Ocean play key roles in global change (1, 2). The coastal Southern Ocean, particularly its shelf and marginal ice zones, is highly productive, with mean rates approaching 300-450 mg C m −2 ·d −1 (3). As such, identifying factors controlling phytoplankton growth in these regions is essential for understanding the ocean's role in past, present, and future biogeochemical cycles. Although irradiance, temperature, and iron availability are often considered to be the primary drivers of Southern Ocean productivity (1, 4), cobalamin (vitamin B 12 ) availability has also been shown to play a role (5, 6). Cobalamin is produced only by select bacteria and archaea and is required by most eukaryotic phytoplankton, as well as many bacteria that do not produce the vitamin (7). Cobalamin is used for a range of functions, including methionine biosynthesis and one-carbon metabolism. Importantly, phytoplankton that are able to grow without cobalamin preferentially use it when available; growth...

Abstract. Relative to inorganic nitrogen, concentrations of dissolved organic nitrogen (DON) are often high, even in regions believed to be nitrogen-limited. The persistence of these high concentrations led to the view that the DON pool was largely refractory and therefore unimportant to plankton nutrition. Any DON that was utilized was believed to fuel bacterial production. More recent work, however, indicates that fluxes into and out of the DON pool can be large, and that the constancy in concentration is a function of tightly coupled production and consumption processes. Evidence is also accumulating which indicates that phytoplankton, including a number of harmful species, may obtain a substantial part of their nitrogen nutrition from organic compounds. Ongoing research includes ways to discriminate between autotrophic and heterotrophic utilization, as well as a number of mechanisms, such as cell surface enzymes and photochemical decomposition, that could facilitate phytoplankton use of DON components.

In oceanic, coastal, and estuarine environments, an average of 25 to 41 percent of the dissolved inorganic nitrogen (NH(4) (+) and NO(3) (-)) 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 NH(4) (+) and NO(3) (-) uptake, were 10 +/- 1, 18 +/- 14, and 4 days for oceanic, coastal, and estuarine sites, respectively.

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 CO 2 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.biogeochemical cycling | jelly carbon shunt | fisheries production

No abstract