Nitrogen cycle

UVR can affect nitrogen cycling in several ways: (1) through effects on nitrogen-related enzymatic activity by microorganisms such as photoinhibition of nitrogen fixation by prokaryotes, principally cyanobacteria and, indirectly, through effects on the biological availability of essential trace elements, such as iron that stimulate the growth of nitrogen fixers; (2) through enhanced decomposition of persistent DON to biologically labile nitrogenous photoproducts. In addition, UV absorption by inorganic nitrogen species such as nitrate and nitrite produce ROS in aquatic environments, including the highly reactive hydroxyl radical [Chapter 8].

Time (days)

Figure 7. Changes in the stable isotopic composition of CDOM in a coastal river water sample on exposure to natural solar radiation . The change is consistent with observed changes in DOC isotopic composition that occur when terrestrially-derived DOM moves from land into the sea.

5.3.1 Nitrogen fixation

All organisms require nitrogen and, although its most common form in the environment, N2 gas, is quite abundant, it can only be used by microorganisms that can "fix" nitrogen by using the enzyme nitrogenase to reduce N2 to ammonium (NH4+). Nitrogenase is rapidly inhibited by exposure to UV-B radiation in vivo [45,147]. Moreover, the growth and survival of most N-fixing cyanobacteria that have been studied can be decreased by only a few hours of UV exposure [45,148]. However, few data are available on the effects of UV on open ocean nitrogen fixers. Damaging effects of UV include those that were discussed earlier, e.g. DNA and photosystem II damage [45]. Presumably, UV exposure can also affect the biological availability of certain organic substrates that affect the growth rates of cyanobacteria, but little attention has been paid to such effects thus far. Nitrogen is a limiting nutrient in remote parts of the open ocean and thus impacts of UV radiation on important nitrogen fixing cyanobac-teria such as Trichodesmium [148] or the newly discovered N-fixing marine nanoplankton [149], although not proven thus far, could be quite ecologically significant.

Furthermore, iron plays a role in stimulating the growth of Trichodesmium [150]. In a later part of this chapter, the possible role of UV in iron cycling and biological availability is discussed in more detail. It has been proposed that the iron deposited in the sea via long range transport of Saharan dust can trigger rapid growth of this organism and that related increases in biologically-available nitrogen can trigger growth in toxic organisms such as Gymnodinium breve [148,151]. Riverine inputs and wet deposition of iron also can be important sources (Figure 1). Intense precipitation events can flush large amounts of riverine iron into coastal areas. Iron limitation of Trichodesmium may be even more pronounced than was originally believed, because much of the iron believed to be 'dissolved' in the upper ocean, is in fact in colloidal form and thus presumably less biologically-available [152].

5.3.2 Photoreactions of persistent DON

Biologically labile nitrogen compounds such as nitrate, ammonium and amino acids are rapidly recycled by the biota in aquatic systems, while N-containing substances whose structures are too complex or randomized to be readily assimilated accumulate in the water column. In lakes, estuaries, and parts of coastal regions, these persistent classes of DON are predominantly terrestrially-derived. In the open ocean, persistent DON is derived from a combination of bacterial and perhaps photochemical transformation of algal-derived organic matter. In oligotrophic waters with limited N fixation or external inputs of labile N, the labile compounds drop almost to immeasurable levels in the photic zone while the persistent DON accumulates [153]. Interactions of UVR and DON provide a pathway for the conversion of persistent DON into compounds that are more easily assimilated by aquatic microorganisms, such as ammonium [9,20,154-158], nitrite [159] and dissolved primary amines [9,20,154,155,160],

Ammonium photoproduction generally is the most rapid photoreaction of DON. Net production or, in some cases, consumption [156] of ammonium appears to be the result of a competition between production, possibly via hydrolysis of reactive organonitrogen intermediates such as Schiff bases [154], and reactions of ammonium with humic substances and their photoproducts [156]. Environmental conditions such as system pH and temperature affect the conversion of the intermediates into ammonium [59,154]. Other factors such as the degree of pre-exposure to solar radiation do not affect ammonium photoproduction, as evidenced by the low production rates observed in irradiated groundwater samples [156]. Under N-limiting conditions, the release of nitrogenous photoproducts from DOM photodegradation was found to significantly increase rates of bacterial growth, and it occurred most efficiently on exposure to

UV-B radiation [154]. Modeling results suggest that photochemically-produced labile nitrogen compounds can be an important source of biologically available nitrogen in coastal regions [154]. Such photoreactions also may be an increasingly important source of labile N in the upper open ocean, where future intensification of stratification in response to global warming may reduce up-welled sources of labile N [73]. However, few data are available on the photo-reactions of open-ocean DON.

5.3.3 Nitrous oxide, nitric oxide, nitrate and nitrite

Nitrous oxide is a greenhouse gas with a poorly characterized, but possibly significant ocean source [161]. N20 is produced in the ocean by nitrifying and denitrifying bacteria via processes that are inhibited by high dissolved oxygen levels and by light. The main production of N20 occurs in the deep ocean where the nitrifiers are well protected from solar UVR. Thus, changes in UV fluxes at the ocean surface would only indirectly change the N20 fluxes through, for example, alterations in N2 fixation or labile N photoproduction.

UV-B radiation is mainly responsible for the photodegradation of nitrate in water [162-164], This photoreaction is likely to be too slow (near-surface turnover in sunlight of about 0.008 day-1 in the tropics and 0.005 day-1 in mid-latitudes [163]) in comparison with the rapid biological uptake of nitrate to have a major effect on the environmental lifetime of nitrate in most aquatic environments, certainly in N-limited regions. Nonetheless, nitrate is an important source of reactive oxygen species in certain freshwaters [163,165]; among other species, hydroxyl radicals are produced in this photoreaction. Nitrite is also degraded by solar UVR to form nitric oxide (NO), hydroxyl radicals, and other products [162], but this reaction is mainly induced by UV-A radiation and thus its photolysis lifetime under solar radiation is considerably shorter than that of nitrate [162,166,167]. The rapid photolysis of nitrite constrains its buildup as a photoproduct from DOM photodegradation [159]. Photochemically produced NO may be an important source of this chemically-reactive gas in the remote marine boundary layer [167], but the buildup in its concentration is constrained by the co-occurrence of photochemically-produced superoxide ions in the upper ocean that rapidly react with NO [5,168, Chapter 8]. Jankowski et al. [162] have taken advantage of the sensitivity of nitrate and nitrite photoreactions to solar UV by using these substances in solar UV actinometers (i.e., photochemical systems for measuring UV irradiance).

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