Natural photolysis by ultraviolet irradiance of recalcitrant dissolved organic matter to simple substrates for rapidbacterial metabolism

Abstract

Whole leachate and humic and fulvic acid fractions of dissolved organic matter (DOM) released from senescent littoral aquatic plants were exposed to varying spectra of ultraviolet radiation as well as natural UV of sunlight over different periods of time. Examination of the DOM by solid-state 13C nuclear magnetic resonance and pyrolytic gas chromatography-mass spectrometry before and after photolysis revealed only subtle changes to the bulk DOM. However, the DOM exposed to natural UV radiation showed immediate stimulation of and sustained bacterial growth. Chemical analyses by high performance liquid chromatography (HPLC) of the small organic fractions generated by photolysis of humic substances showed marked, pro-' gressively increasing release of numerous small fatty acids, particularly acetic, formic, citric, pyruvic, and levulinic, among others. Use of radiolabeled humic substances demonstrated that these small compounds photolyzed from the humic substances were readily metabolized by the bacteria.Dissolved organic matter (DOM) of the open ocean apparently originates largely from planktonic photosynthesis, and concentrations are usually quite low (0.5-2 mg C liter-l) (Wangersky 1984;Wetzel 1984;Meybeck 1993;Hedges et al. 1993). In inland waters, however, phenolic and other aromatic-based humic compounds, largely of terrestrial and higher aquatic plant origin, form a major component of dissolved organic acids of freshwaters and can constitute some 80% of the total DOM, 30-40% of which is composed of aromatic carbons (Malcolm 1990). Concentrations of 4-8 mg organic acids liter-l are common in surface waters and often exceed 25 mg liter-l in organic-rich waters, such as those of wetlands, floodplains of river ecosystems, and interstitial waters of hydrosoils (Wetzel 1984;Mann and Wetzel 1995).The higher concentrations of DOM in inland surface waters clearly result from high photosynthetic productivity within lake ecosystems, particularly associated with littoral and wetland regions, as well as large loadings of DOM from decomposition of plant materials within the drainage basin (Wetzel 1990(Wetzel , 1992. Similarly, the high photosynthesis and decomposition within the extensive Acknowledgments

Photodegraded dissolved organic matter from peak freshet river discharge as a substrate for bacterial production in a lake-rich great Arctic delta

“…Photodegraded DOM molecules are smaller and less aromatic, and therefore more labile and easily consumed by bacteria, than are the unaltered compounds in the initial DOM pool (Wetzel et al 1995;Moran and Zepp 1997;Bertilsson and Tranvik 1998;Obernosterer and Benner 2004). The presence of photodegraded and bioavailable DOM has been observed to fuel increases in BP in the short term, with these effects fading over longer observational periods.…”

“…Although DOC and lignin concentrations declined due to complete carbon photodegradation and mineralization, the changes in DOM quality, such as decreased molecular weight and aromaticity, are likely more important from an ecosystem perspective as they render freshet DOM more bioavailable for microbial consumption and bacterial metabolism (Lindell et al 1995;Wetzel et al 1995). These increases in bioavailable DOM occur at a critical time of year when Mackenzie Delta food web productivity is increasing rapidly due to the breakup of ice cover, warming temperatures, the delivery of nutrient-rich floodwater, and prolonged daylight hours.…”

“…Once present in the water column and exposed to sunlight, however, allochthonous DOM can be photodegraded via the absorption of high-energy ultraviolet (UV) wavelengths (280-400 nm). Absorbed UV breaks down aromatic carbon rings, producing smaller, less aromatic DOM molecules (Bertilsson and Tranvik 2000;Osburn et al 2001;Sulzberger and Durisch-Kaiser 2009) that have lower molecular weights and are more bioavailable for bacterial consumption and decomposition (Wetzel et al 1995;Moran and Zepp 1997;Bertilsson and Tranvik 1998;Obernosterer and Benner 2004). Heterotrophic grazers subsequently consume bacteria, which shunts otherwise unavailable carbon, organic matter, and energy back into the aquatic food web via the microbial loop (Azam et al 1983).…”

Occurrence and bacterial cycling of dissolved nitrogen in the Gulf of Riga, the Baltic Sea

“…In addition, the production of DCAA at the 5.3 km station may also have increased the bacterial production at that station. Sunlight has previously been found to transform natural C and N compounds, including recalcitrant DOC, into substances that can be readily utilized by aquatic bacteria (Lindell et al 1995, Wetzel et al 1995, Bushaw et al 1996, Jnrgensen et al 1998. The reduced bacterial growth in several of the cultures suggests, however, that light changed otherwise labile material into more recalcitrant or inhibitory compounds.…”

SISI: A New Device for In Situ Incubations at the Ocean Surface

“…Photolysis transforms recalcitrant DOM into bioavailable, labile carbon compounds, a process that occurs even at low intensities of visible light [50]. Such labile compounds can be further utilized by heterotrophic microorganisms [51]. On the other hand, labile DOM components under the influence of UV radiation can also undergo crosslinking and, in turn, contribute to the refractory DOM pool [50].…”