278
8.1 Introduction
All natural waters on Earth contain dissolved organic matter (DOM). The ubiquitous presence of organic life in all environments has resulted in its widespread distribution, from
the depths of the oceans, to trapped within polar ice sheets, to aerosol droplets in the upper
atmosphere. Despite the ability of microbes to evolve to degrade all types of complex
structures, an organic residue always remains, owing to it being either energetically unfavorable, at too low a concentration, or preserved as a result of some other external limiting
factor such as auxiliary nutrients.
Large pools of organic carbon exist on land as living biomass (500 Pg C) and soils (2300
Pg C) (Jobbágy and Jackson, 2000; Houghton, 2007). It has recently been estimated that
2.9 Pg C yr
–1
are leached from land to inland waters, with 0.9 Pg C being consequently
exported to the ocean (Tranvik et al., 2009). In the oceans the majority of the organic carbon (663 Pg C) is found as DOM (Hansell et al., 2009). Ocean productivity results in a net
accumulation of 2 Pg C yr
–1
as semilabile DOM (Hansell & Carlson, 1998) that is gradually remineralized during the mixing time of the oceans, whereas there is thought to be a
more or less constant background of oceanic refractory DOM (Hansell et al., 2009). These
carbon reservoir sizes and fluxes are considerable and an essential link in the global carbon
cycle that controls climate. To put these numbers into context, the reservoir of carbon in
the atmosphere as CO 2 is approximately 800 Pg (Houghton, 2007) and fossil fuel emissions
are currently estimated at 7.2 Pg C yr
–1
(Canadell et al., 2007). It is therefore important that
we obtain a better understanding of the production, turnover, and fate of DOM in aquatic
environments, if we are to attempt to understand its role in the global carbon cycle and
the potential feedbacks to and change as a result of climate change (Jiao et al., 2010). To
accomplish this, a battery of chemical techniques is required to quantify and characterize DOM and follow how its two major sinks, microbial and photochemical degradation,
modify and ultimately remineralize organic carbon.
A fraction of the organic compounds present in dissolved organic matter absorbs light
and a subfraction of these also fluoresce (Figure 8.1). For several decades, the optical
(absorption and fluorescence) properties of DOM have been used to study both the distribution of DOM and its characteristics in aquatic environments. The major advantage of
the approach is that it requires very small sample volumes and minimal preparation before
8
Biological Origins and Fate of Fluorescent Dissolved
Organic Matter in Aquatic Environments
Colin A. Stedmon and Rose M. Cory
8.1 Introduction
All natural waters on Earth contain dissolved organic matter (DOM). The ubiquitous presence of organic life in all environments has resulted in its widespread distribution, from
the depths of the oceans, to trapped within polar ice sheets, to aerosol droplets in the upper
atmosphere. Despite the ability of microbes to evolve to degrade all types of complex
structures, an organic residue always remains, owing to it being either energetically unfavorable, at too low a concentration, or preserved as a result of some other external limiting
factor such as auxiliary nutrients.
Large pools of organic carbon exist on land as living biomass (500 Pg C) and soils (2300
Pg C) (Jobbágy and Jackson, 2000; Houghton, 2007). It has recently been estimated that
2.9 Pg C yr
–1
are leached from land to inland waters, with 0.9 Pg C being consequently
exported to the ocean (Tranvik et al., 2009). In the oceans the majority of the organic carbon (663 Pg C) is found as DOM (Hansell et al., 2009). Ocean productivity results in a net
accumulation of 2 Pg C yr
–1
as semilabile DOM (Hansell & Carlson, 1998) that is gradually remineralized during the mixing time of the oceans, whereas there is thought to be a
more or less constant background of oceanic refractory DOM (Hansell et al., 2009). These
carbon reservoir sizes and fluxes are considerable and an essential link in the global carbon
cycle that controls climate. To put these numbers into context, the reservoir of carbon in
the atmosphere as CO 2 is approximately 800 Pg (Houghton, 2007) and fossil fuel emissions
are currently estimated at 7.2 Pg C yr
–1
(Canadell et al., 2007). It is therefore important that
we obtain a better understanding of the production, turnover, and fate of DOM in aquatic
environments, if we are to attempt to understand its role in the global carbon cycle and
the potential feedbacks to and change as a result of climate change (Jiao et al., 2010). To
accomplish this, a battery of chemical techniques is required to quantify and characterize DOM and follow how its two major sinks, microbial and photochemical degradation,
modify and ultimately remineralize organic carbon.
A fraction of the organic compounds present in dissolved organic matter absorbs light
and a subfraction of these also fluoresce (Figure 8.1). For several decades, the optical
(absorption and fluorescence) properties of DOM have been used to study both the distribution of DOM and its characteristics in aquatic environments. The major advantage of
the approach is that it requires very small sample volumes and minimal preparation before
8
Biological Origins and Fate of Fluorescent Dissolved
Organic Matter in Aquatic Environments
Colin A. Stedmon and Rose M. Cory