![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Ocean Dissolved
Organic Matter The biogeochemical behaviour of the large amounts
of organic carbon matter that are stored in the ocean is elusive.
Size-age-composition relations now quantify the production of very small
organic molecules as a major pathway for carbon sequestration. It has been found that the oceans are a significant
sink for carbon. An important part of this sink is biological activity,
which converts CO2 to biomass, which is then degraded into
organic particles and molecules. It is indicated by the size and age
distributions of organic carbon fractions that in excess of 95 % of it
is in the form of dissolved molecules, and that most of these are small
and relatively old. There is, however, little understanding of the
mechanisms that generate this unique carbon pool, its production rate,
or the mechanism by which it is ultimately remineralised to inorganic
forms. Walker et al. (Walker
et al., 2016) has used relationships between the size of organic matter,
14C age and elemental composition (Amon & Benner, 1996;
Benner & Amon, 2014) to estimate that small refractory molecules are
produced by microbes from larger molecules at a rate of 0.24 PgC yr-1,
which is a carbon sequestration pathway of similar magnitude to that of
the burial or organic carbon in the sediments. In the surface waters of the ocean biological
processes modulate the rate of CO2 exchange between the
atmosphere and the ocean, and therefore influence global climate. In the
surface waters of the ocean organic carbon can be remineralised to CO2
and emitted into the atmosphere, or exported to depths at which
biological activity is limited, which effectively sequester it on
timescales of global ocean convection. A spectrum of biological and physical processes
affects organic molecules during their residence in the ocean. The
size-reactivity-continuum model summarised the combined effect of these
processes (Amon & Benner, 1996; Benner & Amon, 2014). According to this
model fresh organic matter is produced and exists in relatively large
particles. This freshly produced organic matter becomes progressively
smaller and less labile as it ages and degrades, which ultimately leads
to the accumulation of smaller, older and more refractory molecules in
the interior of the ocean. This conceptual model has mostly been
supported by observational and experimental studies, which provide
evidence for the rapid conversion of fresh organic matter to molecules
that are more refractory, and that cannot be identified by
molecular-level characterisation (Kaiser & Benner, 2009). Support for
the idea that it is microbial transformation that is responsible for
rendering organic matter progressively more recalcitrant has been lent
by these studies (Jiao et al., 2010). Some insight into the size-reactivity relationship
may be provided by the 14C age of organic carbon fractions
occurring in intriguing patterns. The modern age of dissolved inorganic
carbon is reflected in organic particles in the surface waters of the
ocean, which suggests that the particles were formed recently. It is
indicated that a significant fraction of molecules, possibly those that
are more recalcitrant, must persist through several cycles of global
ocean mixing, as dissolved organic carbon in deep water has an average
of between 4,000 and 6,000 years depending on the location (Williams &
Druffel, 1987). Evidence for a widespread size-age-composition
relationship is provided by Walker et
al. (Walker et al., 2016).
They were also able to quantify the production of recalcitrant dissolved
organic matter (DOM), by refining earlier estimates that had been
derived from environmental gradients in dissolved organic carbon
(Hansell et al., 2009). In their study they analysed the C:N ratio and
14C age or organic matter derived from the coastal, surface
and deep waters of the Pacific Ocean. Larger particles in their samples
tended to be young and enriched in nitrogen, while smaller molecules
tended to be older and depleted in nitrogen, which suggests that the
chemical recalcitrance of DOM is responsible for its persistence in the
ocean. This pattern is consistent with previous work from the Atlantic
and Southern oceans, which suggests this size-age-composition
relationship holds widely (Lechtenfeld et al., 2014). There are alternative explanations for the
persistence of marine DOM in the ocean. According to one of these
hypotheses as microbes tend to be nutrient limited in ocean surface
environments they are not able to utilise all DOM that is available as a
substrate for growth, which therefore enables DOM to persist in the
ocean on a seasonal timescale. This hypothesis does not apply directly
to the interior of the ocean, where there are plentiful nutrients,
though it can explain the delayed utilisation of DOM rich in
carbohydrate in the surface waters of the ocean under nutrient
limitation; instead, the DOM could be transferred to the interior of the
ocean during convection (Hansell et al., 2009). It is suggested by another hypothesis that organic
molecules in the ocean are so diluted that it is not likely they will
encounter a microbe that has the ability to consume them, and this
allows them to persist for long periods (Dittmar, 2015). It is suggested
instead by the relationship that has been observed between radiocarbon
age and the chemical composition of DOM (Walker et al., 2016,
Lechtenfeld et al., 2014) that chemical
composition plays an intrinsic role in determining the persistence of
DOM. The idea that chemical recalcitrance is responsible for DOM
persisting (Jiao et al., 2010; Dittmar, 2015) is most consistent with
the evidence that has been found by Walker et
al. for a strong relationship
between organic carbon age and chemical composition. Though the
possibility is not excluded by the size-age-composition continuum that a
portion of the DOM in the deep ocean is labile, which suggests that
other processes could contribute to the persistence of DOM Hansell et
al., 2009; Dittmar, 2015), The evidence for a strong size-age-composition
relationship in marine organic matter that has been provided by Walker
et al. (Walker et al., 2016)
supports the conclusion that the production of refractory organic
molecules by microbes makes a significant contribution to the
sequestration of carbon in the ocean. Whether the size of this large
carbon pool will change in a warmer ocean that is more stratified in the
future will determine if the refractory organic carbon reservoir will be
an important carbon sink or source in the future.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |