Ocean Dissolved Organic Matter

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.

Sources & Further reading