The ocean assimilates a large amount of atmospheric CO2 and is potentially a buffer for climate change. A fraction of the assimilated CO2 is incorporated into algal biomass and further converted into refractory dissolved organic matter (DOM). Carbon bound in refractory DOM has the potential to be stored in the ocean’s interior for millennia and thereby avoiding being released as atmospheric CO2. In order to understand the role of refractory DOM in ocean carbon sequestration, its formation and removal mechanisms must be investigated. In a synopsis and four scientific papers, this PhD project studies the prokaryotic production and degradation of oceanic refractory DOM and discusses the reasons for the persistent nature of this large DOM fraction.
The first two papers investigate the microbial carbon pump, i.e. prokaryotic transfor-mation of organic carbon into refractory DOM. The results show that prokaryotes can produce refractory compounds from simple substrates such as glucose. Microbial deg-radation of glucose itself only produces small amounts of refractory DOM. However, degradation of prokaryote cell remains (grown from the glucose) produces a considera-ble amount of refractory DOM. In addition, the results show that microbial production of refractory DOM depends on the lability of the substrate. Although both labile and semi-labile DOM can act as a substrate for microbial production of refractory DOM, a larger fraction of the semi-labile substrate is converted to a refractory form. The results of the second paper indicate that the microbial carbon pump also applies for biomole-cules, since certain neutral sugars and amino acids produced by prokaryotes are able to resist long-term degradation. In addition, there is a striking similarity between the com-position of old biomolecules found in the ocean and biomolecules produced by prokar-yotes in our incubations, suggesting that prokaryotes are a significant source of refracto-ry biomolecules in the ocean.
The third paper investigates the removal mechanisms of refractory DOM in the Arctic Ocean, and the fourth paper discusses potential explanations for variations in DOM bio-availability. The results of the third paper show a preferential retention of certain refrac-tory DOM fractions in sea ice during initial ice formation. Other fractions follow a con-servative incorporation into sea ice and subsequent drainage with brine. In addition, the results show that DOM expelled from sea ice is more bioavailable than bulk DOM, sug-gesting that physico-chemical processes related to sea ice formation increase the lability and subsequent microbial degradation of DOM. The calculations in the fourth paper reveal that encounters between prokaryote cells and individual DOM compounds in the ocean are rare—possibly too rare to sustain viable uptake and assimilation. Hence, the dilute concentration of individual compounds is a possible explanation for the apparent refractory nature of most DOM in the ocean.
Understanding the mechanisms that control the quality and quantity of refractory DOM in the ocean is essential to our understanding of the global carbon cycle. Although our knowledge of refractory DOM continually grows, there are still many mysteries left to be solved.