Background and context
“50% of the oxygen we breathe comes from the ocean”
“Every second breath you take comes from the ocean”
Article written by Placido Benzi, IOC-UNESCO.
These mantras are constantly echoing throughout the main scientific and general communication channels on public television and social media, in the wake of what constitutes the largest literacy effort on the Global Ocean in the history of humankind: the United Nations Decade of Ocean Science for Sustainable Development. This global initiative represent an“awesome wave” already flooding an informational desert with a multidimensional approach towards the advancement of Ocean Literacy and Ocean Sciences all over the globe. On parallel, the public and the governments are increasingly partaking in a plethora of initiatives and activities, spending a considerable amount of time and cognitive effort on the diffusion and consolidation of Ocean science and knowledge. Despite the bonafide interest shown by the different stakeholders involved, we’ve observed the diffusion and circulation of many propositions related to the ocean which are, in fact, not correct. Rapid propagation of information through recursive steps of reporting and slight modification, such as in the case of tabloids reporting facts emanated by governments or agencies, could indeed generate unaccounted literacy mistakes. Such process is especially enforced by an “artificial selection” according to which an information considered beautiful or dramatic is collectively stabilized and fixed, notwithstanding its authenticity. In such instances, the false information assumes the connotation of “mantra”, and is perpetually circulated and eventually integrated into public knowledge. A similar course of action characterizes the history of a particular piece of micro-knowledge related to the oceanic Oxygen production. The aforementioned mantras are recursively propagated throughout social media and even important institutions – UN, IUCN- and as positive as they can be, they remain factually incorrect and can lead to false assumptions on the way the Ocean works and regulate life on Earth. There are only few instances of written commentaries addressing this urgent issue, namely from C. Duarte and J-P. Gattuso (1), and despite the clarity and brilliance of such interventions, propagation of incorrect sentences on this topic is still in force.
Addressing the myth
The established common narrative around these factoids sees the Ocean as an exporter of a considerable flux of Oxygen to the atmosphere, which is then readily consumed by terrestrial life forms. The counterpart to the Ocean, which under this lens produces around 50% of the Oxygen we breathe, is the Land compartment, which through plants generates the remaining quota of available Oxygen. While it is true that around 50% of the Oxygen currently produced on Earth comes from the Ocean, it is indeed false to state that this quota is actually available to humans or to any other terrestrial life form. What the aforementioned narrative is ignoring is the whole ocean metabolism and its net trophic status. Comparing the Ocean to a giant organism, we could assert that the Oxygen produced via primary production is also mostly consumed by the Ocean itself through respiration. Thus, the current net Oxygen production from the Ocean is close to zero. System thinking comes to the rescue in many other similar instances, for example in the case of the Amazon rainforest, which “reportedly” produces 20% of the Oxygen we breathe. If we again account for whole-ecosystem processes, we have to highlight that the Amazon rainforest ecosystem also consumes most of the Oxygen it produces, mainly via respiration and oxidation of organic compounds by plants, fungi, animals and microbes. Thus, the contribution in terms of Oxygen we breathe is again close to zero. The natural question arising from these considerations is thus: “if the Oxygen we breathe doesn’t currently come from the Ocean, where did it come from?”. Of the whole organic carbon produced in the Ocean, only a minuscule fraction escapes the carbon cycle remineralization and is thus sequestered over relevant time scales. Although being very small, this fraction has been historically vital, considering its role in the accumulation of oxygen in the atmosphere over geological timescales. The burial of organic carbon and its preservation over long time scales prevents its oxidation, meaning that the oxygen generated during the same photosynthetic process which also generated organic carbon, is actually spared. Therefore, we can imagine an invisible link between carbon burial in the Ocean and its related Oxygen sparing effect. Thus, the low and slow carbon burial process in the Ocean through time led to a gradual buildup of Oxygen in the atmosphere, starting from values less than 0.001% of the present atmospheric level -21%- during the first half of the history of this planet (2).
Summary:
1) Approximately half of the Oxygen currently produced on Earth is generated by the Ocean.
2) The Oxygen currently produced by the Ocean is mostly consumed by the Ocean itself.
3) Currently, the Ocean doesn’t produce any significant fraction of the Oxygen we breathe.
4) Burial and preservation of organic carbon in the Ocean and in part on Land over geological timescales is responsible for the current reservoir of atmospheric Oxygen and the related quota respired by humans.
