Humans will always have oxygen to breathe, but we can’t say the same for ocean life

There is nothing more fundamental to humans than the availability of oxygen. We give little thought to the oxygen we need, we just breathe, but where does it come from?

To shed light on this, statements such as “the ocean provides 50% of the oxygen we breathe”, or its equivalent, “every second breath we breathe comes from the ocean”, have become common mantras to highlight human dependence on the ocean and the risk of lower oxygen supply due to climate change and environmental degradation.

These mantras are repeated by high-profile politicians, including US climate envoy John Kerry and French president Emmanuel Macron, international organisations such as Unesco and the European Commission, and even prominent reports from the IPCC and other reputable scientific institutions.

While they may be good fodder for speeches, these claims misrepresent where the oxygen we breathe actually comes from, and in doing so, mislead the public as to why we should step up our role as ocean custodians.

Where do we get our oxygen?

The Earth’s atmosphere has not always been as rich in oxygen as it is today. The atmosphere is now made up of 21% oxygen, but it accounted for just 0.001% of today’s levels during the first 2 billion years of Earth’s history.

It is the advent of microscopic ocean bacteria and plants (phytoplankton) and, later, larger plants on land which caused the staggering increase of oxygen in our atmosphere. This oxygen is derived from photosynthesis – the process by which plants turn carbon dioxide and water into organic matter and oxygen.

Oxygen has been relatively stable at a high level for the past 500 million years. Today, roughly half of photosynthesis takes place in the ocean and half on land.

So yes, the ocean is responsible for about 50% of the oxygen produced on the planet. But it’s not responsible for 50% of the air we humans breathe. Most of the oxygen produced by the ocean is directly consumed by the microbes and animals that live there, or as plant and animal products fall to the seafloor. In fact, the net production of oxygen in the ocean is close to 0.

Figure legend. Oxygen budget for the period 1960-2014 (redrawn from Grégoire et al., 2019)
Oxygen produced by photosynthesis (i.e. net primary production) in the upper ocean is roughly consumed by respiration within the water column, except for a small excess production of 0.002 Pmol O2 per year which corresponds to burial in the ocean floor. Redrawn from Grégoire et al., 2019, Fourni par l’auteur

A tiny fraction of the primary production, roughly 0.1%, escapes degradation and is stored as organic carbon in marine sediments – a process referred to as the biological carbon pump. This organic carbon may eventually turn into fossil fuels such as coal, oil and gas. The tiny amount of oxygen which had been generated to produce this carbon store can later be released to the atmosphere. A similar process occurs on land too, with some carbon stored in soils.

Therefore, the oxygen we currently breathe comes from the slow accumulation of O2 in the atmosphere supported by the burial of organic matter over very long time-scales – hundreds of millions of years – and not from the contemporary production by either the land or ocean biosphere.

The Conversation, 12 August 2021. Full article.

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