The world’s oceans are losing oxygen – and rapidly. The principal cause is the increasing warming of the oceans, which reduces the solubility of oxygen in water and increases respiratory activity. In addition, it strengthens stratification of the water column, making it harder for deep and surface water to mix.

“Within just 50 years – from 1960 to 2010 – the global oxygen content of the oceans fell by two percent and the volume of anoxic waters increased fourfold,” says biogeochemist Dr. Gonzalo Gomez Saez from the Department of Earth and Environmental Sciences at LMU. “This massive decrease in oxygen is changing elemental cycles, on which marine ecosystems are based.”

Less oxygen, more sulfur

An important aspect is the metabolization of sulfur compounds by the microorganisms living in the water, as Gomez Saez explains: “In waters with low oxygen concentration, organic sulfur compounds play a particularly important role, as they tend to accumulate in these waters. To date, however, we have no clear explanation for this phenomenon.” A team led by Gomez Saez published a study that appeared recently in The ISME Journal, which takes a closer look at what happens with sulfur-containing compounds in oxygen-deficient waters.

The main focus of the investigation was on a molecule that ordinary people may know as part of red-meat diet and as an additive in energy drinks: taurine. This bioavailable organic sulfur compound also plays an important role in nature, as it is utilized by ocean microbes for nutrient exchange, energy production, and growth.

“Many ocean microbes have the genetic potential of consuming taurine, but it is unclear how this activity is linked to the worldwide decline in oxygen levels” notes Gomez Saez. “We aimed, therefore, to understand how different concentrations of dissolved oxygen in the water influence the microbial processing of taurine.”

Samples from Mariager Fjord

To this end, the research team – which had members from Aarhus (Denmark), Munich, Oldenburg, and Bremen – investigated water samples from the Danish Mariager Fjord, which loses huge amounts of oxygen in summer. The inlet is connected to the Kattegat strait by means of a flat sill. This limits how much deep water it mixes with and favors the formation of oxygen-deficient conditions.

These conditions intensify in summer when warmer weather conditions lead to temporary stratification and higher respiratory activity by marine organisms. As such, Mariager Fjord serves as a valuable natural laboratory for investigating how biogeochemical processes react to changing oxygen levels.

Immediately after taking water samples from the fjord, Dr. Ömer Coskun, a former LMU researcher and first author of the article, incubated oxygen-deficient water and fully oxygen-enriched water with substrates to which stable isotopes had been added – they ‘fed,’ as it were, the microorganisms contained in the water with labeled nutrients. “In this way, we were able to follow which microbes incorporated carbon from the various compounds under different oxygen concentrations in the fjord,” says Gomez Saez.

The future of marine cycles

The main result: taurine was assimilated only in the low-oxygen (hypoxic) conditions of the fjord waters. By using DNA sequencing, the team was also able to determine which microorganisms are responsible for taurine metabolization in Mariager Fjord: mainly Flavobacteria from the phylum Bacteroidota. “As oxygen-deficient areas are spreading worldwide and compounds like taurine are becoming increasingly common in the oceans as a result, microorganisms that utilize such compounds will probably play a larger role in marine carbon and sulfur cycles in the future,” concludes Gomez Saez.