Swirls of photosynthetic algae thrive in the Arabian Sea in February 2015.
Swirls of photosynthetic algae (green) thrive in the Arabian Sea in February 2015. Algae blooms like these create low-oxygen environments around the world. New research could improve predictions of how much these “dead zones” will grow in the future. Credit: Norman Kuring, NASA’s Ocean Color web
Source: Global Biogeochemical Cycles

Certain parts of Earth’s oceans are so oxygen depleted that they can hardly sustain life. Climate models predict that these “dead zones” will expand as global warming progresses, affecting ecosystems, fisheries, and the climate itself. Now Lengger et al. provide new evidence that such predictions do not adequately account for the activity of anaerobic microbes that consume inorganic carbon within dead zones.

Dead zones form where photosynthetic algae rapidly flourish in surface waters. As vast numbers of algae die and sink through the water column, aerobic microbes break them down, consuming nearly all available oxygen in the process. With so little oxygen left in deeper waters, microbes are unable to completely decompose much of the sinking organic matter before it settles on the seafloor.

The amount of organic matter in dead zone sediments can inform predictions of climate models, which usually assume that all this matter initially came from algae. However, in recent years, evidence has emerged that some of the organic matter in these sediments is instead produced by anaerobic microbes that eat dissolved inorganic carbon dioxide in oxygen-depleted waters.

To better understand this process, the authors studied microbes in the Arabian Sea, home to the largest dead zone in the world. They used the R/V Pelagia to collect sediment cores in the dead zone and conducted an isotopic analysis to investigate the origins of the organic matter in the cores.

The analysis revealed that anaerobic microbes could be responsible for one fifth of the organic matter found in seafloor sediments of the Arabian Sea dead zone. Climate models that do not account for the influence of such microbes may underestimate the extent to which dead zones will expand as global temperatures rise.

In this investigation, the researchers developed a new strategy for evaluating anaerobic consumption of inorganic carbon in deep waters. The method, which relies on detecting a distinct chemical signature of the microbes known as the “bacteriohopanetetrol stereoisomer,” could aid future investigations of dead zones around the world, they noted. (Global Biogeochemical Cycles, https://doi.org/10.1029/2019GB006282, 2019)

—Sarah Stanley, Science Writer


Stanley, S. (2020), Anaerobic activity is a big contributor in marine “dead zones”, Eos, 101, https://doi.org/10.1029/2020EO138726. Published on 17 January 2020.

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