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Saudi Arabia: Marine Biology

Fish Migrations Keep the Global Carbon Cycle Running

| Editor: Alexander Stark

This echogram section displays the acoustic data showing the vertical and temporal distribution of fish within a 24-hour cycle.
This echogram section displays the acoustic data showing the vertical and temporal distribution of fish within a 24-hour cycle. (Source: Kaust)

The biological carbon pump is a cyclical process by which inorganic carbon from the atmosphere is fixed by marine lifeforms and transported through ocean layers into the deepest waters and ocean sediments. Fish that feed at the surface at night and retreat to the mesopelagic zone by day were thought to influence carbon cycling, but the extent of their contribution has never been explored.

Thuwal/Saudi Arabia — The first in-depth analyses of dissolved organic carbon (DOC) cycling in the Red Sea highlights the important role of migrating shoals of fish in sustaining deep-ocean microorganisms and potentially the global carbon cycle.

Maria Calleja and Xosé Anxelu Morán at Kaust’s Red Sea Research Center, and coworkers, demonstrate the impact of this daily migration on the vertical movement of carbon in the Red Sea and how it fuels the metabolism of single-celled heterotrophic prokaryotes belonging to the domains Bacteria and Archaea.

In a previous study, the co-authors Anders Røstad and Stein Kaartvedt discovered a community of fish in the Red Sea that migrate every night from around 550 m depth to the surface waters to feed. The scientists wondered how this fish migration might affect the microbial community inhabiting the same depths. The two projects sought to clarify this by collecting data from a single Red Sea sampling site.

The first study examined vertical differences in DOC concentration and the flow of carbon through microbial communities at three specific layers in the water column during the day. Over eight days, the team monitored features such as DOC consumption, prokaryote growth and community composition in natural water samples taken from the surface, the deep layer where the fish rested during the day, and an intermediate layer at 275 m .

Bacteria That are More Active in Deeper Waters

Bacterial growth efficiency in the deepest layer was significantly higher than previously estimated, “suggesting a labile DOC source — one that is tasty and easily broken down by the bacteria — that helps generate larger cells,” explains Calleja. Heterotrophic bacterial communities in the mesopelagic layer were also found to be more active than those in the surface waters.

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The second paper, led by a former Kaust post-doc who is now at the University of Exeter, Francisca García, followed changes over 24 hours along the whole water column, sampling 12 different depths (from 5 to 700 m) every two hours. The team analyzed the dynamics between DOC, bacteria and fish movements during the 24-hour cycle.

The researchers used flow cytometry to analyze microbe cell sizes and community structure at high temporal resolution, showing higher microbial diversity in the mesopelagic zone than expected. These deep microbial communities may be more dynamic than previously thought, thanks to this active carbon transfer of labile DOC by fish.

If this is happening in the Red Sea, could it be happening in other marine basins and the open ocean? It may have unprecedented implications for the global ocean carbon cycle.

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