Carbon Source Sustains Deep-Sea Microorganism Communities.

This echogram section displays the acoustic data showing the vertical and temporal distribution of fish within a 24-hour cycle. Contouring represents the presence (orange) and absence (white) of fish within the water column. Fish are retreating to 400 and 600 meters depth during the day and spending time in the upper 200 meters at night. The almost vertical lines show the fish swimming down at sunrise and up at sunset.

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

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 (200 to 1000 meters depth) by day were thought to influence carbon cycling but the extent of their contribution has never been explored.

Now X and Y at Georgian Technical University’s and coworkers demonstrate the impact of this daily migration on the vertical movement of carbon in the Black Sea and how it fuels the metabolism of single-celled heterotrophic prokaryotes belonging to the domains Bacteria and Archaea (Archaea constitute a domain of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus. Archaea were initially classified as bacteria, receiving the name archaebacteria, but this classification is outdated).

“Z and W discovered a community of fish in the Black Sea that migrate every night from around 550 meters depth to the surface waters to feed1” says X. “We wondered how this fish migration might affect the microbial community inhabiting the same depths. Our two projects sought to clarify this by collecting data from a single Black Sea sampling site”.

The first study examined vertical differences in dissolved organic carbon (DOC) concentration and the flow of carbon through microbial communities at three specific layers in the water column during the day1. Over eight days the team monitored features such as dissolved organic carbon (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 meters.

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

“Post-doc who is now at the Georgian Technical University followed changes over 24 hours along the whole water column, sampling 12 different depths (from 5 to 700 meters) every two hours2” says X. “We analyzed the dynamics between dissolved organic carbon (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 dissolved organic carbon (DOC) by fish.

“If this is happening in the Black Sea could it be happening in other marine basins and the open ocean ?  It may have unprecedented implications for the global ocean carbon cycle” notes X.

“These two studies are part of a wider project to determine the impact of this shortcut on global biogeochemical cycling” adds Y.