The bathypelagic zone of the ocean, which spans depths between 1,000 and 4,000 meters (3,300 – 13,100 feet) below the ocean’s surface, is characterized by permanent darkness, low temperatures, and high pressure. In this hostile environment, slow-growing bacteria survive by relying on sinking organic matter, including proteins and carbohydrates called polysaccharides, from algae in the sun-lit surface waters of the ocean. 

Much of this organic matter is heavily degraded by the time it reaches the deep sea, and intact polysaccharides are hard to come by. For bacteria living in the bathypelagic zone, survival means getting the most out of every rare polysaccharide that reaches these depths – and a new study suggests that for some bacteria, selfishness may be key to their survival.

Bacteria typically feed by releasing enzymes into the water to break down their food into small enough pieces to be taken into the cell. However, by releasing these enzymes into the surrounding water, bacteria naturally lose some of the products of this process. While breaking down food externally can be profitable when resources are in high abundance, it is a much less successful strategy in environments where resource availability is low, such as the deep sea.

In a recent pre-print that I am a co-author on, we suggest that some bacteria at these depths may be using a selfish method of polysaccharide uptake. This method allows them to bring large pieces of polysaccharides into their cells without first breaking them down externally, enabling the bacteria to selfishly keep all the food to themselves.

To make this discovery, we incubated bathypelagic bacteria with fluorescently-labeled polysaccharides. By staining the bacteria with a DNA-binding dye and viewing them under microscopes, we were able to see intact pieces of polysaccharides inside the cells, indicating that these bacteria had not used external enzymes to break them down prior to uptake.

These results provide the first example of selfish behavior in deep-sea bacteria, suggesting that selfishness may be more common among bacteria than previously thought.