The US military’s Naval Research Laboratory (NRL) is developing genetically engineered organisms that it hopes will be able to detect the presence of enemy submarines, ships, or divers, according to a new report published in Defense One.

The work is part of a $45 million initiative across all the branches of the US armed forces that is investigating ocean-based military applications for genetic engineering.

Motherboard has previously reported on the military’s plans to use larger aquatic animals to detect enemy ships, but now it appears that they are also looking to weaponize biology on the microscale.

As the Naval Research Laboratory told Motherboard in an email, researchers plan on using microorganisms commonly found in the ocean and genetically modify them so they react to various substances left by enemy vessels or equipment, such as fuel exhaust and trace amounts of metals.

So what would these organisms’ reactions look like? NRL researcher Sarah Glaven told Defense One that they could take the form of a chemical reaction in which microbes give up some of their electrons. The electrons would then be detected by a submarine drone, which could use them to determine what kind of enemy vessel the microbes had encountered.

“The reason we think we can accomplish this is because we have this vast database of info we’ve collected from growing these natural systems,” Glaven said.

The NRL told me that Glaven is currently focusing on marinobacter bacteria as a suitable candidate for this type of intelligence, but “other microbes might also be suitable.”

Genetically modifying bacteria and other microorganisms to express certain traits in the lab, effectively creating a biological sensor, is a well worn path by now. But replicating results in the field adds a number of difficulties. One of the main challenges is modifying the microorganisms to withstand the stressors of the environment.

“If you want to move a biological bio-based sensor to the field...you try to protect them,” Dimitra Stratis-Cullum, a biomaterials scientist at the Army Research Laboratory, told Defense One. “You try to encapsulate them...to basically increase their longevity in these harsh environments. It’s very difficult to do that with the organisms that have some bio tools in them now.”

Stratis-Cullum said right now NRL researchers are focusing on developing new types of coatings that can be applied to genetically modified microorganisms to increase their ruggedness in the field. Ideally, the coating would be applied with a 3D printer to allow soldiers to readily produce militarized microbes in the field.

For now, however, a spokesperson for the NRL told me in an email that the work is “largely at the basic research stage, and [is] thus confined to the laboratory.”