Kharaa bacterium

The Kharaa bacterium is an alien pathogen that drives the plots of both Subnautica and Below Zero. It infected the ecosystem of Planet 4546B and killed over 143 billion individuals across the Architects' interconnected civilization before they quarantined the planet.

Origins

The Kharaa bacterium was not native to Planet 4546B. The Architects encountered it on another world during their galactic exploration. They brought samples to 4546B for study, building research facilities across the planet to contain and analyze the pathogen. Their goal was to develop a cure. The bacterium had already devastated their core worlds, killing 143 billion individuals according to data approximately 1,000 years old.

The outbreak on 4546B

A containment breach at the Disease Research Facility in the Lost River released the Kharaa into 4546B's ocean. The cause: a Sea Dragon Leviathan attacked the facility to recover its stolen egg, ripping apart the anchor cables and breaching the containment systems. Al-An, an Architect scientist, had disobeyed orders by attempting to hatch Sea Dragon eggs, provoking the attack.

The bacterium spread rapidly through the planet's ecosystem. The Architects activated the Quarantine Enforcement Platform to prevent any organism from leaving the planet, then abandoned 4546B.

Infection stages

In the original Subnautica, Kharaa infection progresses through four visible stages:

• Stage 1 Initial infection. Symptoms are not visible. Slight green discoloration begins to appear on the player's skin.

• Stage 2 Green cyst-like growths appear on the player's hands (the only visible skin in first-person). Green veiny discoloration surrounds the growths.

• Stage 3 The cysts grow larger and glow more brightly. Veiny discoloration spreads further.

• Stage 4 Advanced infection. Large, prominent glowing green pustules cover the visible skin. The glow intensifies.

The infection becomes noticeable to the player after visiting the Disease Research Facility, when the Quarantine Enforcement Platform's scanner identifies Ryley as infected and denies access. Infected wildlife displays the same green cysts and veiny discoloration.

Enzyme 42 and the cure

The cure for Kharaa is Enzyme 42, a substance naturally produced by the Sea Emperor Leviathan. The Architects captured the Sea Emperor after discovering this property but could not synthesize a stable version from the deteriorating adult.

Ryley cures the infection by hatching the Sea Emperor's five eggs. The hatching enzyme requires five ingredients: Eye Stalk, Ghost Weed, Fungal Sample, and Bulb Bush Sample (gathered through the Primary Containment Facility's Alien Arch teleporters) plus a Sea Crown from the aquarium's trenches. The juvenile Sea Emperors produce stable Enzyme 42 and release it into the ocean, curing the entire planet.

In Below Zero

Two years later, Alterra Corporation discovers a Frozen Leviathan in the Glacial Basin still carrying the Kharaa infection. Rather than destroying it, Alterra researches the pathogen for potential weaponization. Sam Ayou sabotages the research and Robin Ayou later completes the job by injecting the Frozen Leviathan with a Kharaa antidote crafted from Frost Vase Plant and Fevered Pepper.

In Subnautica 2

Whether the Kharaa bacterium exists on Planet Zezura hasn't been confirmed. The Voices from Beyond audio drama describes havoc spreading across the Ariadne Arm, and the Steam store page mentions a bacterium spreading on Zezura, but no direct connection to Kharaa has been established.