Abstract Fiber-optic sensors for dissolved CO 2 are an emergent technology for monitoring marine geologic CO 2 sequestration sites. Fiber- optic sensing technology has been used successfully in the oil and gas sector and is advantageous because it is capable of cost- effective, instantaneous, distributed sensing. This is an improvement over current practice, which normally requires samples to be pumped to the surface for analysis. Biofouling of fiber-based sensors is a concern for the marine environment, as the biofouling can cause signal drift and also adversely impact the sensor's ability to detect dissolved CO 2 . Single mode optical fibers with long period gratings etched onto the core of the fiber were used for this study. Pseudoalteromonas sp. NCIMB 2021 was cultured and grown on sensing elements using nutrient-dense synthetic seawater. Biofouling was shown to cause shifts in the baseline signal. Post-fouling sensitivity of the sensor was also reduced relative to pre-fouling levels. Mechanical cleaning of the sensors restored sensor sensitivity to that seen prior to bacterial colonization.