Dry micrometer-thick crystalline photosystem I (PSI) has been shown to generate unprecedented large photovoltage under illumination. We use variable temperature Kelvin probe force microscopy to show that deep acceptor centers are responsbile for this anomalous photovoltage. We assumed that these centers are located close to the positively charged F-B(2+) clusters, forming a coupled center that effectively captures the photoexcited electron into a deep state. We extract the main inherent parameters of the deep centers, which are extremely important in the potential use of photosynthetic proteins in various optoelectronic devices.