In this paper, the complexities involved in both microbiologically influenced corrosion and deterioration of reinforced concrete structures by algae are explained. In this regards, the five possible corrosion/deterioration mechanisms that may be expected are addressed and described. These mechanisms are as follows: Absorption of some chemicals necessary for the algae from within the cement paste of the concrete: this mechanism can finally result in drying out the concrete and developing cracks as a result of formation of internal cavities and voids. Biofilm formation and increasing the likelihood of attracting more micro-/macroorganisms that can either deteriorate the concrete itself or the reinforcement steel inside, or both. An example as such can be development of an environment favorable for the acid-producing sulphur oxidizing bacteria that through generating very acidic conditions are capable of doing harm to both metallic and non-metallic phases of the reinforced concrete. Development of electrochemical cells such as differential aeration cells due to the photosynthetic driven oxygen production and consumption features of algae. In this way, it is likely that spots of varying oxygen partial pressures will be created, tus facilitating the corrosion of the steel inside. Production of acids that can be detrimental to the mechanical integrity of both concrete and steel reinforcement. Production of alkaline conditions that upon varying and fluctuating due to the chemistry of the system, can protect the concrete but do damage to the steel phase via mechanisms such as caustic embrittlement. The aim of this review paper is to gather all possible mechanisms that may be involved in explaining the contribution of algae to the bio-corrosion/bio-deterioration of reinforced concrete
