Isolated photosystems and sub-cellular photosynthetic fractions may be able to directly reduce the anode if the biological redox components are close enough to the electrode for electron transfer to occur.  In contrast to organisms such as dissimilatory metal reducing bacteria, algae and cyanobacteria are poorly adapted for extracellular electron export - no molecular mechanisms enabling direct reduction of an insoluble extracellular electron acceptor have been conclusively identified.  Nevertheless, a low rate of anode reduction has been observed from whole photosynthetic organisms without the addition of exogenous redox-active compounds. It has been speculated that electron transfer occurs through the release of low concentrations of endogenous redox mediator compounds.  Improving the electron export activity of cyanobacteria for use in biological photovoltaic systems is a topic of current research.