Study the following and select the appropriate option.

I) Dark reaction occurs only in the absence of light.

II) Light reaction occurs in the presence of light.

III) Dark reaction occurs even in absence of light when assimilatory power is supplied.

IV) In all photosynthetic organisms water acts as the hydrogen donor.

The term "dark reaction" is often used interchangeably with the term "light-independent reaction" or "Calvin cycle" in photosynthesis. These reactions do not occur only in the absence of light; rather, they can occur in the presence of light as well.

In oxygenic photosynthesis water is indeed used as the electron and proton donor. However, there are some types of photosynthetic bacteria that carry out anoxygenic photosynthesis, which does not produce oxygen as a byproduct. These bacteria use different electron donors other than water. Some of these bacteria use compounds like hydrogen sulfide (H₂S), sulphur, or even organic molecules as their electron donors in the photosynthetic process.