Chloroplasts contain the green pigment chlorophyll which is needed for photosynthesis. Plants require magnesium to synthesize chlorophyll, essential for photosynthesis. In plants, magnesium is necessary for synthesis of chlorophyll and photosynthesis. The photosynthetic system of brown algae is made of a P700 complex containing chlorophyll a. These spores tend to affect photosynthesis and reduce chlorophyll. The antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins. Carotenes contribute to photosynthesis by transmitting the light energy they absorb to chlorophyll. Plants absorb light primarily using the pigment chlorophyll. On the thylakoid membranes are photosynthetic pigments, including chlorophyll "a". There are steep gradients in nutrient concentrations, chlorophyll concentrations and primary productivity in the Mediterranean. The rings of porphyrin are components of hemoglobin, myoglobin, vitamin B12, chlorophyll, and cytochromes. Plants that lack chlorophyll cannot photosynthesize. Ratio fluorescence emission can be used to measure chlorophyll content. Such chelating agents include the porphyrin rings in hemoglobin and chlorophyll. Chloroplasts and cyanobacteria contain the blue-green pigment chlorophyll "a".