Plant present at various depth in ocean are mostly algae. These show great variations in its photosynthetic pigments. These can absorb different wave lengths of light and performs photosynthesis.

Green algae-chlorophyll-a, (absorbs red) and b(absorbs blue violet).

Brown algae-chlorophyll-a, c and fucoxanthin (absorbrs yollow).

Rhodophyceae-chlorophyll-a, $d$ and phycoerythrin.

The plants carry out photosynthesis in presence of light. The quality and intensity of light affect only the rate of photosynthesis. So, plants growing in different canopy will carry out photosynthesis, but rate could be different depending upon the intensity and quantity of light received.

RuBisCo is an enzyme which has dual nature. It acts as carboxylase, when $\mathrm{CO}_2$ concentration is good enough in atmosphere. But if $\mathrm{O}_2$ concentration increases, its nature changes and it binds with $\mathrm{O}_2$ and acts as oxygenase enzyme, which forces $\mathrm{CO}_2$ to enter $\mathrm{C}_2$ cycle thus leading to photorespiration and loss of $\mathrm{CO}_2$.

Photosynthesis is an enzyme specific process. All enzymes act at an otptimum temperature (i.e., $25-35^{\circ} \mathrm{C}$ ). If temperature increases, enzyme get denature, thus leading to fall in the rate of photosynthesis.

In light reaction plants trap solar radiation by photosynthetic pigments which convert light energy into chemical energy. Main event of light reaction is photophosphorylation, i.e., formation of ATP from ADP + Pi by using energy of excited electron movement through electron transport chain, present in thylakoid membrane.

Chemiosmosis is the movement of ions across a selectively permeable membrane, down the electrochemical/ proton gradient.

Chemiosmosis hypothesis of ATP formation was first proposed by Mitchell (1961) according to which the enzyme ATP synthase generales ATP via a membrane, proton pump and proton gradient. ATP synthase allows ions $\mathrm{O}_2$ protons to pass through membrane and proton pump. This creales a high cocentration of protons ( $\mathrm{H}^{+}$) in the lumen and hence diffuses across the membrane to activale ATPase, releasing ATP molecules. One molecule of ATP is released for every two $\left(\mathrm{H}^{+}\right)$ions passing Through ATPase.