section_path: Chemical properties/Oxygen, photosynthesis and carbon cycling
section_name: Oxygen, photosynthesis and carbon cycling
paragraph_text: Oxygen, photosynthesis and carbon cycling
  
  Photosynthesis in the surface ocean releases oxygen and consumes carbon dioxide. Phytoplankton, a type of microscopic free-floating algae, controls this process. After the plants have grown, oxygen is consumed and carbon dioxide released, as a result of bacterial decomposition of the organic matter created by photosynthesis in the ocean. The sinking and bacterial decomposition of some organic matter in deep ocean water, at depths where the waters are out of contact with the atmosphere, leads to a reduction in oxygen concentrations and increase in carbon dioxide, carbonate and bicarbonate. This cycling of carbon dioxide in oceans is an important part of the global carbon cycle.
  
  The oceans represent a major carbon sink for carbon dioxide taken up from the atmosphere by photosynthesis and by dissolution (see also carbon sequestration). There is also increased attention on carbon dioxide uptake in coastal marine habitats such as mangroves and saltmarshes. This process is often referred to as " Blue carbon ". The focus is on these ecosystems because they are strong carbon sinks as well as ecologically important habitats under threat from human activities and environmental degradation.
  
  As deep ocean water circulates throughout the globe, it contains gradually less oxygen and gradually more carbon dioxide with more time away from the air at the surface. This gradual decrease in oxygen concentration happens as sinking organic matter continuously gets decomposed during the time the water is out of contact with the atmosphere. Most of the deep waters of the ocean still contain relatively high concentrations of oxygen sufficient for most animals to survive. However, some ocean areas have very low oxygen due to long periods of isolation of the water from the atmosphere. These oxygen deficient areas, called oxygen minimum zones or hypoxic waters, will generally be made worse by the effects of climate change on oceans.