Ultraviolet radiation, particularly UV-B, reduces photosynthesis in phytoplankton and damages DNA. This can lower aquatic productivity and cause mutations, cancer or cell death in exposed organisms.
The Mechanism
This is part of IB ESS subtopic 6.4 Stratospheric ozone at both SL and HL. Stratospheric ozone normally absorbs much of the Sun's harmful UV radiation, limiting the amount reaching Earth's surface.
When ozone-depleting substances reduce stratospheric ozone concentrations, more UV-B reaches surface ecosystems. Its effects include:
| Target | Mechanism and harm |
|---|---|
| Phytoplankton | UV-B damages photosynthetic pigments, proteins and cellular DNA. This reduces photosynthesis, growth and reproduction, lowering primary productivity. |
| DNA | DNA absorbs UV energy, which can create abnormal bonds between adjacent bases, such as thymine dimers. These distort the DNA molecule and interfere with replication and transcription. |
Cells may repair this DNA damage. However, if repair fails, the cell may die or reproduce with a mutation. In multicellular organisms, accumulated mutations can cause cancers; UV exposure also contributes to skin damage and cataracts in humans.
Damage to phytoplankton has system-wide consequences because they are producers at the base of many aquatic food webs. Reduced phytoplankton productivity means less energy and biomass are available to consumers, while reduced carbon uptake may weaken the ocean's role as a carbon sink.
The Montreal Protocol reduced the production, trade and use of many ozone-depleting substances, helping prevent the stratospheric ozone planetary boundary from being crossed. However, vulnerability remains unequal because access to monitoring, healthcare and UV protection differs between societies.
A common misconception is that ozone depletion causes climate change. They are distinct environmental issues: ozone depletion mainly increases surface UV exposure, whereas climate change results primarily from the enhanced greenhouse effect.
Exam Technique
For an explain question, give a causal chain: ozone depletion leads to increased UV-B, which damages phytoplankton or DNA, producing ecological or health consequences. Do not merely state that UV radiation is “harmful”; identify the mechanism and its effect.
