Impacts of Ozone Depletion and UV-B Radiation on Marine Life
UV-B is a harmful form of the Sun's radiation that reaches Earth's surface due to depletion of the protective ozone layer.
UV-B affects the gene activity, photosynthesis and metabolism of a plant, therefore affecting the whole plant growth.
Marine Primary Producers
The main primary producers of marine environments are phytoplankton.
Phytoplankton are plankton consisting of microscopic plants.
Phytoplankton are restricted to the photic zone, which is the layer of water where enough sunlight reaches to sustain photosynthesis and plant growth.
In the photic zone however, there is simultaneous exposure to harmful UV-B, which penetrate up to 70m deep into clear water.
How does UV-B affect photosynthesis?
UV-B impairs photosynthetic processes involved in primary production.
UV-B is absorbed by nucleic acids and induces DNA damage and mutations.
It also damages proteins and renders membranes as “leaky”.
Strategies to protect against UV stress
Phytoplankton engage in strategies to protect themselves against solar UV radiation
Two main strategies include:
- Production of UV screening pigments which absorb UV-B radiation to a certain extent reaching genetic material
- Vertical migration within the water column by active or passive movement
- Active: swimming
- Passive: changing buoyancy
Experimental methods used by scientists
There are different ways that scientists have tested the effects of UV radiation on phytoplankton communities.
1. Lab experiments
- Under standardized conditions using artificial light for enhanced UV treatments
- Cultures grown in lab may have developed different sensitivity levels and stress behaviours that are not representative of natural phytoplankton communities
2. Exposing phytoplankton to brighter solar radiation at the water surface and applying filters to screen off certain wavelengths, or enhance irradiation by supplemental light sources
- Confirmed detrimental effects of UV-B
- Suggested some beneficial effects of UV-A because it induces photo-repair of damaged DNA by using photolyases
3. Mesocosm studies
- Experimental tool that brings a small part of the natural environment under controlled conditions
- Showed no reduction in biomass by UV-B but showed altered species composition - possibly because of different sensitivities of inoculated phytoplankton or different UV sensitivity of phytoplankton grazers such as ciliates
- Conditions mimicked 30% ozone depletion and there were significant reductions in biomass productivity
4. Field work
- Our best option but not easy
- You need sensitive instrumentation which has to be reliable in the rough environments on research vessels
- For there to be ecological significance, large areas need to be sampled
Study by Li et al.
- Aim: to analyze the impact of solar UV-A and UV-B on photosynthetic carbon uptake by phytoplankton
Water samples were exposed to solar radiation in quartz tubes after adding a carbon source
- Sample set 1: full solar radiation
- Sample set 2: UV-B component was removed by an appropriate filter foil
- Sample set 3: whole UV wavelength band was excluded
- Under clear skies: UV-B impaired photosynthetic carbon fixation by up to 27% in coastal and open-ocean waters
- Inhibition by UV-A, which increased from near-shore to open ocean water eventually reaching a similar reduction of carbon uptake as UV-B
- Under cloudy skies: carbon fixation enhanced by up to 25% in coastal waters, but not in open ocean
One effect of global climate change may be change in cloud cover, thereby different exposure intensities to UV-B radiation occurs.
Phytoplankton in open ocean waters are more sensitive to UV-B than near-shore populations because near-shore populations have higher efficiency in repair processes and are more shaded.
Any significant changes in the phytoplankton productivity and species composition will definitely affect the rest of the aquatic food web
UV-B radiation and seagrasses
What are seagrasses?
- Flowering plants that can live underwater
- They grow in sediment on the sea floor
- They have elongated leaves and buried root-like structures, or rhizomes
- They serve as feeding and nursery habitats for many fish and food for waterfowl
- Their rhizome systems bind and stabilize bottom sediments, while their leaves improve water quality by filtering suspended matter
Effects of UV-B on seagrasses
The reaction of seagrasses to UV-B ranges from inhibition of photosynthesis to increased production of UV-B blocking compounds within plant tissue.
Relatively little emphasis has been given to the possible effects on aquatic plants, including these seagrasses, when compared to terrestrial research.
Seagrasses, are influenced by UV-B since its wavelengths can penetrate depths up to 70m.
UV-B radiation mostly affects marine plants in shallow, intertidal environments because they are often at or above the water during low tide.
Study by Trocine et al.
The study looked at 3 tropical seagrass species and how increased UV-B radiation negatively impacted their photosynthetic capacity. There was species variation present with respect to the response to increased UV-B.
1. Halodule wrightii (or Shoalgrass): greatest photosynthetic tolerance to UV-B
2. Syringodium filiforme Kutz (or Manatee grass): slightly less tolerance
3. Halophila engelmannii (or Star grass): little to no tolerance
Why were some seagrasses more tolerant?
Tolerance levels were due to UV-B blocking by anthocyanin or other flavonoids (plant pigments).
There was evidence that epiphytic growth on seagrasses also shielded them from harmful UV-B. Epiphytic growth is when a plant grows on another plant but is not considered parasitic.
Increased UV-B is most likely to affect tropical seagrasses in areas where the greatest amount of direct UV light reaches the Earth’s surface. Increased effects of UV-B are also seen occurring beneath the growing ozone hole above Antarctica.
Can UV-B exposure be good?
An increase in UV-B --> increase in plant phenols and flavonoids --> increase plants' resistance to herbivores + pathogens --> decrease rates of decomposition
The indirect impact of increased UV-B: a plant’s sequestration of carbon. Sequestration of carbon is when a plant takes carbon dioxide from the atmosphere and puts it into long-term storage. Under rising, global carbon dioxide levels, plants have a greater supply of carbon for secondary metabolism indirectly increases their ability to block harmful UV-B.