IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 172

IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT
DONAT-P. HÄDER
Friedrich-Alexander-Universität, Institut für Botanik und
Pharmazeutische Biologie, Staudtstr. 5, D-91058 Erlangen, Germany
1. Abstract
Aquatic ecosystems produce about 50% of the global biomass and play an
important role in atmospheric carbon dioxide cycling. Since the primary producers are
confined to the euphotic zone for energetic reasons, they are simultaneously exposed to
short wavelength radiation. Solar UV affects growth, reproduction, photosynthetic
production and many other physiological processes. Cyanobacteria are important
ubiquitous prokaryotes which populate terrestrial and aquatic habitats. They account for
up to 40 % of the marine biomass production and are important components of wet land
ecosystems such as rice paddy fields. These organisms are also highly impaired by solar
UV, but they and other motile microorganisms have developed mitigating strategies to
protect themselves from this stress. One protection strategy is based on vertical
migrations within the water column or a microbial mats. However, both motility and
orientation are impaired by UV radiation. Another means of protection is achieved by
the production of screening pigments including mycosporine-like amino acids (MAA)
or scytonemins. MAAs are also produced by phytoplankton and macroalgae. In several
organisms action spectra were measured which indicate that MAA synthesis is induced
by UV in most cases. These sunscreen pigments prevent short wavelength radiation
from reaching the UV sensitive DNA where it induces thymine dimers. Remaining
dimers are removed by photorepair which involves the enzyme photolyase. The
photosynthetic apparatus is another main target in primary aquatic biomass producers.
Inhibition of the photosynthetic electron transport chain can be determined by oxygen
measurements or by pulse amplitude modulated (PAM) fluorescence. Plants reduce the
potentially deleterious effects of solar UV by decreasing the photosynthetic electron
transport in photosystem II, a process called photoinhibition. Despite the dramatic
effects of even ambient solar UV on individual species and physiological responses, the
effect of ozone depletion on whole ecosystems is surprisingly low and close to the noise
level induced by all other environmental factors such as mixing layer depth, cloud cover
and temperature.
2. Introduction
Marine and freshwater aquatic ecosystems account for about half of the primary
biomass production on our planet. The aquatic primary producers incorporate about as