IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 176

it is difficult to use short-term observations to predict longer-term (days to years)
ecological responses 23-24 .
The targets of solar UV in phytoplankton are manifold, and short wavelength
radiation affects DNA and cellular proteins, impairs growth and reproduction,
photosynthesis 25-27 as well as motility and orientation in phytoplankton 28 . Ammonium
and nitrate uptake is also affected by solar radiation 29 . The organisms respond by the
synthesis of heat-shock proteins and changes in the cellular amino acid pools. DNA is a
central target in the cell. It has a high absorption in the UV, and solar short wavelength
radiation both damages DNA and delays DNA synthesis in many organisms 30 .
Many phytoplankton use protective and mitigating strategies to prevent excessive
harm by solar radiation. In addition to vertical migration and DNA repair, many (but not
all) phytoplankton organisms produce UV screening pigments. In the marine
dinoflagellate Gyrodinium dorsum short wavelength radiation induces the synthesis of
mycosporine-like amino acids (MAA). HPLC and spectroscopy revealed that G. dorsum
contains a complex mixture of several MAAs. A polychromatic action spectrum for the
induction of MAA has a major peak at 320 nm 31-32 . However, exposure to excessive
short wavelength UV-B radiation resulted in decreased overall MAA production. In
addition, there was a shift in the absorption of the MAA mixture towards shorter
wavelengths indicating that short wavelength UV-B may change the MAA
composition 31 .
MAAs may serve several functions in the cell and operate as osmotic regulator,
as antifreeze, in addition to UV absorption. The protective effect of MAAs against UV
radiation was demonstrated in G. dorsum . In one batch of cells MAA synthesis was
induced by UV-A + PAR, while in the control (irradiated with PAR only) no MAAs
were induced. Both samples (with high and low MAA content, respectively) were
subsequently subjected to high UV-B irradiation. Cells with low MAA content suffered
a complete loss of motility within 3 h, while the cells with high MAAs content survived
at least two times longer.
A library of photoprotective compounds in cyanobacteria, phytoplankton and
macroalgae has been compiled and made available in a database accessible on the
Internet (http://www.biologie.uni-erlangen.de/botanik1/index.html) to provide easy
access to the various photoprotective compounds reported in the literature 33 . The data
base includes absorption maxima, molecular structures and extinction coefficients of
some of the important photoprotective compounds.
To identify a MAA, both the absorption peaks and the R f values (HPLC) need to
be determined using a standard procedure. MAAs are water-soluble substances
characterized by a cyclohexenone or cyclohexenimine chromophore linked to the
nitrogen substituent of an amino acid or its imino alcohol. Their absorption maxima
range from 310 to 360 nm, and they have an average molecular weight of around 300 34 .
5. Macroalgae
While phytoplankton is free to move in the water column 9 , most macroalgae are
sessile and restricted to their growth site with its specific environmental properties 35 .