Ecology of UV
UV introduction and basic optics
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Temperature Effects on UV Exposure in Aquatic Ecosystems
prepared by Craig
Williamson
Aquatic ecosystems are exposed to high levels of UV radiation at a wide range of temperatures. The reason for this is that temperature and UV radiation generally vary along the same environmental gradients but changes are not always parallel over space and time. For example, the thermal inertia of aquatic systems lead seasonal changes in temperature to generally lag behind changes in UV in a way that creates an early season peak in the UV: temperature ratio (UV:T, Fig. 1), with potential implications for plankton populations (Fig. 2).
Inverse relationships between temperature and UV can cause even greater variation in seasonal UV:T ratios. For example, UV increases while temperature decreases with increasing elevation such that alpine lakes with low CDOM are subjected to some of the highest UV:T ratios of any aquatic ecosystems (Sommaruga and Psenner 1997, Laurion et al. 2000). Cloud cover may increase at high elevations, reducing total UV exposure (Caldwell 1968, Byron 1982), but also making sharp peaks in UV exposure at any one time less predictable.
So why is the temperature at which an organism is exposed to UV radiation
important?
Because UV damage to DNA is thought to be largely temperature independent, but molecular repair of this damage is temperature dependent; thus variations in UV:T ratios are likely to alter UV impacts on aquatic organisms and ecosystems.
There is developing evidence that many organisms at all trophic levels from bacteria and phytoplankton to protozoa, zooplankton, and fish depend heavily on photoenzymatic repair (PER) for their defense against damage from UV radiation. Since PER is an enzyme-driven process, and enzyme kinetics are temperature dependent, organisms that depend on PER for their defense are likely to be less tolerant of UV exposure at lower temperatures (Fig. 3). This interaction between UV exposure and temperature is one of the core relationships behind this NSF IRCEB "UV-Lakes" project.
There are also potentially important indirect interactions between UVR and temperature in the vertical habitat gradients of lakes. For example, many organisms derive a distinct demographic advantage if they are able to maximize the amount of time spent in the warm surface waters of lakes (Orcutt and Porter 1983, Stich and Lampert 1984). Warmer temperatures speed up rates of growth and reproduction. More rapid growth to maturity will in turn reduce vulnerability to predation and starvation during the smaller developmental stages.
The direct absorption of UV-B radiation and resulting DNA damage is generally considered to be unaffected by temperature. In contrast, UV-A radiation produces oxygen free radicals, including singlet oxygen, superoxide anion, and hydroxyl and peroxide radicals that can react with DNA and be more temperature-dependent (Mackey and Derrick 1986). Changes in temperature can effect PER, NER, and BER as well as modulate these responses through induction of molecular chaperones (heat shock proteins) from heat and cold stress. Early work showed that the formation of photoreactivating enzyme-damaged DNA complexes was temperature-dependent (Fukui et al. 1981). Temperature affects DNA-protein and protein-protein interactions involved in NER; binding of the prokaryotic exinuclease component UvrA to itself (dimerization) and to DNA was significantly reduced at non-optimal temperature (Mazur and Grossman 1991). The incision stage of NER is more labile to the effects of temperature than subsequent excision-resynthesis steps in prokaryotes (Morimyo et al. 1975) and eukaryotes (Hjertvik et al. 1998). In the latter study, effects of temperature on incision were significantly more pronounced in NER compared to BER. This is supported by studies in mammalian cells in culture where incubation of cells at lower temperatures significantly reduced NER (Wheeler et al. 1992). This reduction was significantly greater for the second, slower phase of the biphasic NER process (5-fold reduction) relative to the initial rate of repair (2-fold reduction).
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