Ultraviolet radiation and natural sunscreens in Antarctic krill
Creators: name: Newman, SJ
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis examines the adaptations of Antarctic krill (Euphausia superba Dana) to ultraviolet radiation in terms of susceptibility, behavioural avoidance and the presence/utility of UV-absorbing mycosporine-like amino acids (MAAs). In order to test the susceptibility of krill to UVB (280-320 nm) radiation, groups of animals in laboratory tanks were irradiated with various light treatments in order to determine lethal dose and effect on generalised activity. It was found that krill were killed within 3 days (8 hours of irradiation per day) by levels of UVB radiation equivalent to that at 5-15 m depth in the Southern Ocean in Spring. UV A (320-400 nm) was found to have no more effect on mortality and activity than visible light (PAR, 400-700 nm) only. This showed that krill are remarkably susceptible to low levels of UVB radiation, and are therefore at risk given their photic zone habitat. The behaviour of krill in response to gradients of visible light (photosynthetically active radiation, PAR), ultraviolet-A (UVA) and ultraviolet-B (UVB) was examined. It was found that krill swim away from high intensities of UV A and PAR, moving to areas of low intensity, but that they appear insensitive to high levels of UVB radiation. In a vertical tank, irradiance from above with UV A, but not UVB or PAR, was found to cause krill to significantly increase their depth. I conclude that krill can detect UVA but not UVB. Although detection of UVA may indirectly signal the presence of invisible UVB radiation, the fact that the spring ozone hole allows more UVB radiation through relative to UV A radiation, means that this response may not be sufficient to alert krill to initiate behaviour to avoid harmful radiation. The source, location and function of UV -absorbing MAAs were examined. Krill were fed algae irradiated under different UV conditions, with the different MAAs produced serving as a biological marker. In addition to MAA analysis of UV-killed krill, it was found that krill acquire MAAs from dietary algae, and not by de novo synthesis. Analysis of different krill tissues showed that some MAAs are selectively accumulated in certain body parts - highest concentrations were measured in eye tissues, and asterina-330 was found only in the eyes. This concentration of UV absorbing compounds in the eyes may aid in protection from UVB radiation, or may filter out UV A which can disrupt crustacean vision. An analysis of MAAs in krill caught during a large-scale biological survey revealed that concentration of MAAs was not directly correlated with the abundance of sympatric algae in the upper 100m suggesting, along with experimental data, that MAAs can be retained in tissues for long periods. This work also found that concentrations of MAAs decline as krill become larger, which may mean that susceptibility to UV changes with time. Variation of MAA concentrations along the survey also suggests that susceptibility also varies geographically, and is most likely influenced by abundance of phytoplankton. Krill fed on high-MAA diets did not appear to survive high UVB radiation any longer than those fed on low-MAA diets, although starved krill survived for a significantly shorter time than either. However, a diet high in MAAs seemed to reduce the detrimental effects of UVB radiation on activity. Further statistical analyses suggested a relationship between mortality and MAA concentration independent of feeding, specifically, survival time increased as concentrations of total MAAs and mycosporine-glycine:valine increased. It thus appears that Antarctic krill are equipped with a range of strategies to counteract the effects of ultraviolet radiation that penetrates to significant depths (20 m and deeper) in Antarctic waters during spring. It is equivocal whether krill have been impacted by UVB radiation in the past, due to a lack of data on krill populations from periods before the onset of ozone depletion. However, it is likely that other environmental factors are of more immediate concern in terms of the observed variation in krill stocks.