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.J. Newman et al. J. Exp. Mar. Biol. Ecol. 255 2000 93 –110 Table 5
Euphausia superba. MAA concentrations in krill tissue after the final feeding 63 days and following 35 days
21
of starvation 98 days. Data in nmol g approx. dry wt. Figures in parentheses are S.D. Student’s t-test
performed on individual MAA concentrations from days 63 and 98. Bold P-values indicate statistical significance P ,0.05
MAA Treatment
Starved Fed PAR-only algae
Fed PAR1UVA B
algae 63
98 63
98 63
98 Mycosporine-
2.25 1.68
46.50 32.80
379.54 113.13
glycine:valine 1.9
0.68 7.63
4.25 76.54
16.00 df510, t 5 0.448
df58, t 5 1.568 df56, t 5 3.406
P , 0.332 P , 0.078
P , 0.007
Porphyra-334 273.12
268.91 446.77
356.42 295.69
174.92 28.84
25.07 22.53
38.16 27.80
14.21 df510, t 5 0.109
df58, t 5 2.038 df56, t 5 3.868
P , 0.457 P , 0.038
P , 0.004
Shinorine 135.25
129.05 187.09
156.60 129.20
87.75 16.50
15.61 17.89
13.73 11.13
8.27 df510, t 5 0.268
df58, t 5 1.352 df56, t 5 2.989
P , 0.397 P , 0.107
P , 0.012
Mycosporine- 143.07
128.24 133.91
129.44 164.54
106.18 glycine
24.34 27.78
34.06 27.26
20.44 21.27
df510, t 5 0.381 df58, t 5 0.102
df56, t 5 1.978 P , 0.356
P , 0.460 P , 0.048
Palythine 0.78
1.78 0.93
1.74 1.26
2.30 0.37
0.91 0.92
0.25 1.26
0.81 df510, t 5 0.636
df58, t 5 1.887 df56, t 5 1.579
P , 0.270 P , 0.048
P , 0.082 Asterina-330
2.69 0.93
1.98 0.68
2.11 1.19
0.49 0.26
0.55 0.43
0.54 0.44
df510, t 5 3.434 df58, t 5 1.842
df56, t 5 1.302
P , 0.003 P , 0.051
P , 0.120
4. Discussion
Many marine invertebrates have high tissue concentrations of MAAs but lack MAA-producing symbionts. It has been suggested that these animals acquire MAAs
from dietary sources Chalker et al., 1988; Shick et al., 1992; Karentz et al., 1997. The first experimental evidence of this was the accumulation of shinorine in the ovaries of
the sea urchin Strongylocentrotus droebachiensis from the red alga Mastocarpus stellatus Carroll and Shick, 1996. Adams and Shick 1996 further showed that this
accumulation protects the eggs of S
. droebachiensis from UV-induced damage. MAAs have since been shown to trophically accumulate in the spawn of the sea hare Aplysia
dactylomela Carefoot et al., 1998 and in the eyes of medaka fish Oryzias latipes Mason et al., 1998.
S .J. Newman et al. J. Exp. Mar. Biol. Ecol. 255 2000 93 –110
105
These studies, and the observation that wild-caught krill were found to contain proportions of MAAs similar to the content of dietary phytoplankton Karentz et al.,
1991b; Dunlap et al., 1995, prompted us to examine the trophic transfer of MAAs in the Antarctic marine food web. We chose the prymnesiophyte Phaeocystis antarctica as the
dietary algal species since krill consume and survive well on Phaeocystis spp. in captivity Haberman et al., 1993; Virtue et al., 1993. In the ecological context, P
. antarctica is a major component of Antarctic phytoplankton Marchant et al., 1991;
Karentz and Spero, 1995; Riegger and Robinson, 1997, especially at the marginal ice edge in spring when UV incidence and penetration are high and krill are actively feeding
Hamner et al., 1983; Holm-Hansen and Huntley, 1984. In our original design we expected that P
. antarctica grown under PAR-only irradiation would have low MAA concentrations and would therefore serve as the low MAA diet, while P
. antarctica grown under PAR1UVA B would provide the high MAA diet for comparing dietary
accumulation rates. We observed, however, that P . antarctica grown under PAR-only
produced substantial concentrations of MAAs dominated by porphyra-334 with low concentrations of mycosporine-glycine:valine. Culturing P
. antarctica under PAR1 UVA B did indeed increase the total MAA content, but also increased the synthesis of
mycosporine-glycine:valine to be the dominant MAA component over porphyra-334 Fig. 2a. Given this difference, the manipulation of mycosporine-glycine:valine and
porphyra-334 concentrations in P . antarctica grown under different light regimes served
as a marker for differential accumulation of MAAs in krill maintained on diets having different MAA compositions.
Shinorine and porphyra-334 differ in their chemical structures by only one methyl group at the imino-group substitution by the amino acids serine and threonine,
respectively Dunlap and Shick, 1998. Synthesis of shinorine is stimulated in P .
antarctica grown under UV light Fig. 2b, whereas porphyra-334 concentrations decline under UV light relative to the MAA content of cultures grown under PAR-only Fig. 2a.
In contrast, the production of mycosporine-glycine:valine in P . antarctica is accelerated
almost exclusively by addition of UVA B wavelengths Fig. 2a. Specific pathways for the biosynthesis of mycosporine-glycine:valine in P
. antarctica are unknown as for other MAAs but the reduction of porphyra-334 by UV light would indicate that
synthesis of this MAA may be competitive with the UV-induction of mycosporine- glycine:valine biosynthesis. Our results are not directly comparable with recent work on
MAAs in P . antarctica Riegger and Robinson, 1997; Jeffrey et al., 1999 due to both
papers’ lack of specific information on MAA identity. However, our results broadly concur with those of Riegger and Robinson 1997, who found that synthesis of
UV-absorbing compounds presumed to be MAAs is maximal under UV wavelengths 280–400.
The significant pattern of change in the bioaccumulation of mycosporine- glycine:valine Fig. 4 and its subsequent loss during starvation Table 5 demonstrates a
significant effect of dietary availability on trophic accumulation of this MAA component in krill tissues Tables 3 and 4. The reason that the dominant MAA accumulated in krill
fed algae grown under PAR1UVA B was mycosporine-glycine:valine is most likely because of its extremely high concentrations relative to the other MAAs. However, it
may be that some MAAs, such as mycosporine-glycine:valine, are more readily
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.J. Newman et al. J. Exp. Mar. Biol. Ecol. 255 2000 93 –110
absorbed into krill tissues than others, as was observed by Mason et al. 1998 in the accumulation of algal MAAs by medaka fish Oryzias latipes. Repetition of the present
study with other dietary species having different selection of MAA components would allow further exploration of this question.
The feeding of P . antarctica grown under PAR1UVA B to krill caused an initial, but
significant, loss of porphyra-334 and shinorine compared with starved animals or krill fed algae grown under PAR-only. This suggests that feeding krill PAR1UVA B P
. antarctica causes a metabolic disturbance of unknown stress, forcing an initial
absorption or release of these MAA components. However, while the initial loss was unexpected, subsequent accumulation of the MAAs by day 63 Fig. 4 and a drop in
concentrations during starvation Table 5 fits with what might be expected given the observed patterns of change in mycosporine-glycine:valine levels. In krill fed PAR-only
algae, porphyra-334 and shinorine followed the same general pattern as the group fed PAR1UVA B algae, but remained at or above concentrations in the control animals
Fig. 4.
The mycosporine-glycine content of krill remained at or near initial levels during feeding and starvation, despite enhanced quantities of mycosporine-glycine available in
both diets. One possible explanation could be that experimental krill were already saturated in mycosporine-glycine from algae consumed in the wild, or that some
mechanism maintained a homeostatic balance of mycosporine-glycine in these consum- ers, particularly during starvation. Such a mechanism could involve the conversion of
cellular imino-mycosporines in a process analogous to the conversion of shinorine and porphyra-334 to mycosporine-glycine, similar to what occurs by the metabolism of
certain marine bacteria Dunlap and Shick, 1998. Mycosporine-glycine, unlike imino- mycosporines, has moderate antioxidant activity Dunlap and Yamamoto, 1995 and
maintaining consistent levels may have a functional but untested significance in preventing photooxidative stress.
Insufficient levels of palythine and asterina-330 were present in extracts of P .
antarctica to allow quantification. Peaks heights allowed only determinations of presence absence in a small fraction of algal extracts see Results. As such, the paucity
of palythine and asterina-330 in the krill diet is most likely due to background concentrations that were already present before the onset of feeding Fig. 5.
Some of our results show that, following cessation of feeding, concentrations of some MAAs return to initial values. Specifically, concentrations of the major MAAs porphyra-
334, shinorine and mycosporine-glycine:valine in krill fed PAR1UVA B algae de- creased toward initial levels Table 5. This implies that sequestered MAAs cannot be
retained indefinitely, although concentrations in krill starved for several months suggest that MAAs can remain in body tissues for long periods with minimal dietary
supplementation.
The total MAA concentrations obtained in krill from our experimental treatments are broadly consistent with published results from wild-caught specimens Karentz et al.,
1991b. In the present study, the mean total MAA concentration for starved krill was
21
3863 nmol g approx. dry mass, which compares favourably with the mean con-
21
centration of 4670 nmol g dry mass eight specimens reported by Karentz et al.
1991b, when one considers that animals used in this experiment, although caught in
S .J. Newman et al. J. Exp. Mar. Biol. Ecol. 255 2000 93 –110
107
Fig. 5. Euphausia superba. Effect of 63 days of feeding on concentration of trace MAAs palythine, asterina-330 in krill fed P
. antarctica grown under various light conditions. Bars represent one standard error n 5 4. See Tables 3 and 4 for results of two- and three-way ANOVA.
late summer, had experienced a prolonged period of low-food availability during transport and subsequent captivity 52 days which may account for the low con-
centrations in comparison to freshly-caught krill. Antarctic phytoplankton have been shown to quickly acclimatise to high UV-ir-
radiance by increasing production of MAAs in a matter of days Helbling et al., 1992, ˜
1996; Montecino and Pizarro, 1995; Villafane et al., 1995. This rapid response suggests that krill that feed on phytoplankton following a period of high UV irradiance, such as
that incident on a surface bloom during spring, will consume more MAAs than krill feeding on the same species of phytoplankton shaded from UV by sea ice cover or by
depth in the water column. In addition, it is commonly believed that increasing levels of UVB radiation will favour succession of UV-resistant species and alter the floristic
composition of phytoplankton communities Marchant, 1993; Davidson et al., 1994. Given that krill have such a varied diet, it can be expected that if MAA-rich species
become more abundant in response to environmental UV stress, the increase in dietary availability will enhance MAA levels in consumer organisms. Therefore, the accumula-
tion of MAAs, being dependent upon the recent irradiation history of prey algae, may provide a functional mechanism whereby MAA concentrations in krill are increased by
an indirect response to enhanced UVB exposure. This ‘co-acclimation’ can occur by ingesting greater amounts of MAAs synthesised by phytoplankton in response to UV
exposure and or by UV-mediated change in floristic composition to favor the presence
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of UV-tolerant species having a greater capacity for the production of UV-absorbing MAAs.
5. Conclusions
