Latitudinal and seasonal variability

Aquatic organisms can utilize a number of defense systems to overcome UVR stress (DNA repair, UVR screening, antioxidant enzymes and compounds, see Chapter 10). It can therefore be hypothesized that long term exposure to high incident UVR doses promotes UVR resistance in cells, species, populations or communities. For this reason it can also be hypothesized that organisms inhabiting low latitude regions have developed more efficient UVR defenses than organisms from higher latitudes [17,110,118-120]. Still, recent studies have clearly demonstrated that tropical aquatic organisms also suffer from in situ UVR stress [2,11,52,87,94,102,121]. It remains to be tested whether tropical organisms are more resistant to UVR as compared with organisms inhabiting higher latitudes. Few latitudinal or other large scale comparisons of UV-B vulnerability in aquatic organisms have been carried out [17,122]. One study revealed large differences in UVR vulnerability between regions and seasons [122], based on absolute biological weighting functions established for a large number of phytoplankton assemblages. Literature data allow for minimal comparison of UV effects due to the fact that different physiological parameters were considered (14C incorporation, variable fluorescence Fv/Fm, growth rate reduction, DNA damage). The use of various types and brands of UVR meters (spectroradiometers, broad band meters, dosimeters) further complicates comparisons of field studies. Finally, even very small differences in methodological approach, for instance the application of different antibodies, may prevent direct comparisons. Therefore, proper comparisons can only be done by applying both identical methods and instruments.

In an effort to reveal latitudinal patterns of vulnerability for UV-B-mediated DNA damage accumulation, we compared CPD induction patterns for a variety of marine plankton assemblages (Table 2). For additional comparison, three high altitude lakes (Argentinean Andes) were included. Identical experiments were performed at all locations. Plankton assemblages were exposed to natural solar radiation for a daily period, after which the accumulated CPDs were measured. Simultaneously, daily DNA (biological) effective doses (BED) were measured using DNA dosimeters. Vulnerability for CPD induction in the various size fractions was assessed by calculating the Mean Damage Ratio (MDR), by normalizing CPD abundance to the level of incident biologically effective irradiance, according to

CPDs (accumulated in plankton) CPDs (accumulated in DNA dosimeter)

In order to distinguish between heterotrophic bacteria and the main phyto-plankton groups, size fractionation was done differently for the different regions; for the (sub)tropical sites bacteria and phytoplankton were separated in fractions 0.2 to 0.8 fim and 0.8 to 10 pm (Table 2). For the other sites the large, diatom dominated fractions >10 ¿¿m were considered. This approach for comparing UV-B vulnerabilities in a variety of regions has some obvious drawbacks. First of all, samples for the experiments were kept at the surface, receiving full solar radiation for the whole experimental period. This may have introduced (overexposure) artifacts since cells may have otherwise been subjected to vertical mixing, thereby receiving a different exposure regime in situ. Furthermore, differences in species dominance at the various locations forced us to compare different floristic groups as well as geographical regions. While being aware of these important drawbacks, the advantage of the present approach was that accurate measurements of DNA damage as well as DNA effective UV-B allowed for realistic MDR calculations. Also, in spite of the floristic differences, natural representative assemblages were tested for the various geographical regions.

MDR values were significantly (p <0.05) higher in the bacteria than in the larger size fractions judging from the consistently higher MDR values (Table 3). Only at the Caribbean and the Andes sites was no significant difference between small and large cells observed. Although contradicted by the Caribbean site, the bacterial fractions showed some correlation with latitude and altitude: the lowest MDR values were found for the highest mean BED (Andes) and the highest MDR for the lowest mean BED (Antarctic site). On the other hand, the Caribbean site showed relatively high MDR values, despite the fact that BEDs were high. As shown here, the observed high UV-B vulnerability does not implicate that bacteria are incapable of adjustment to high incident UVR levels. In particular, the low MDR value for the high altitude lakes hints at physiological acclimation or community change in favor of more UV-B resistant species.

The phytoplankton fractions also showed a large variability in MDR values,

Table 2. General information on the MDR experiments. MDR is defined as CPDs accumulated in plankton divided by CPDs accumulated in dosimeter DNA. Data are taken from Boelen et al. [94,95], Buma et al. [89,96], Helbling et al., unpublished results (Andes)

Location (number of

Experimental

Table 2. General information on the MDR experiments. MDR is defined as CPDs accumulated in plankton divided by CPDs accumulated in dosimeter DNA. Data are taken from Boelen et al. [94,95], Buma et al. [89,96], Helbling et al., unpublished results (Andes)

Location (number of

Experimental

experiments)

Lat./Long.

period

Habitat mean water T

Fractions tested

Main plankton groups

Caribbean

12°N

April 1998

marine/oligotr. 27 °C

small: 0.2-0.8 fim

small: Prochlorophytes,

(10)

69°W

large: 0.8-10 /im

bacteria large: Synechococcus spp.

Red Sea

29°N

September 1998

marine/oligotr. 28 °C

small: 0.2-0.8 fim

small: Prochlorophytes,

(2)

35°E

large: 0.8-10 /xm

bacteria large: Synechococcus spp.

Andes

41°S

January 1999

high altitude lakes; 19°C

small: 0.2-2 fim

small: bacteria

(3)

72°W

large: >10 fim

large: diatoms, colonial cyanophytes, dinoflagfellates

Argentinean Sea

45°S

January 1999

marine/eutrophic 19°C

small: 0.2-2 fim

small: picophytoplankton,

(4)

66°W

large: >10 /im

bacteria large: diatoms

Antarctic

67°S

January 1998

marine/eutrophic 2°C

small: 0.2-2 fim

small: bacteria

(4)

68°W

large: >10 fim

large: diatoms

Table 3. Mean damage ratios (MDR) for the various regions and size fractions

Small

Large

Location

Caribbean Red Sea Andes

Argentinean Sea Antarctic

0.32 + 0.07 0.15 + 0.03 0.04 + 0.02 0.41 ±0.22 0.55 + 0.18

0.33+0.06 0.13 ±0.01 0.03 ±0.02 0.28 ±0.21 0.20 ±0.09

with the Caribbean, Synechococcus spp. dominated fraction showing the highest MDR value. Strikingly, a significant (p <0.05) difference was observed between MDR values of the Caribbean and the Red Sea, the latter also dominated by Synechococcus spp. The temperate and Antarctic, diatom dominated assemblages gave intermediate MDR values of 0.28 and 0.20, respectively. These values were not significantly different (p < 0.05). Again, phytoplankton from the high altitude lakes showed very low CPD accumulation, giving the lowest MDR value of 0.03 recorded for all experiments, suggesting acclimation to the prevailing high incident UV-B doses (Table 3). No clear latitudinal pattern was observed for the marine autotrophic assemblages. In addition, no clear relation with cell size was observed for phytoplankton. The diatoms from the temperate and the Antarctic site showed intermediate MDR values as compared with the two (sub)tropical sites. Cell size related vulnerability for CPD induction, therefore, was not demonstrated for the phytoplankton fractions.

There are several factors that could likely contribute to the vulnerability for in situ CPD induction as measured using our approach. One important factor that could affect UV-B vulnerability is the nutrient status of the cells. Theoretically, nutrient starvation could strongly hamper UV-B defense mechanisms because the synthesis of repair enzymes and screening substances requires energy and nutrients. Both picophytoplankton-dominated systems (Caribbean, Red Sea) were oligotrophic [123,124]. Therefore, at least at both picophytoplankton-dominated sites, differences in UVR vulnerability could not be explained by nutrient availability. Finally, the long and short term UVR history of the cells may have determined the level of UVR defenses prior to the incubation experiments. For example, deep vertical mixing (Caribbean) may have failed to induce UVR defense systems due to the low mean UVR exposure levels. Wind induced vertical mixing was strong in the Gulf of Aqaba too, but the sampling season (September) may have allowed the community to adjust to high UVR levels over the prolonged summer period. It is clear from the high altitude lake experiments that communities acclimate to the prevailing UVR regimes, judging from the low MDR levels recorded here. We conclude that factors other than cell size and latitude seem to determine CPD vulnerability in phytoplankton. Large parts of the world's oceans are characterized by high incident UVR levels in combination with low UVR attenuation. At the same time, nutrient (nitrate, iron) starvation commonly occurs in oceanic surface waters. The interaction between various growth limiting and stress factors (nutrient limitation, UVR stress) should therefore deserve further attention in future marine photobiology studies.

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