Short-term radiation effects on Antarctic macroalgal propagules

In various laboratory studies the impact of UVR on isolated macroalgal propagules (Paper VI to VIII) was studied. Laboratory experiments should, however, not be extrapolated to determine community responses but they still provide valuable information of underlying mechanistic processes.

Generally, the impact of UVR to different macroalgal propagules reflected the zonation patterns of the adult algae on the shore. Reproductive cells of eulittoral algae were less affected and recovered better from UVR induced stress. Propagules of sublittoral algae on the other hand showed a higher degree of photodamage and DNA damage, also seen in slower or less effective recovery and repair mechanisms.

Photosynthesis-irradiance (P-I) curves

The P-I curves measured in the different species showed that the photosynthesis of reproductive cells of Antarctic macroalgae is shade adapted with saturating irradiances varying from 33 to 83 µmol m^-2 s^-1. Low light adaptation of photosynthesis is observed to be the general characteristic feature of reproductive cells of macroalgae (Amsler &

Neushul 1991; Roleda et al. 2004; 2005; 2006b). This might be related to the chlorophyll antenna size and number of chloroplasts present in reproductive cells compared to multicellular macroscopic stages. Red algal spores had lower Ik values than brown or green algae gametes or spores. These results are in agreement with measured Ik values for the adult thalli. Weykam et al. (1996) showed that the Ik values of adult Antarctic Rhodophyta are low compared to Chlorophyta or Heterokontophyta.

PAR effects on photosynthesis

Optimum quantum yield (Fv/Fm) of zoospores of Adenocystis utricularis was not affected by 8 h exposure to PAR (a dose of 136 kJ m^-2). Investigations of spores and gametes of the other studied species revealed a reduction of photosynthetic efficiency with increasing dose. Strongest inhibition was found in subtidal species (A. mirabilis and I. cordata). Reduction of photosynthetic efficiency while exposed to PAR is a protective mechanism to dissipate energy absorbed by PSII as heat via the xanthophyll cycle to avoid photodamage (dynamic photoinhibition; Osmond 1994). This process occurs mostly in algae from the intertidal or the upper subtidal and enables them to recover rapidly after the offset of the stressful condition (Hanelt et al. 1994). In contrast,

impairment of the D1 protein leading to a decrease in photosynthetic capacity is called chronic photoinhibition. This occurs in shade-adapted macroalgae growing in the lower sublittoral zone when exposed to high irradiances and is only reversible by a replacement of this protein which can take several hours (Matteo et al. 1984). These species have a lower ability to down-regulate photosynthesis through the protective dynamic photoinhibitory process (Hanelt et al. 2003). Reproductive cells of all macroalgae can be exposed to high PAR values during their planktonic phase while they stay in the euphotic zone. The potential for acclimation and recovery of the photosynthetic apparatus to high PAR conditions is therefore an important pre-requisite for the recruitment and ecological success of a shallow water alga (Roleda et al. 2006b).

UVR effects on photosynthesis

UVR exhibited an additional negative effect on the photosynthetic efficiency in all species tested. Monospores of the supralittoral species P. endiviifolium were the most tolerant to UVR. Although the measurable effects of both PAR and UVR in the reduction of photosynthetic efficiency are similar, the mechanisms behind are different.

UVR damage of the photosynthetic apparatus occurs in a more direct way, due to its absorption by biomolecules (Vass 1997; Franklin et al. 2003). Photosynthetic performance may be additionally depressed in light treatments supplemented with UVR by possible damage to the oxidizing site and reaction center of PS II (Grzymski et al.

2001; Turcsányi & Vass 2002). Especially Antarctic macroalgae can therefore suffer from the increased UV-B radiation during spring (due to stratospheric ozone depletion) exhibiting adverse effects on photosynthesis (Hanelt et al. 2003).

Recovery of photosynthesis

After photoinhibition, recovery of photosynthesis often requires exposure to low white light (Hanelt et al. 1992). Optimum quantum yield of all eulittoral species recovered completely after 48 hours post-cultivation under low white light. An incomplete recovery was observed in sublittoral propagules, especially when pre-exposed to UVR.

Recovery of photosynthetic efficiency of zoospores of different kelp species varied between 8 and 24 hours in upper and lower sublittoral species, respectively (Roleda et al. 2006b). Exposure to UVR was further observed to delay photosynthetic recovery of Arctic kelp zoospores (Roleda et al. 2006b). Comparison between species showed that intertidal I. cordata tetraspores had higher recovery rates compared to tetraspores isolated from subtidal algae, especially when pre-exposed to UV-B. Depth related sensitivity of reproductive cells was previously reported in kelp zoospores isolated from sporophytes collected at different depth gradient (Swanson & Druehl 2000).

DNA damage and repair

The absence of DNA damage in P. endiviifolium spores and minimal CPD formation in A. utricularis, I. cordata and M. hariotii propagules indicate effective shielding of the DNA and/or fast repair mechanism in the Antarctic intertidal propagules. Damage in subtidal A. mirabilis was significantly higher and no complete DNA repair was observed.

The degree of damage due to UVR was observed to be related to the zonation of the adult algae at the coastline. An effective DNA repair mechanism was also observed in spores of Arctic and temperate Laminariales and Gigartinales (Roleda et al. 2004, 2005). DNA damage can be repaired through photolyase enzyme (light-dependent), nucleotide excision and recombination repair (light-independent; van de Poll et al.

2002). The small amount of DNA damage in the tested intertidal Antarctic species might therefore be related to a high photolyase activity. Another possibility is shielding due to UV absorbing compounds. Brown algae are known to be able to produce phlorotannins which absorb in the UV-B range of the spectrum (Pavia et al. 1997). If phlorotannins occur in the exposed brown algal spores remains to be studied. On the other hand adult I. cordata was shown to produce two different types of MAAs (shinorine and palythine; Hoyer et al. 2001; Karsten et al. in press), absorbing in the UV-A wavelengths, both of these were also found in tetraspores indicating a protective role of MAAs already in reproductive cells. The ability of the propagules to cope with UV-B induced DNA damage seems to be crucial for the vertical zonation of the macrothalli at the coastline. If not repaired, DNA lesions can disrupt metabolism, cell division and impair growth and germination.

In general, exposure to the UV doses used in our laboratory experiment should not affect the survival and success of the investigated intertidal algae on short term view as all species recovered effectively from UV induced damage. Subtidal species on the other hand, were more affected and might especially suffer from an increasing UV-B penetration into the water body due to stratospheric ozone depletion. In the field, maximal light intensities can be much higher than the ones applied in the laboratory experiments, especially when low tide coincides with times of highest ozone depletion, noon, cloudless weather conditions and a high water transparency. All these factors can occur together in austral spring, when most macroalgae start to grow and reproduce.

Moreover, longer exposure times to ambient radiation over more than 8 h are possible in the austral summer when cells are suspended within the euphotic layer of the water column.