Abstract
Lichens produce various carbon-based secondary compounds (CBSCs) in response to abiotic conditions and herbivory. Although lichen CBSCs have received considerable attention with regard to responses to UV-B exposure, very little is known about intra-specific variation across environmental gradients and their role in protection against herbivory in the Antarctic. Here we report on the variation in CBSCs of two widely distributed and common Antarctic lichens, Usnea antarctica and Umbilicaria antarctica, between sites with different solar exposure (NW–SE) and along natural nitrogen (N) gradients which are associated with changing lichen-invertebrate associations on Signy Island (South Orkney Islands, maritime Antarctic). Fumarprotocetraric and usnic acid concentrations in Usnea showed no relationships with solar exposure, lichen-N or associated invertebrate abundance. However, fumarprotocetraric acid concentration was 13 times higher at inland sites compared to coastal sites along the N-gradients. Gyrophoric acid concentration in Umbilicaria was 33% lower in sun-facing (northerly exposed) habitats compared to more shaded (south-facing) rocks and declined with elevation. Gyrophoric acid concentration was positively correlated with the abundance and species richness of associated microarthropods, similar to the patterns found with lichen N. This initial investigation indicates that there can be large intraspecific variation in lichen CBSC concentrations across relative short distances (< 500 m) on Signy Island and raises further questions regarding current understanding of the role of CBSCs in Antarctic lichens in relation to biotic and abiotic pressures.
Highlights
Lichens produce various carbon-based secondary compounds (CBSCs) which often vary in response to environmental gradients (Swanson et al 1996; Bjerke et al 2004; Vatne et al 2011), in particular in relation to solar radiation (Bjerke et al 2002; McEvoy et al 2006, 2007), and nitrogen availability (Solhaug and Gauslaa 2012)
Fumarprotocetraric acid concentration was c. 13 times higher (Tukey HSD p < 0.05) at inland sites (18.7 mg g−1 DW) compared to sampling transects affected by penguins (1.4 mg g−1 DW) while usnic acid concentration did not differ between sites and were on average 15.87 (SE ± 0.20) mg g −1 DW (Table 1, Fig. 2)
There was great variability in the CBSC concentrations measured in Usnea and Umbilicaria, but their intra-specific variation in relation to habitat exposure and nitrogen gradients did not support our hypotheses
Summary
Lichens produce various carbon-based secondary compounds (CBSCs) which often vary in response to environmental gradients (Swanson et al 1996; Bjerke et al 2004; Vatne et al 2011), in particular in relation to solar radiation (Bjerke et al 2002; McEvoy et al 2006, 2007), and nitrogen availability (Solhaug and Gauslaa 2012). Nutrients can become limiting, as is clear from biodiversity and ecosystem process rate increases in the vicinity of marine vertebrate concentrations that supply external nutrients from the ocean (Erskine et al 1998; Ball et al 2015; Bokhorst et al 2019a). These aggregations generate volatilised ammonia which is deposited further inland where the nitrogen is taken up by lichens and other primary producers (Crittenden et al 2015; Bokhorst et al 2019a). Lichen tissue nitrogen concentration is known to be affected by this nitrogen input from marine vertebrates it is currently unclear whether CBSC production in Antarctic lichens is affected
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