Induction, exudation and the UV protective role of kelp phlorotannins

Abstract

This study examined the ability of kelp phlorotannin exudate to attenuate artificial UVB radiation and whether exposure to UV could affect short-term phlorotannin levels within Macrocystis integrifolia Bory, the surrounding seawater or affect the seawater UV transparency. A dilution series of seawater that had contained M. integrifolia, reduced UVB radiation in an inverse relationship. Phlorotannin levels detected in this contact seawater were also inversely related to UVB exposure. Increasing concentrations of seawater previously containing M. integrifolia with phlorotannin concentration from 0.5 to 10.5 μg ml ^-1, significantly increased (P<0.05) the survivorship (from 0 to 100%) of germinating Laminaria groenlandica Rosenvinge meiospores grown under an artificial UVB radiation source. Short-term exposure of M. integrifolia blades to UVA and UVB radiation significantly (P<0.05) increased phlorotannins within tissues (∼10% of dry tissue weight) as compared to PAR or dark exposed algae (∼8.5-7.5%, respectively). UVA radiation was found to significantly (P<0.05) increase contact seawater UVB absorbance from ∼0.01 to 0.025 (Abs _{300 nm} g ^-1 wet blade), which was mirrored, though not significantly (P>0.05), by concentrations of phlorotannins within contact liquid. Results suggest that phlorotannins from macroalgal sources are produced and released into seawater during periods of UVA stress but degraded by UVB and can, at low concentrations (>0.84 μg ml ^-1), reduce the impact of UVB exposure to UV-sensitive kelp meiospores. Kelp beds are significant point sources of water soluble UV-screening compounds, and may form natural UV-refugia along coastal shores.