An assessment of temporal variation in the low molecular weight phlorotannin profiles in four intertidal brown macroalgae

Phlorotannins are a group of highly bioactive marine polyphenols found exclusively in brown algae (Phaeophyceae) where they play an integral role in chemical defence. Owing to their high degree of reactivity, phlorotannins have attracted recent commercial interest with potential for application in areas such as pharma-, nutra- and cosmeceuticals. However, to date, little is known regarding their biosynthesis and the factors that drive their natural variability, particularly in relation to their chemical properties such as polymerisation and isomerisation - two factors which directly influence their degree of activity. Low molecular weight (LMW; < 3.5 kDa) phlorotannin-enriched fractions have exhibited a high level of antioxidant activity yet their degree of natural variability within macroalgae is not known. This study aimed to, for the first time, assess the temporal variation of LMW phlorotannin chemical profiles of four intertidal brown macroalgae dominant along the west coast of Ireland - Pelvetia canaliculata, Fucus vesiculosus, Ascophyllum nodosum and Himanthalia elongata. Samples were collected at six different time points over two years (2014-2015) and assessed for temporal shifts in the chemical profile, content and antioxidant activity. Species-specific profiles were detected, with each species displaying varying degrees of polymerisation and isomerisation, which were linked to the in vitro antioxidant activity observed. While some temporal shifts were observed in the molecular weight composition of the LMW phlorotannin profiles, they, along with the degree of isomerisation, did not appear to correspond with large-scale temporal changes in irradiance or temperature regime of Galway Bay. The results emphasise the strong degree of natural variability associated with phlorotannin profiles which, in situ, are likely to be influenced by several parameters synergistically; future studies should investigate small-scale effects of physio-chemical parameters, as well as biological factors, at microhabitat level, which could be aided by a laboratory-based assessment of individual parameters.