Microtopographic cues for settlement of zoospores of the green fouling alga Enteromorpha

Enteromorpha, the most important macroalga that fouls ships, produces very large numbers of swimming spores that respond to a number of settlement cues. Responses to topographic cues have been investigated using surfaces with defined microtopographies fabricated from polydimethyl siloxane elastomer (PDMS). The topographic features were based on two designs, (a) a series of 5 or 1.5 mum deep valleys with valley floors and ridges varying between 5 and 20 mum, and (b) pillars of 5 mum diameter and 5 or 1.5 mum height, spaced 5-20 mum apart. The features were arranged in blocks to provide the swimming spores with a 'choice' of where to settle. Swimming spores settled preferentially in the valleys and against the pillars. The number of spores that settled increased very substantially as the width of the valley decreased. The majority of spores settled in the angle between the valley floor and sidewall. Lower numbers settled on the surfaces with lower profile features. Silica beads of similar dimension to the spore body were used to determine whether the spatial relationships between settled spores and the topographic features were a consequence of active settlement behaviours. The results are discussed in relation to the energy needs for spores to adhere to various surface features.