The ups and downs of life in a benthic biofilm: Migration of benthic diatoms.

Benthic diatom communities have been documented many times to exhibit a predictable pattern of migration entrained to diurnal and tidal cycles forming transient biofilms in the estuarine intertidal. The highly visible nature of these biofilms means that many anecdotal descriptions of this behaviour exist, yet much still remains to be elucidated about these behaviours and what drives them. There has been a resurgence in work examining the migratory patterns of benthic diatoms because of the important roles they play in the overall functioning of the ecosystem (e.g. primary production). It is the aim of this paper to provide a comprehensive overview of the migratory literature to date. The paper will review the methodology that was historically used to examine patterns in diatom migration as well as introduce new technologies (e.g. remote sensing). The patterns and cues for migration are discussed, highlighting the fact that whilst a "classic" pattern of migration is often found (migration of cells to the sediment surface as the tide leaves the site followed by a downwards migration in anticipation of the incoming tide), these cannot be considered to be consistent, with many site and taxon specific idiosyncrasies. Furthermore, it has been suggested that cells also migrate within the biofilm, cycling their position at the sediment surface therefore maximising their overall fitness, and this hypothesis is discussed. The vertical migration strategy of the benthic diatoms is critical to their pivotal role in the structure and functioning of estuarine intertidal mudflats and the costs and benefits of a microphytobenthic existence are discussed in this context.