Microphytobenthos (MPB) on intertidal mudflats is a major component of primary producers in some estuarine ecosystems. To sustain photosynthesis, MPB migrate through the upper sediment layer and form transient biofilms during emersion periods, and thus may be exposed to high irradiance and ultraviolet radiation (UV-R), possibly resulting in photodamage to the photosynthetic apparatus. In contrast, downard migration could allow cells to optimize position in the photic zone, avoiding photoinhibitory light levels. Engineered biofilms with inhibited migratory capacity were used to distinguish between possible strategies (photoacclimation or migration) evolved by MPB to cope with photoinhibitory irradiances, when a series of UV filters with different cut-off wavelengths was used to estimate the respective contribution of visible light and UV-R. Engineered biofilms with full migratory capacity maintained a high relative electron transport rate (rETR), in contrast to engineered non-migratory biofilms, which showed a decrease in rETR under high irradiance, with a greater decrease under UV-B radiation. Migration thus appeared to be the principal short-term mechanism allowing MPB to avoid or minimize UV-R and high PAR photodamage in situ. Nevertheless, physiological acclimation processes to different light levels ('light-shade' patterns) seem to occur in the long term, and probably superimpose on migratory capacity, making light history an important component of MPB photoacclimation strategies.
KEY WORDS: Microphytobenthos · Migration · Photodamage · Photoinhibition · Ultraviolet radiation
Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 52: [223][224][225][226][227][228][229][230][231][232] 2008 benthic algae show complex patterns of vertical migration within sediments, further complicated by variations in light environment and other environmental variables (temperature, salinity, nutrient and water availability, alternation of seasons). Moreover, water currents and meteorological events can disturb biofilm structure and dynamics. MPB photosynthesis is limited to the emersion periods, and daytime emersion can occur once (around midday) or twice a day (morning and afternoon), as a function of tidal and nycthemeral rhythms. MPB can thus be exposed to irradiances up to, and sometimes in excess of, 2000 µmol m -2 s -1 , when emersion coincides with midday maxima in irradiance (Underwood & Kromkamp 1999, Perkins et al. 2001). At such high irradiance, planktonic microalgae usually show a decrease in photosynthesis, the extent of which varies according to irradiance level, time or length of exposure, and light history (the accumulated light dose to which the cells have been exposed previously). The decrease in photosynthetic activity caused by high irradiances can be attributed to different phenomena (photoprotection, photoinactivation, photoinhibition, photoacclimation), depending on the time scale considered, the possible existence of photodamage to the components of...