The survival strategies of microphytobenthos : behaviour and physiology

Microphytobenthos (MPB) need photoadaptive strategies to survive the highly dynamic light environment in which they reside. They are able to adjust their photosynthetic activity by physiological regulation or behaviourally by migrating vertically through the sediment. This study investigates the effects of the time of day on the vertical migration of benthic diatoms at two sites near Hobart, Tasmania using a pulse amplitude modulation fluorometer (Water PAM; Walz, Effeltrich) to measure chlorophyll fluorescence. Chlorophyll a content and maximum quantum yield (Fv/Fm) were used to examine the profiles of microalgal biomass and photosynthetic properties within sediment cores, both diurnally and over a 12 month experimental period. The results show a seasonal pattern of chlorophyll a biofilm development, with maximum values attained in summer at Pipe Clay Lagoon and in spring at Browns River. A greater amount of biomass was observed in the muddier sediment at Browns River with a sharper peak of chlorophyll a compared to the gradual incline then decline at Pipe Clay Lagoon. Fv/Fm values changed throughout the day with the cells more quenched at midday than sunrise while experiencing the highest illumination. Xanthophylls can provide photoprotection to MPB cells by cycling between epoxide and de-epoxide forms to dissipate excess light energy as heat. The second part of the study examined the xanthophyll cycle in microphytobenthos on tidally exposed sediment at Browns River. The goal of this work was to examine whether microphytobenthos at Browns River used the xanthophyll cycle as a physiological defence against photoinhibition during a natural light-dark cycle (day-night). A High Pressure Liquid Chromatography (HPLC) system was used as a pigment separation technique followed by pigment detection using a photodiode array and quantification against pure pigment standards. A pulse amplitude modulated (PAM) fluorometer was used to determine the chlorophyll fluorescence and assess photosynthetic performance in terms of maximum PSII quantum yield (Fv/F,,,), non-photochemical quenching and Ek in the field. Changes in PAM fluorescence and xanthophyll: chlorophyll a ratios suggests that MPB were under physiological stress at noon. The results indicate that the MPB cells exposed to light at the surface migrated deeper into the •sediments to replenish the epoxide form of their xanthophylls. Overall the result suggests that MPBs utilise both behavioural and physiological strategies to survive in the dynamic intertidal environment. This research highlights the importance of the photoadaptive strategies of MPB in a changing light environment with particular reference to the need for more than one strategy. This research on MPB ecology helps to form a more accurate picture on survival strategies while it underlines the fact that previous research has shown inconsistencies. Further research is needed in this area, particularly in the southern hemisphere, to lessen these inconsistencies and build on current knowledge