Tolerance of tropical marine microphytobenthos exposed to elevated irradiance and temperature

The photosynthetic productivity of tropical microphytobenthos (MPB) is largely driven by changes in light intensities and temperature at the surface of sediment flats during emersion. Here, the response of the MPB community to temperature and light was examined. Changes in temperature and irradiance during tidal cycles in the Tanjung Rhu estuary, Langkawi, Malaysia, in 2007 significantly affected the photosynthetic capacities of the MPB. Higher photosynthetic parameters, such as the maximum relative electron transport rate (rETRmax), photosynthetic efficiency (α), maximum quantum yield (F_v/F_m), and effective quantum yield (ΔF/F′_m), were recorded at high tide when the temperatures were lower. However, when the community was experimentally exposed to irradiances of 1800 µmol photons m⁻² s⁻¹, it was only able to photosynthesize at temperatures < 50 ∘C. Above this temperature, no photosynthetic activity was observed. Not only did high temperatures at high irradiance affect the algal communities, but limited photosynthetic activity was also observed in samples when exposed to limited irradiance. Recovery rates were highest at the lowest temperatures and decreased as the temperature increased. The recovery rates for samples exposed to temperatures of 40 ∘C were 4.01×10⁻³±0.002 s⁻¹ and decreased to 1.01×10−5±0.001 s−1 at 60 ∘C, indicating irreversible damage to Photosystem II (PSII). These characteristics suggest that the MPB communities in this estuary were able to adapt to temperature variation. However, enhanced photoinhibition would result if exposed to elevated temperatures, especially during low tide where in situ temperature was already 43 ∘C. Hence, if in situ temperature were to further increase during tidal emersion, 50 ∘C could be a temperature threshold for photosynthetic performance of tropical estuarine benthic microalgal communities.