Abstract

Abstract. 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 (Fv/Fm), and effective quantum yield (ΔF/Fm′), 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.

Highlights

  • Microphytobenthos (MPB) makes a significant contribution to the primary production of shallow coastal and estuarine environments, regularly contributing more than 50 % of the total annual production (Underwood, 2002; Rajesh et al, 2001)

  • We found that the MPB communities inhabiting the Tanjung Rhu estuary were not inhibited by the high- and variable-light environment or temperatures (28.5 to 43 ◦C) during tidal exposure

  • An increase in temperature and irradiance during low tide caused a decline in Fv/Fm at all sites in the Tanjung Rhu estuary; these were low compared with optimum values of ∼ 0.650 for healthy microalgae (McMinn and Hegseth, 2004)

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Summary

Introduction

Microphytobenthos (MPB) makes a significant contribution to the primary production of shallow coastal and estuarine environments, regularly contributing more than 50 % of the total annual production (Underwood, 2002; Rajesh et al, 2001). High turbidity, which is a characteristic of many tropical estuaries, reduces the irradiance reaching the microalgae mats which limits photosynthesis (Underwood, 2002). Intertidal ecosystems in the tropics are affected by large changes in solar irradiance and temperatures, caused by tidal cycles. Variation in irradiances, temperatures, and salinity can adversely affect the photosynthetic activity of MPB communities (Underwood, 2002; Laviale et al, 2016). To overcome these problems, many taxa have developed adjustment and avoidance strategies. Some can protect themselves physiologically against the damaging effects of exposure (Blanchard et al, 2004; Perkins et al, 2010) by dissipating the excess energy absorbed by light-harvesting centers, a process

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