Influence of temperature on the relationship between oxygen- and fluorescence-based estimates of photosynthetic parameters in a marine benthic diatom (Cylindrotheca closterium)

In this paper we investigate the temperature sensitivity of the photosynthetic process of the benthic diatom Cylindrotheca closterium grown in light-limited turbidostat cultures at two different growth rates. Photosynthesis was measured as the rate of oxygen evolution and as the photosystem II (PSII) electron transport rate (ETR). The photosynthetic efficiency (a), as measured by both methods, was rather insensitive to temperature, and decreased significantly only at the extreme temperatures used (5 and 35°C). The maximum PSII quantum efficiency (F v /F m ) showed a small but significant trend of reduction with increasing temperature. However, the maximum rate of photosynthesis (P B max and ETR max ) was extremely temperature sensitive. The effect of temperature on the relationship between P B and ETR was limited to the most extreme temperatures investigated; deviations from linearity were most extreme at 5°C and different conversion factors were observed at 5 and 35°C. A short-term change in temperature (10-30°C), as might be experienced during emersion on a European tidal flat, will not significantly affect the relationship between P B and ETR. However, care should be taken when using a single conversion factor between P B and ETR at the extremes of the temperature range. We have also shown that algal absorption measurements are important for correct calculation of ETR. The facts that different species seem to have different conversion factors and that changing environmental conditions will affect the absorption capacity and growth rate of the microphytobenthos (MPB) community suggest that it is wise to perform further calibrations of the relationship in the field before use in primary production modelling. Variable fluorescence measurements are quick and non-invasive and, with knowledge of the absorption properties of the MPB community, allow the quantification of photosynthetic parameters across large areas. Hence they are potentially useful for improving our estimates of ecosystem scale primary production.