This study presents a methods evaluation and intercalibration of active fluorescence-based measurements of the quantum yield (/ 0 PSII ) and absorption coefficient (a PSII ) of photosystem II (PSII) photochemistry. Measurements of / 0 PSII , a PSII , and irradiance (E) can be scaled to derive photosynthetic electron transport rates (P e ), the process that fuels phytoplankton carbon fixation and growth. Bio-optical estimates of / 0 PSII and a PSII were evaluated using 10 phytoplankton cultures across different pigment groups with varying bio-optical absorption characteristics on six different fast-repetition rate fluorometers that span two different manufacturers and four different models. Culture measurements of / 0 PSII and the effective absorption cross section of PSII photochemistry (r PSII , a constituent of a PSII ) showed a high degree of correspondence across instruments, although some instrument-specific biases are identified. A range of approaches have been used in the literature to estimate a PSII ðkÞ and are evaluated here. With the exception of ex situ a PSII ðkÞ estimates from paired r PSII and PSII reaction center concentration (½RCII) measurements, the accuracy and precision of in situ a PSII ðkÞ methodologies are largely determined by the variance of method-specific coefficients. The accuracy and precision of these coefficients are evaluated, compared to literature data, and discussed within a framework of autonomous P e measurements. This study supports the application of an instrument-specific calibration coefficient (K R ) that scales minimum fluorescence in the dark (F 0 ) to a PSII as both the most accurate in situ measurement of a PSII , and the methodology best suited for highly resolved autonomous P e measurements.V C 2014 Association for the Sciences of Limnology and Oceanography Improved monitoring of phytoplankton productivity (PP) is a core goal across the aquatic sciences and underpins long term management plans for coastal seas and the global ocean (European Marine Board 2013). Following the success of global ocean observatory systems such as the free-drifting Argo profilers (http://www.argo.ucsd.edu/), scientists are now looking to integrate instruments that are capable of autonomous biological rate and flux measurements into environmental sensor networks (Claustre et al. 2010). Unlike traditional in vitro photosynthetic assays, active fluorescence-based photosynthetic measurements are well suited for environmental sensor networks as many of these optical instruments can operate autonomously providing high resolution in situ photosynthesis measurements. Bio-optical models scale active fluorescence measurements to generate estimates of electron transport rates by photosystem II (P e ), whose reductant yield fuels carbon fixation and growth. The derivation of P e is shown in Eq. 1 as the product of photon irradiance (EðkÞ), the absorption coefficient of photosystem II (PSII) light-harvesting pigments (a LHII ðkÞ), and E-dependent measurements of the quantum yield of PSII pho...

Heat tolerance of plants related to cell membrane thermostability is commonly estimated via the measurement of ion leakage from plant segments after defined heat treatment. To compare heat tolerance of various plants, it is crucial to select suitable heating conditions. This selection is time-consuming and optimizing the conditions for all investigated plants may even be impossible. Another problem of the method is its tendency to overestimate basal heat tolerance. Here we present an improved ion leakage method, which does not suffer from these drawbacks. It is based on gradual heating of plant segments in a water bath or algal suspensions from room temperature up to 70-75°C. The electrical conductivity of the bath/suspension, which is measured continuously during heating, abruptly increases at a certain temperature T (within 55-70°C). The T value can be taken as a measure of cell membrane thermostability, representing the heat tolerance of plants/organisms. Higher T corresponds to higher heat tolerance (basal or acquired) connected to higher thermostability of the cell membrane, as evidenced by the common ion leakage method. The new method also enables determination of the thermostability of photochemical reactions in photosynthetic samples via the simultaneous measurement of Chl fluorescence.