Photosynthetic rates of seagrasses have until recently been measured a s gas exchange of chamber-enclosed leaves mainly in the laboratory, and in situ measurements under natural conditions are scarce. In this work we explore the possibility of rneasunng such rates by pulse amplitude modulated (PAM) fluorometry, using a newly developed underwater device. This was done by first comparing photosynthetic O2 evolution (net photosynthesis corrected for dark respiration) with rates of electron transport (ETR) derived from fluorescen.ce measurements of the effective quantum yield of photosystem I1 multiplied with the estlnlated photon flux of photosynthetic active radiation absorbed by this photosystem. In the field, ETRs were then measured both as rapid light curves (RLCs) and by in situ point measurements under ambient light during the day. Photosynthetic O2 volution showed a linear relationship with ETR within a range of irradiances for the Mediterranean seagrass Cymodocea nodosa, while the tropical Halophila stipulacea and a temperate intertidal population of Zostera marina exhibited decreasing O2 evolution rates relative to ETRs at high lrradiances. These differences are likely due to photorespiration, w h~c h is absent in C. nodosa. The molar ratio between photosynthetic O2 evolution and ETR within the range of their linear relationship was found to be 0.3 for C. nodosa, which is close to the theoretical stoichiometric ratio of 0.25, but was higher and lower for 2. manna and H. stjpulacea, respectively. Point measurements of ETR in the field showed good agreements wlth rates derived from RLCs for H. stipulacea and Z. marina, but values varied greatly between replicate measurements for C. nodosa a t high irradiances. It is speculated that this variation was partly due to lightflecks caused by waves in the shallow water where these measurements were done. In all, this work shows that PAM fluorometry can efficiently yield photosynthetic rates for seagrasses in the laboratory, without the typical lag experienced by O2 electrodes, a s well a s in situ under natural conditions which are not disturbed by enclosures.
In this work, we attempt to quantify pulse amplitude modulated (PAM) chlorophyll fluorescence measurements in marine macroalgae in terms of photosynthetic rates. For this, the effective electron transfer quantum yield of photosystem II measured for two Ulva species, at various irradiances and inorganic carbon (Ci) concentrations, was multiplied by the estimated flux of photons absorbed by the photosynthetic pigments associated with this photosystem. The rates of electron transport (ETR) calculated in this way were then compared with rates of photosynthetic O # evolution as measured in association with the fluorescence measurements. It was found that the calculated ETRs correlated linearly with rates of ' gross ' O # evolution (net O # exchange corrected for dark respiration as measured immediately after turning off each irradiance level) within the range of irradiances applied (up to 608 µmol photons m −# s −"). The average molar O # \ETR ratio was 0n238 for Ulva lactuca and 0n261 for Ulva fasciata, which is close to the theoretical maximal value of 0n25. Rates of O # evolution at various concentrations of Ci also showed linear correlations with ETR, and the average molar O # \ETR ratio was 0n249. These results show that PAM fluorometry can be used as a practical tool for quantifying photosynthetic rates at least under moderate irradiances in thin-bladed macroalgae such as Ulva possessing a CO #-concentrating system. A comparison between the PAM-101 (which was used in Sweden for the light-and Ci-response measurements of Ulva lactuca) and the newly developed portable Diving-PAM (used for Ulva fasciata in Israel) showed that such fluorescence-based photosynthetic rate measurements can also be carried out in situ.
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