Measuring photosynthetic rates in seagrasses by pulse amplitude modulated (PAM) fluorometry

Abstract

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.