Testing the possibility for photosynthetic compensation in an alga-invertebrate symbiosis under thermal stress: implications for carbon production and translocation

Abstract

Warner, Mark E.Thermal stress is driving the global decline in reef coral growth and survival. These cnidarians possess different levels of thermal tolerance driven, in part, by the symbiotic dinoflagellates they host. Chlorophyll a fluorescence is often used to assess photo-stress in these symbiotic algae. However, the underlying implications of photo-stress and how it affects downstream carbon production and translocation to the host have yet to be investigated. Active PSII reaction centers were manipulated in a thermally tolerant and thermally sensitive strain of Breviolum minutum by chemical inhibition with DCMU to examine how chlorophyll fluorescence, photosynthetic efficiency (α), and maximal photosynthesis (Pmax) changed. These parameters were then used to examine possible compensatory electron flow in steady-state electron turnover through functional PSII reaction centers (1/τPSII). In addition, acute heating response in vitro, as well as chronic heating of intact symbioses, were used to compare the effects of thermal stress and the relationship between PSII reaction center inactivation and other measures of photosynthesis. Values for α and Pmax declined with increasing DCMU more rapidly in the thermally susceptible symbiont strain than in the thermally tolerant strain with similar results seen both in vitro and in hospite. The thermally tolerant symbiont maintained carbon translocation during experimental heating, while translocation decreased in the thermally susceptible symbiont. Manipulation of PSII photochemistry by chemical titration or experimental heating provided evidence that the thermally tolerant symbiont sustained photochemical efficiency better than the thermally susceptible alga by maintaining or increasing compensatory electron flow.University of Delaware, School of Marine Science and PolicyM.S