Effects of free amino acids on the photosynthetic carbon metabolism of symbiotic dinoflagellates

Abstract

Synthetic host factor (SHF) was used in parallel with crude cnidarian’s host factor (CHF) to investigate their effects on both photosynthetic carbon metabolism and the first step of lipid synthesis in the symbiotic dinoflagellates Symbiodium purchrorum of the sea anemone Aiptasia pulchella. Several species have been studied, namely, sea anemone A. pulchella, reef coral Pocillopora damicornis, and green alga Chlamydomonas reinhardii. Both short-term and long-term experiments with radioactive carbon have shown a higher rate of the alga 14C photoaccumulation with host factor(s) than of dinoflagellates located in artificial sea water (ASW) alone. In dinoflagellates incubated with both ASW and CHF, 14C-labeled glycerol was detectable after 15 s of alga illumination. In dinoflagellates isolated from P. damicornis and incubated in CHF and SHF and in dinoflagellates isolated from A. pulchella and incubated in CHF, a higher percentage of 14C was found in the glycerol as compared to the ASW trial. At the same time, in ASW trial the radioactive label was primarily located in ethanol-soluble lipid fraction. Similar results were observed when dinoflagellates isolated from P. damicornis were incubated with aspartate or glutamate. But there was no effect with taurine, serine, valine, glycine, or lysine. C. reinhardii, incubated in salt-free CHF, partitioned a greater percent of 14C into the glycerol and less into the ethanol-soluble lipids as compared to the corresponding control incubations. The amount of 14C in neutral and polar lipids was identical in that in A. pulchella dinoflagellates incubated in ASW or CHF. The arrays of neutral 14C-lipids produced under both ASW alone and ASW with CHF conditions, and over time were not significantly distinguishable. Host factors appeared to provide an optimum environment to sustain maximum metabolic efficiency. A biochemical model, based on the quantitative and qualitative assessment of carbon pathways in dinoflagellates incubated both in host factor and sea water alone, is presented.