Respiratory metabolism in the marine macroalga, Ulva spp: exploratory studies with the respiratory electron transport system (ETS)

Abstract

Respiration is an indicator of metabolism in individual organisms and in ecological populations, and along with other data can provide carbon flow rates through food webs. Here, we studied the metabolism of the green algae, Ulva rotundata Blidingand Ulvarigida C.Agardhthatinhabit intertidal pools along the coast of Gran Canaria. To do this we explored the use of the respiratory electron transport system (ETS) in a pilot study of the variability of potential respiration in Ulva and its relationship with chlorophyll and with dry mass. We studied both field and culture samples. In the initial part of our exploration we learned that homogenization with a Potter-Elvehjen tissue grinder yielded much more enzyme activity per sample than grinding in liquid nitrogen with a mortar and pestle. Furthermore, we learned that NADPH dehydrogenase, not NADH dehydrogenase, was the dominant contributor to the overall ETS activity. The contribution of succinate dehydrogenase activity is negligible. ETS activity as a metabolic proxy has been successfully applied to study many different organisms in the ocean including bacteria, phytoplankton and zooplankton, but it has not been used to study marine macroalgae. These neritic and littoral macrophytes have major ecological and industrial importance, yet little is known about their respiratory physiology. Such knowledge would strengthen our understanding of the resources of the coastal ocean and facilitate its development and best use. In this first phaseof our research we have learned that potential respiration (φ) in Ulva ranges from 0.56μmol O2min-1(g dry mass)-1to 1.24 μmol-1O2min-1(g dry mass)-1.We havelearned that the relationship between changes in potential respiration and dry mass in Ulva follows the equation:φ=0,36*DM +0,013, r2= 0,8363(n = 29);changes in potential respiration and chlorophyllain Ulvaspp. follows the equation: φ= 1,19*Chla -1,8, r2= 0,8556(n =29); changes in potential respiration and chlorophyllbin Ulvaspp. follows the equation: φ= 0,70*Chlb -1,93, r2= 0,8643(n =29). Finally, in a pilot time-course experiment to determine the impact of nutrient-limitation on the metabolic capacity in Ulva, we learned that Ulvadoes not have sufficient reserves to maintain its potential respiration for a week. It would lose between 25 to 60% of its initial potential respiration over this time period. In this first phase of our research we have developed the methodology for the homogenization of Ulva spp., and used a standard spectrophotometric based kinetic enzyme assay to describe the impact of nutrient limitation on the metabolic capacity in Ulva spp. samples collected in the wild and maintained in controlled cultures during a week, being not enough to bring the algae to total starvation conditions.