Comparative functional characteristics of DMSP lyases extracted from polar and temperate Phaeocystis species

Abstract

Members of the marine phytoplankton genus Phaeocystis (Prymnesiophyceae) produce large amounts of the intracellular osmolyte DMSP and they are known to also produce lyase enzymes that cleave DMSP into the biogeochemically important trace gas DMS. The functional characteristics of DMSP lyase activity in Phaeocystis spp. are not well known. We characterized DMSP lyase activity in extracts from 2 ecologically important species from this genus, the mesophile P. globosa (strain CCMP629) and the psychrophile P. antarctica (strain CCMP1374). Results from whole cell extracts showed that both algal species were potent producers of DMSP lyase, with Michaelis-Menten constant (K m ) and maximum reaction velocity (V max , respectively, for P. antarctica. The optimal DMSP lyase activity was recorded at pH 4 and 30°C for P. globosa, and at pH 5 and 20°C for P. antarctica. The half-life of the DMSP lyase of P. globosa was 210 min at 25°C, which was longer than that of the P. antarctica enzyme (61.9 min). First-order kinetic analysis of DMSP lyase thermal denaturation demonstrated that the activation energy, free energy, enthalpy and entropy of denaturation in P. antarctica extracts were lower than for P. globosa extracts, confirming that the P. antarctica DMSP lyase was more thermolabile than the lyase from the temperate strain. Inhibitor tests with metals, a chelator (EDTA) and a serine binding agent (PMSF) suggested that the DMSP lyases from both Phaeocystis species were metalloenzymes with serine and sulfhydryl groups at the active site. The acidic pH optima for the Phaeocystis strains are consistent with findings from other Prymnesiophyceae, and we speculate that this may reflect adaptation to an acidic sub-cellular location for the DMSP lyase. KEY WORDS:Dimethylsulfide · Dimethylsulfoniopropionate · DMSP · DMSP lyase · Prymnesiophyte · Psychrophile Resale or republication not permitted without written consent of the publisherAquat Biol 18: 185-195, 2013 186 via the enzyme DMSP demethylase to 3-(methylmercapto) propionic acid (MMPA). In a subsequent reaction, MMPA either undergoes demethylation to form 3-mercaptopropionic acid and/or demethiolation to form methanethiol (Reisch et al. 2011). In the second pathway, DMSP lyase (EC 4.4.1.3) catalyzes the lysis of DMSP into dimethylsulfide (DMS), acrylate and a proton. There is worldwide interest in evaluating the biochemical and molecular mechanisms involved in the regulation of enzymatic cleavage of DMSP to DMS because DMS is an important component of the global sulfur cycle and a potential climate-active trace gas. DMS is supersaturated in all surface ocean waters and, once transferred to the atmosphere, it rapidly oxidizes to dimethylsulfoxide, methanesulfonate (MSA) or sulfate. Sulfur-containing, acidic aerosols derived from these compounds can affect the Earth's climate system by scattering incoming sunlight and contributing to the population of aerosol particles that act as cloud condensation nuclei (Charlson et al. 1987, Simó 2001.DMSP lyases are the key...

Effects of iron limitation and UV radiation on Phaeocystis antarctica growth and dimethylsulfoniopropionate, dimethylsulfoxide and acrylate concentrations

“…4b) further supports this supposition and net conversion of DMSP to acrylate and DMS, with much of the DMS converted to DMSO c , followed by active transport or diffusion of acrylate and DMSO into the dissolved phase. This conclusion would hold irrespective of whether DMSP lyase is primarily located intracellularly as suggested by Mohapatra et al [23] or extracellularly as proposed by Stefels and Dijkhuizen [22] ; however, if extracellular lysis is important then there would need to be a significant diffusive flux of DMS into the cell to support the cellular DMSO concentrations that were observed, especially in the growth curve experiment conducted in the Percival incubator where cultures were not exposed to UVR, making the possibility for oxidative conversion of dissolved DMS to DMSO unlikely.…”

“…[19,20] P. antarctica is a high-DMSP producer with average cellular DMSP concentrations between 120 and 310 mmol L À1 that increase under iron limitation. [3] DMSP can undergo enzymatic cleavage to acrylate and dimethylsulfide (DMS), [21][22][23][24] and DMS can be further oxidised to dimethylsulfoxide (DMSO), largely through reactions with ROS in the cell. [25] Several hypotheses have been proposed to explain high cellular DMSP concentrations in marine algae including cryoprotection, [26,27] osmoregulation, [28] prey deterrence, [29,30] an antioxidant function [25] and an overflow mechanism for excess carbon and energy.…”

Effect of PAR irradiance intensity on Phaeocystis antarctica (Prymnesiophyceae) growth and DMSP, DMSO, and acrylate concentrations

“…Since there is currently no evidence for the biochemical production of DMSO c by marine phytoplankton, the only pathway for DMSO c production would be through the abiotic reaction of DMS and DMSP with ROS as part of the antioxidant mechanism proposed by Sunda et al (2002). The constitutively high DMSP‐lyase activity in Phaeocystis (Mohapatra et al, 2013, 2014; Stefels & Dijkhuizen, 1996) allows for some DMSP to be converted to DMS. Dimethylsulfide, and to a lesser extent DMSP, react with the hydroxyl radical and other ROS (e.g., singlet oxygen) to form DMSO as a main reaction product (Adewuyi & Carmichael, 1986; Amels et al, 1997; Foote & Peters, 1971; Sunda et al, 2002).…”

Modelling dimethylsulfide diffusion in the algal external boundary layer: implications for mutualistic and signalling roles

“…Intracellular production of DMS from DMSP lysis is suggested from several recent studies (Franklin et al, 2010;Mohapatra et al, 2013;Mohapatra et al, 2014;Alcolombri et al, 2015). However, work by Stefels and Dijkhuizen (1996) suggests that DMSP-lyase is located on the cell surface in some algae, which would affect cell-size scaling of the DMS production flux.…”