Deep-Sea Research II 49 (2002) 3001–3016 Vertical and temporal variability of DMSP lyase activity in a coccolithophorid bloom in the northern North Sea Michael Steinke a, *, Gill Malin a , Stuart W. Gibb b , Peter H. Burkill c a School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK b North Highland College, University of the Highlands and Islands, Thurso KW14 7EE, UK c Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK Received 18 June 2001; received in revised form 14 November 2001; accepted 29 January 2002 Abstract The climatically relevant trace gas dimethyl sulphide (DMS) is produced within the microbial food-web from the algal metabolite dimethylsulphoniopropionate (DMSP). The presence of DMSP lyase isozymes is necessary for this process. Measurements of in vitro DMSP lyase activity (DLA) were conducted in the northern North Sea in June 1999 in order to investigate the vertical and temporal variability of activity in a Lagrangian time-series process study. DLA ranged from 4 to 207nMh 1 , with maximum values close to the surface and between 30 and 50m depth. DLA increased towards the surface relative to chlorophyll a, as did the non-photosynthetic but photoprotective pigment diadinoxanthin, DMS and dissolved dimethylsulphoxide, a likely oxidation product of DMS. These observations support the hypothesis that DMSP lyases can be affected by irradiance levels, and that DMSP and its cleavage products could be involved in scavenging oxygen radicals; hence, they may function as antioxidants in marine algae. Linear regression analysis of our field data showed reduced biomass of some oligotrich and non-oligotrich ciliates at higher levels of DLA, a finding that could be supportive of a role for phytoplankton DMSP lyases in chemical defence. r 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction The marine algal secondary metabolite dimethyl- sulphoniopropionate (DMSP) accumulates to high concentrations in some unicellular phytoplankton (Keller et al., 1989), and enzymatic cleavage of DMSP is the primary source of the dimethyl sulphide (DMS) found in the sea. This volatile trace gas is of importance for the biogeochemical cycling of sulphur at the global scale, since its emission to the air provides a link between the oceanic sulphur pool and the terrestrial environ- ment (Lovelock et al., 1972; Andreae, 1990). Atmospheric oxidation products of DMS can result in an increased formation of cloud condensation nuclei (Liss et al., 1997). Hence, algal production of DMSP in the remote open ocean and subsequent formation of DMS may be linked to climate via a negative feedback mechanism that could counteract global warming (Charlson et al., 1987). The quantity of marine biogenic DMS available for sea-to-air exchange results from the balance between several production and consumption processes that involve the transformation of *Corresponding author. Tel.: +44-1603-593399; fax: +44- 1603-507719. E-mail address: m.steinke@uea.ac.uk (M. Steinke). 0967-0645/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0967-0645(02)00068-1