Antifreeze proteins in polar sea ice diatoms: diversity and gene expression in the genus Fragilariopsis

Abstract

Fragilariopsis is a dominating psychrophilic diatom genus in polar sea ice. The two species Fragilariopsis cylindrus and Fragilariopsis curta are able to grow and divide below freezing temperature of sea water and above average sea water salinity. Here we show that antifreeze proteins (AFPs), involved in cold adaptation in several psychrophilic organisms, are widespread in the two polar species. The presence of AFP genes (afps) as a multigene family indicated the importance of this group of genes for the genus Fragilariopsis, possibly contributing to its success in sea ice. Protein phylogeny showed the potential mobility of afps, which appear to have crossed kingdom and domain borders, occurring in Bacteria, diatoms, crustaceans and fungi. Our results revealed a broad distribution of AFPs not only in polar organisms but also in taxa apparently not related to cold environments, suggesting that these proteins may be multifunctional. The relevance of AFPs to Fragilariopsis was also shown by gene expression analysis. Under stress conditions typical for sea ice, with subzero temperatures and high salinities, F. cylindrus and F. curta strongly expressed selected afps. An E/G point mutation in the Fragilariopsis AFPs may play a role in gene expression activity and protein function.

Chlorophyll‐normalized isoprene production in laboratory cultures of marine microalgae and implications for global models

“…That said, the highest isoprene producer in this study, P. capsulatus CCMP, is also a member of the Chlorophyta, and similarly Shaw et al (2003) showed that the chlorophyte Micromonas putida CCMP 489 produced relatively high concentrations of isoprene. This suggests that the similarly low production rates measured here and elsewhere for D. tertiolecta (Bonsang et al 2010) appear to be species-specific, rather than a broader taxonomic phenomenon among chlorophytes, which is further supported by the relatively high isoprene production demonstrated by the macroalga Ulva intestinalis (Broadgate et al 2004). Clearly, given the ecological and biogeochemical importance of chlorophytes in aquatic environments, the nature and extent of variability within this class requires further attention.…”

“…Data shown are the means for all strains analyzed in triplicate during this study (white bars) from each taxonomic group 6 standard error (SE) normalized to Chl a. Results from previous research are shown in gray bars for comparison (Shaw et al 2003;Gantt et al 2009;Bonsang et al 2010). The numbers of samples are included above each data point, and refer to the number of strains analyzed.…”

“…This decision was based on the shorter incubation periods used in this study compared to others (Shaw et al 2003;Bonsang et al 2010), and thus the fact that diurnal variation was not accounted for here so that isoprene production measurements reported here are likely to be maximal values. Data from the literature have been converted to hourly production rates for comparison, by dividing by 14 based on the corresponding light cycle used for incubations and the fact that most if not all isoprene is produced during the light period (Shaw et al 2003;Sharkey et al 2008).…”

“…Isoprene production rates have been determined for 30 strains from 15 species of microalgae (Table 1; Shaw et al 2003Shaw et al , 2010Bonsang et al 2010), 10 species of temperate macroalgae (Broadgate et al 2004), and temperate intertidal microbial communities along an estuarine gradient (Acuñ a Alvarez et al 2009;Exton et al 2012). Additional studies have identified isoprene production in a further 11 strains from six microalgal species, but without quantification (Moore et al 1994;McKay et al 1996;Shaw et al 2010).…”

“…While these studies cover a generally broad taxonomic range of algae, the extent to which taxonomic and environmental variability regulates isoprene production is unclear. In situ studies have demonstrated several patterns of marine isoprene production, including seasonal variations (Broadgate et al 1997; Liakakou et al 2007;Exton et al 2012), and a positive correlation with chlorophyll a (Chl a; Bonsang et al 1992;Milne et al 1995;Moore and Wang 2006), but these are based on measurements from natural samples containing mixed algal communities. By resolving the uncertainty surrounding the taxonomic and environmental regulation of marine isoprene production, more accurate predictions can be made on how future environmental change may alter the contribution of marine isoprene production to global emissions.…”