Occurrence of chlorophyll allomers during virus-induced mortality and population decline in the ubiquitous picoeukaryote Ostreococcus tauri.

During viral infection and growth limitation of the picoeukaryote Ostreococcus tauri, we examined the relationship between membrane permeability, oxidative stress and chlorophyll allomers (oxidation products). Chlorophyll allomers were measured in batch-cultures of O. tauri in parallel with maximum quantum efficiency of photosystem II photochemistry (Fv /Fm ), carotenoids, and reactive oxygen species and membrane permeability using fluorescent probes (CM-H2 DCFDA and SYTOX-Green). Viral infection led to mass cell lysis of the O. tauri cells within 48 h. The concentration of the allomer hydroxychlorophyll a peaked with a 16-fold increase (relative to chlorophyll-a) just after the major lysis event. In contrast, cell death due to growth limitation resulted in a twofold increase in allomer production, relative to chl-a. Two allomers were detected solely in association with O. tauri debris after viral lysis, and unlike other allomers were not observed before viral lysis, or during cell death due to growth limitation. Conversely, the component chl-aP276 was found in the highest concentrations relative to chl-a, in exponentially growing O. tauri. The components described have potential as indicators of mode of phytoplankton mortality, and of population growth.

Protection of cells from salinity stress by extracellular polymeric substances in diatom biofilms.

Diatom biofilms are abundant in the marine environment. It is assumed (but untested) that extracellular polymeric substances (EPS), produced by diatoms, enable cells to cope with fluctuating salinity. To determine the protective role of EPS, Cylindrotheca closterium was grown in xanthan gum at salinities of 35, 50, 70 and 90 ppt. A xanthan matrix significantly increased cell viability (determined by SYTOX-Green), growth rate and population density by up to 300, 2,300 and 200%, respectively. Diatoms grown in 0.75% w/v xanthan, subjected to acute salinity shock treatments (at salinities 17.5, 50, 70 and 90 ppt) maintained photosynthetic capacity, Fq'/Fm', within 4% of pre-shock values, whereas Fq'/Fm' in cells grown without xanthan declined by up to 64% with hypersaline shock. Biofilms that developed in xanthan at standard salinity helped cells to maintain function during salinity shock. These results provide evidence of the benefits of living in an EPS matrix for biofilm diatoms.