Cellular viability and the occurence and significance of chlorophyll allomers during phytoplankton turnover.

Abstract

Phytoplankton can exist in the water column, whole but non-functional, and the percentage of these dead cells in highly variable. These dead cells can contain chlorophyll and contribute to ocean colour, and hence estimates of oceanic primary productivity. The aim of this project was to assess indicators of phytoplankton physiological state, focusing on the formation of chlorophyll a oxidation products (allomers) and a chlorophyll precursor. Initially, to establish an appropriate method for the identification and quantification of chlorophyll allomers, a method selection and optimisation study was carried out. This assessment revealed that chlorophyll was prone to oxidation during sample analysis. Instrumentation, sample manipulation, method duration and HPLC solvent composition were all contributors to sample oxidation. The application of a method by Zapata et al. (2000) was found to produce minimal and consistent chlorophyll oxidation and was applied in subsequent studies. During a culture study of the picoeukaryote Ostreococcus tauri (Prasinophyceae), two chlorophyll allomers were formed solely during viral-infection, and not during environmental limitation of growth. Allomers began to increase 24 hours post viral-infection (hpi), simultaneously with decreases in population density and Fv/Fm, and an increase in membrane permeability. During viral-infection allomers reached a maximum level 48 hpi, which was 10-fold higher than the maximum level of allomers formed during environmental limitation. Chlorophyll a allomers were measured over an annual cycle for the first time, at the Western Channel Observatory (UK). Allomer occurrence (relative to chl-a) was maximal during April with a total allomer to chl-a ratio of 0.093 in surface water. Peaks in allomers were associated with blooms of Phaeocystis spp., Guinardia delicatula, Chaetoceros socialis and Emiliania huxleyi and associations were dependent on the cause of the taxas’ declines. In situ allomer measurements were also taken during a research cruise in the central and southern North Sea, where the maximum ratio of allomers to chl-a (0.15) was measured at the Flamborough Front.