Cell death in phytoplankton: correlation between changes in membrane permeability, photosynthetic activity, pigmentation and growth

Abstract

Different stages of the automortality in phytoplankton have been studied applying flow cytometry. These stages are, in order of expression : (1) compromised cell membranes, (2) degradation of the photosynthetic pigments and reduction of the photosynthetic activity, (3) fragmentation of the genomic DNA. The integrity test of the cell membranes is based on the inability of the DNA-specific stain SYTOX Green to pass into cells with intact plasma membranes. The reduction in photosynthetic activity was examined by sorting "%C-labelled phytoplankton cells differing in viability. Finally, DNA fragmentation was traced by measuring changes in genomic DNA. The different phytoplankton species tested showed a great variety in response when grown under the same conditions, but there was also considerable intraspecific variation. Unstained cells, fully stained cells (equivalent to full staining of genomic DNA in fixed cells) and cells with intermediate fluorescence signal occurred together within the same culture. The photosynthetic activity in cells with a reduced viability dropped by as much as 60 % relative to that of the viable cells. In the subsequent stage, when photosynthetic pigments were fully degraded, this value dropped further to around 10 %. Cells in this stage also showed subdiploidy as a result of genome fragmentation. Field tests using samples of phytoplankton collected in the North Atlantic Ocean (40m N, 23m W) during spring showed staining properties similar to those found in cultures grown at suboptimal growth conditions. The percentage of non-viable cells varied considerably (ranging from 5 % to 60 %) between the various phytoplankton groups present. The lowest value was observed for Synechococcus, but some pico-eukaryotes showed percentages as high as 60 %. Moreover, the viability varied with depth (light level) and over a light-dark cycle. The present findings suggest the existence of a (genetically based) uniform process of automortality in phytoplankton. Non-viable cells are a substantial component of the oceanic phytoplankton, affecting the food-web structure and species succession.

Differences in cell viabilities of phytoplankton between spring and late summer in the northwest Pacific Ocean

“…4). In the North Atlantic Ocean (40ºN, 23ºW) during spring, Veldhuis et al (2001) also reported that cell viabilities of Synechococcus ranged from 75 to 90%, using the SYTOX Green membrane permeability assay. It is known that temperature is the dominant factor controlling the growth and loss of Synechococcus in colder waters (Li, 1998).…”

“…It should be noted that field studies of the cell death of marine phytoplankton remain limited to several regions: the North Sea (van Boekel et al, 1992;Brussaard et al, 1995;Riegman and Winter 2003), the Adriatic Sea (Baldi et al, 1997), the Mediterranean Sea (Agustí et al, 1998;Agustí and Duarte, 2000;Agustí and Sánchez, 2002;Alonso-Laita et al, 2005), and the Atlantic Ocean (Veldhuis et al, 2001;Agustí, 2004;Alonso-Laita and Agustí, 2006;Llabrés and Agustí, 2006). No information was available on the cell death of phytoplankton in the Pacific Ocean.…”

Size-fractionated phytoplankton biomass and production in the tropical Atlantic

“…Furthermore, recent studies have demonstrated that a significant fraction of the picophytoplankton community in the surface oligotrophic waters is non-viable, i.e. incapable of cell division (Veldhuis et al, 2001), while viability of picophytoplankton cells increases significantly at the DCM depth (Veldhuis et al, 2005). In agreement, our results demonstrate that picophytoplankton surface P/B was positively correlated with K d , thus revealing the negative effect of high solar radiation on picophytoplankton metabolic dynamics, especially in the oligotrophic oceanic tropical waters, where nutrient scarcity should also negatively affect the picophytoplankton capacity to resist solar radiation and repair associated damages (Banaszak, 2003).…”

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

“…Autocatalytic cell death is a process that is indicated by multiple changes within the phytoplankton cell and it is often difficult to characterize such cells as clearly dead or alive (Franklin et al, 2006(Franklin et al, , 2012. Cell membrane integrity is used as an early indicator of cell death (Veldhuis et al, 2001). Veldhuis et al (2001) showed that a significant proportion of phytoplankton populations grown in culture and in the field (North Atlantic during March) had compromised cell membranes, as indicated by SYTOX Green staining (Fig.…”

“…These processes include sloppy feeding by predators (Møller et al, 2003;Møller, 2007), lytic viral infections (Bratbak et al, 1993;Gobler et al, 1997, Bettarel et al, 2005, and cell death (Veldhuis et al, 2001). Moreover, living phytoplankton cells exude a significant proportion, and under some circumstances the majority, of their photosynthate into the surrounding medium (Fogg, 1983;Wood & Van Valen, 1990).…”