Physiological stress and cell death in marine phytoplankton: Induction of proteases in response to nitrogen or light limitation

Abstract

The physiological processes of natural phytoplankton mortality due to environmental stress (vs. that caused by sedimentation and predation) are poorly understood. Cell survival was examined in batch cultures of the diatom Thalassiosira weissjogii and the chlorophyte Dunaliella tertiolecta during deprivation of fixed nitrogen or light. Despite severe impairment of photosynthetic efficiencies, both species remained viable during 2 weeks of N starvation. Under N stress, a specific protease was induced in the diatom, overall activity of proteases doubled, and there was gradual, selective loss of certain proteins, especially ribulose 1,5bisphosphate carboxylase/oxygenase. Light-deprived diatoms were virtually unaffected, but the chlorophyte underwent catastrophic cell death after about 6 d of darkness. Cell death coincided with a large increase in protease activity and the induction of a specific protease. Although we cannot completely rule out roles for viruses or bacteria in the losses of cells, the consistent timing, the unique response to stress, and the coincident expression of a specific protease strongly suggest that the process is a form of autocatalyzed cell death, such as apoptosis. While of uncertain adaptive significance, phytoplankton cell death may have implications for species succession and cycling of organic matter in aquatic ecosysterns.

Toxin release in response to oxidative stress and programmed cell death in the cyanobacterium Microcystis aeruginosa

“…Exposure of cells to low levels of physical and chemical stressors have been known to trigger apoptosis-like conditions in a variety of photosynthetic organisms including vascular plants (Lam and del Pozo, 2000;Lam et al, 2001), unicellular chlorophytes (Berges and Falkowski, 1998;Segovia et al, 2003), dinoflagellates (Vardi et al, 1999;, and cyanobacteria (Berman-Frank et al, 2004). Characteristics of cells undergoing PCD include chromatin condensation, shrinkage of the nucleus and cytoplasm (Danon et al, 2000), and ordered DNA fragmentation (Hoeberichts and Woitering, 2003).…”

Transformation of iodate to iodide in marine phytoplankton driven by cell senescence

“…Nitrate reduction in phytoplankton strongly depends on light energy and will be hampered under these conditions (Berges & Falkowski 1998).…”

Bridging the Gap between Knowing and Modeling Viruses in Marine Systems—An Upcoming Frontier

“…In the ERSEM model, for instance, it was formulated that lysis products from phytoplankton groups were partly particulate and partly dissolved, with the particulate fraction dependent on the actual nutrient cell quota (Baretta-Bekker et al, 1995, 1997. For primary producers, lysis was defined to included factors such as mechanical damage (e.g., sloppy feeding Imai et al, 1993;Strom et al, 1997), nutrient shortage (Berges and Falkowski, 1998), virus and bacteria activity (Imai et al, 1993(Imai et al, , 1995. A way to achieve this consisted in defining a background lysis rate, an increased lysis rate proportional to nutrient stress and a phytoplankton density dependent term.…”