Project: A Matter of Life or Death? Assessing the physiological roles of PCD-related genes to stress adaptation in diatoms

From NSF Award Abstract:
Diatoms are a class of unicellular phytoplankton that account for ~40% of total marine primary productivity in the modern ocean. Since downward fluxes of biogenic silica and organic matter in the modern ocean derive largely from diatom productivity, there is increased interest in the mechanistic processes that regulate their fate. Indeed, the physiological state and life history of diatom assemblages dictate whether diatom Si and its associated organic C are either recycled in the euphotic zone, or exported to depth, placing mechanistic importance on cellular processes mediating bloom to post-bloom transition in response to nutrient stress. Programmed cell death (PCD) triggered by specific environmental stresses (e.g., Fe starvation, viral infection, high light, oxidative stress, and UV exposure) has now been documented in a variety of prokaryotic and eukaryotic unicellular phytoplankton, including diatoms. It provides a mechanistic framework to help explain lysis rates independent of grazing. The expression and activation of metacaspases, putative death proteases, in stressed diatoms, suggest PCD is an integral pathway in these organisms. Currently, the ecological role(s) of PCD-related genes in unicellular phytoplankton and the evolutionary drivers selecting for their retention remain unknown. Their preservation would seem to provide a negative selection pressure, yet their retention and maintenance suggests some sort of ancient, selective advantage. Metacaspases (and other PCD-related genes) may have co-evolved with other metabolic pathways whereby retention and low-level expression served housekeeping or regulatory functions. Elucidation of the ecological role(s) of PCD-related genes, like metacaspases, in diatom field populations requires an understanding of cellular function(s) and induction under relevant stressors. The premise of this proposal is that a subset of putative, PCD-related genes plays heretofore-unappreciated roles in stress adaptation in marine diatoms. The PIs will investigate whether Fe and N availability differentially regulate their expression and activity in T. pseudonana and in coastal diatoms from the California coast. The researchers will elucidate whether distinct PCD-related genes confer increased fitness under Fe or under N-limitation. This research follows from the investigators previous results that a subset of PCD-related genes is differentially expressed in T. pseudonana cells in response to Fe stress.

Specific hypotheses are:
(1) Expression and activity of PCD-related genes are controlled by Fe or N availability;
(2) Overexpression of putative PCD-related genes confers increased fitness under nutrient limitation;
(3) Diverse T. psuedonana metacaspases share functional epitope similarities; and
(4) PCD-related genes in coastal diatoms display differential responses to steady-state Fe and N limitation.

This work integrates physiology, biochemistry, genetics, ultra-trace metal clean techniques and field-based sampling in order to elucidate the ecological function of metacaspases in diatoms and to identify their roles and regulation in natural diatom populations under Fe versus N limitation. This novel interdisciplinary approach is particularly suited to address the roles of these fascinating enzymes.

Peer-reviewed Publications:
Kimberlee Thamatrakoln, Benjamin Bailleul, Christopher M. Brown, Maxim Y. Gorbunov, Adam B. Kustka, Miguel Frada, Pierre A. Joliot, Paul G. Falkowski, and Kay D. Bidle. 2013. Death-specific protein in a marine diatom regulates photosynthetic responses to iron and light availability.  Proceedings of the National Academy of Sciences; Early Edition, EE. DOI: 10.1073/pnas.1304727110