Alga versus Diazotroph: How Did Prasinophytes Outcompete Nitrogen Fixers in Stratified, N-limited Devonian Epeiric Seas?
Abstract
Black shale forming ecosystems in stratified Late Devonian epeiric seas were chronically nitrogen limited due to the anoxic regeneration of mineralized organic P and the cellular stoichiometry of prasinophyte green algae. Although depleted nitrogen isotope values in black shales have been interpreted as representing a significant contribution by diazotrophic N fixation to the overall N budget, more recent analyses point to redox as the dominant control on δ15N. Despite low water column N:P, the abundance of prasinophyte reproductive cysts suggests a dominantly eukaryotic phytoplankton community rather than one dominated by diazotrophic cyanobacteria. Based upon elemental and stable isotope evidence from the Late Devonian Indiana Basin, we propose that prasinophyte green algae consistently outcompeted diazotrophs by means of physiological adaptations to an N-limited environment. The initiation of a CO2 concentrating mechanism allowed prasinophytes to optimise the carboxylase function of the N-rich enzyme RUBISCO which resulted in the δ13C enrichment of algal biomass. A shift in biomolecular production from N-rich proteins to N-poor lipids permitted prasinophytes to dominate planktonic biomass in spite of N-limitation. Extraordinary N fixation was not the inevitable consequence of low N:P where the stoichiometric plasticity of eukaryotic algae enabled adaptation to a broad range of relative macronutrient abundances.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPP33C2129T
- Keywords:
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- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 4950 PALEOCEANOGRAPHY / Paleoecology