Reactive oxygen production and scavenging in marine phytoplankton

Abstract

Phytoplankton are unicellular photosynthetic microorganisms adapted to live in open water. They use reactive oxygen species (ROS) for cell to cell signalling, although biotic or abiotically produced ROS can be harmful in higher concentrations and therefore phytoplankton must maintain ROS homeostasis. The Black Queen Hypothesis suggests that loss of function mutations carry on until the cost outweighs the benefit [1]. As it is harder for larger phytoplankton to get rid of ROS as it does not readily diffuse out of the cell, we investigated whether they would have more ROS detoxification genes, and fewer ROS producing genes. We indeed found a breakpoint at 3μm above which cells carry more genes encoding ROS scavenging. We could not determine whether there is an effect of cell size on the total genetic capacity for ROS production. The number of genes associated with ROS production and scavenging increased with larger genome size, although the slope was smaller than that of gene model size vs. genome size, indicating that less of the genome is allocated to ROS production and scavenging in larger genomes.