Identification of copper-regulated proteins in an oceanic diatom, Thalassiosira oceanica 1005†
Copper (Cu) is an essential cofactor of photosynthetic and respiratory redox proteins in phytoplankton and a scarce resource in parts of the open sea. Although its importance for growth is well recognized, the molecular mechanisms by which phytoplankton respond and acclimate to Cu deficiency are not well known. In this study, we identified the dominant Cu-regulated proteins and measured key physiological traits of Thalassiosira oceanica (CCMP 1005) under Cu-limiting and sufficient conditions. Growth limitation of T. oceanica occurred at environmentally relevant Cu concentrations (1 nM) as a result of decreased photosynthetic efficiency (ΦPSII). In Cu-limited cells, levels of plastocyanin decreased by 3-fold compared to Cu-replete cells and rates of maximum photosynthetic electron transport were reduced. Proteins associated with light harvesting complexes also declined in response to Cu limitation, presumably to adjust to reduced photosynthetic electron flow and to avoid photodamage to the photosystems. Key enzymes involved in carbon and nitrogen assimilation were down-regulated in low-Cu cells, as were steady state rates of C and N uptake. Relatively fewer proteins were up-regulated by Cu limitation, but among them were two enzymes involved in fatty acid oxidation (FAO). The increase in FAO may be a sign of increased turnover of cellular lipids caused by damage from oxidative stress. A putative transcription factor containing three, repetitive methionine motifs (MpgMgggM; MpgMggM) increased significantly in Cu-limited cells. The collective results provide a general description of how plastocyanin-dependent diatoms adjust metabolism to cope with chronic Cu deficiency.