Abstract
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.
Highlights
Most metabolically active Cu is contained in three metalloproteins: plastocyanin (PC), an electron carrier involved in the light reactions of photosynthesis;[8,9] cytochrome c oxidase (COX), the terminal enzyme in the respiratory electron transport chain;[4] and multicopper oxidase (MCO), part of the high-affinity iron assimilation pathway.[4,10]
More than 65% of down-regulated proteins we identified were predicted to function in photosynthesis, including light harvesting, electron transport and carbon fixation (Table 1)
The results reported here allow us to describe the primary metabolic adjustments of T. oceanica 1005 to Cu deficiency (Fig. 6)
Summary
Most metabolically active Cu is contained in three metalloproteins: plastocyanin (PC), an electron carrier involved in the light reactions of photosynthesis;[8,9] cytochrome c oxidase (COX), the terminal enzyme in the respiratory electron transport chain;[4] and multicopper oxidase (MCO), part of the high-affinity iron assimilation pathway.[4,10] Other essential cuproproteins, present in smaller amounts, include: amine oxidase, Cu–Zn superoxide dismutase, urate oxidase and tyrosinase.[11]. 1106 | Metallomics, 2020, 12, 1106--1117 Paper. Metallomics proteins in metabolic pathways that depend on or feed into pathways requiring Cu. Field experiments show Cu addition increases phytoplankton biomass and production rates, suggesting Cu is in short supply.[12,13] These results are consistent with lab studies showing diatom growth is limited by Cu concentrations similar to the open sea.[6,13,14] Copper-limited diatoms have significantly slower rates of Fe uptake and reduced photosynthetic efficiency compared to Cu-replete cells. The decrease in Fe uptake is attributed to a decrease in multicopper oxidase that provides Fe to the uptake system[10,13] and slower rates of photosynthetic electron flow are thought to result from a decrease in plastocyanin.[6,9] Cu deficiency affects specific physiological functions of diatoms that should be reflected in their proteomic profiles
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