Although algae are responsible for ~40% of global CO2 fixation, this process is limited by iron (Fe) availability in about 30% of the World Ocean. We still don’t understand how algae compete for this scarce nutrient or how they minimize their Fe demand in Fe poor waters. In other regions of the ocean where Fe is not limiting, algae exert energy to concentrate CO2 from seawater, yet we have a poor understanding of the mechanism(s) employed, which groups use which mechanisms or how a CO2 rich world may affect the relative fitness of these groups.
Our research is interdisciplinary by combining emergent genome- enabled methods with classic physiological and kinetic approaches in the lab and field. An exciting era is upon us where the environmental physiology and underlying biochemistry of algae can be studied in unprecedented ways due to advances in –omics and reverse genetics. As described below, our lab is increasingly utilizing the power of proteomics to help address some of these questions.
Carbon Assimilation in Marine Phytoplankton
During photosynthesis, RuBisCO is ironically inefficient at converting CO2 into sugars in the presence of O2, and the observed fast photosynthetic rates of diatoms are only achieved when CO2 levels are somehow elevated near RuBisCO. Establishing the mechanism for this process has been elusive – with controversies borne from important differences in experimental design. Prof. Kustka tackled this problem with a combination of Q-PCR, photo-physiological and so-called shotgun (or discovery-based) proteomic approaches. We constructed a putative and novel pathway for single-celled C4 metabolism, most notably involving the decarboxylation of the C4 compound OAA to pyruvate and bicarbonate via a reversible pyruvate carboxylase (PYC). Much more needs to be done to test this novel model, and we are actively pursuing this.
Iron Uptake Mechanism
Fe supply to the oceans is a critical driver of global biogeochemistry and climate. Resolving the key physiological question regarding which forms of Fe are actually taken up will help constrain global biogeochemical models linking Fe supply and C sequestration. Our preliminary data with the diatom Thalassiosira pseudonana reveals two key low Fe-responsive, cell surface proteins, a finding that challenges our canonical understanding of iron uptake. We have recently been funded by NSF to further investigate this by integrating newly developed reverse genetics tools with proteomic and kinetic approaches. Also, with collaborators at Scripps Institute of Oceanography, we have recently published work showing that other diatoms utilize an iron uptake mechanism that resembles that used by human red blood cells, through an fascinating case of convergent evolution.
Metal Substitution in Phytoplankton
Our group is generally interested in metabolism of nutritive as well as toxic (pollutant) metals. Some metals are toxic because they either competitively inhibit uptake of nutrient metals, interfere with proper intracellular enzyme function, or both. We have a particular interest in the nutritive use of cadmium in marine coccolithophores. These phytoplankton grow faster when supplied with cadmium and low zinc compared to low Zn conditions alone. This use of Cd as a nutrient would be consistent with the low Zn environments that coccolithophores occupy and would have important ramifications for the use of foraminiferal Cd/Ca ratios as paleoproxies for primary production. This use of Cd does not seem to be related to CO2 metabolism (which, in diatoms, is the only known biological role for Cd). David Shire (a PhD candidate in my group) is using metallomic approaches to isolate, identify and quantify Cd-containing proteins. He is interested in the evolutionary pressures leading to selection of Cd as a micronutrient and the impact of this use on Cd biogeochemistry. A significant portion of his work involves quantitative proteomics and metalloproteomics.
Micronutrient Deficiencies and Oxidative Stress in Phytoplankton
Iron and manganese are required for photosynthesis and for dealing with oxidative stress as a normal facet of aerobic metabolism. We’ve disrupted the expression of a protein in the diatom Thalassiosira pseudonana that has altered their metabolism of these metals. We are testing hypotheses involving the role of this protein in supplying these metals to the chloroplast for photosynthetic reaction centers and antioxidants.