Aquatic Organic Geochemistry

This research is a collaboration with Ginger Armbrust and Jennifer Cherrier. 

Photorespiration occurs when O2 outcompetes CO2 for binding with the carbon-fixation enzyme RUBISCO. Fixation of O2 rather than CO2 results in the formation of a two-carbon molecule, 2-P-glycolate, that cannot enter the Calvin cycle. Instead, 2-P-glycolate is metabolized in the photorespiratory pathway, producing a succession of organic carbon and nitrogen compounds including glycolate, glycine, and serine. Ultimately, 3-P-glycerate is generated and enters the Calvin cycle. However, one of every four carbons that enters the photorespiratory pathway is used to produce CO2, and the conversion of glycine to serine results in the formation of NH3. In marine algae, high light and high concentrations of O2 are known to enhance photorespiration and promote leakage of organics such as the photorespiratory-specific compound glycolate. Thus, photorespiration may play a key role in mediating the release of fixed carbon and nitrogen from cells under conditions of high light or high O2.

This diagram illustrates the linakges between phytoplankton and bacteria
that is mediated through the transfer of energy during photorespiration.
From Winnie Lau, PhD Candidate in the MMBL.

We are collaborating with scientists in the UW School of Oceanography’s Marine Molecular Biology Laboratory and at Florida A and M University to investigate glycolate cycling in the upper ocean. The goal in the MOG group is to develop sensitive techniques to measure organic compound cycling in response to photorespiration.

Glycolate concentrations are measured by HPLC using a modification of the 2-nitrophenylhydrazide method of Albert and Martens (1997 Mar. Chem. 56:27-37). Samples are derivatized with 2-nitrophenylhydrazide for 1.5 h at room temperature and loaded onto a 1.5-cm concentrating column (C-18, Beckman Coulter Inc.) within the injection loop of a Gilson (model 231) autosampler (Gilsen Inc., Middleton, WI, USA). After washing the concentrator column with water, samples are injected onto a 25-cm Beckman Ultrasphere C18 column (Beckman Coulter Inc.), and glycolate is separated from other organic acids using the ion-pairing solvent system of Albert and Martens (1997). An HPLC system with an SPD-10Avvp uv-vis detector set to 400nm is used for sample detection (Shimadzu, Columbia, MD, USA).

This is Figure 6 from Parker et al. (2004) (see our publications page for this article from Micaela Parker). It shows extracellular concentrations of glycolate in the culture media of T. Weisflogii. (A) Cells were maintained in continuous light of 15 mmol photons m–2 s–1 (open triangles) and 100 mmol photons m–2 s–1 (open squares) or acclimated to 100 mmol photons m–2 s–1, transferred to the dark for 24 h, and reintroduced to 100 mmol photons m–2 s–1 (closed squares). (B) Cells were acclimated to 15 mmol photons m–2 s–1 (closed triangles) or 50 mmol photons m–2 s–1 (closed circles) of continuous light, transferred to the dark for 24 h, and introduced to 400 mmol photons m–2 s–1. Shading indicates 2 SDs from the mean of the 15 mmol photons m–2 s–1 continuous light culture. Error bars are SDs for three replicate measurements from the same flask. Duplicate symbols indicate duplicate measurements from the same flask. Black bar represents the time the cultures (closed symbols) spent in the dark.

As is shown in Panel B, when the phytoplankton are light stressed, they release glycolate into the media. But how do the bacteria respond? Winnie Lau has been exploring the genetics of bacterial utilization of glycolate. She has evaluated this in several contrasting marine environments and discovered that the diversity of glycolate utilization is locationally-specific. Check out Winnie’s web page for more information on her PhD research.