Defining the Molecular Basis for Energy Balance in Marine Diatoms under Fluctuating Environmental Conditions

The effectiveness of photorespiration as a protective mechanism against photooxidative stress and as a necessary pathway for the dissipation of excess photochemical energy in vascular C3 plants has been established (Kozaki and Takeba 1996, Wingler et al. 2000). Evidence for and against a functional photorespiratory pathway in photosynthetic chromoalveolates such as diatoms has been extensively debated along physiological and theoretical grounds. The extent of photorespiratory activity in marine diatoms is a critical physiological parameter with major implications for phytoplankton growth efficiency and pelagic carbon (C) and nitrogen (N) biogeochemistry. In vascular plants, photorespiratory CO2 release is estimated to be approximately 25% of the rate of net CO2 assimilation and NH3 loss (in the absence of recycling mechanisms) far exceeds primary NH3 assimilation from NO3 ! reduction (Sharkey 1988, Keys et al. 1978). Photorespiration is the light-dependent release of CO2 that results from glycolate metabolism. Light-dependent glycolate production occurs as a result of substrate competition between O2 and CO2 for the active site of RUBISCO, the enzyme responsible for photosynthetic CO2 fixation. Photorespiratory glycolate is subsequently converted to serine in the mitochondria through a pathway that involves the decarboxylation of glycine and release of CO2 and NH3. In C3 vascular plants, the rate of photorespiration increases at elevated O2/CO2 ratios and at higher temperatures because the specificity of RUBISCO for CO2 is diminished under these conditions (Brooks and Farquhar 1985, Tolbert et al. 1995). Other studies have documented a decrease in CO2 affinity and an overall reduction in Calvin cycle activity at colder temperatures (Yokota et al. 1989, Hutchison et al. 2000). It is likely that deviation above and below the optimal temperature for CO2 incorporation causes inefficiency in CO2 uptake mechanisms that lead to increased fluxes of carbon and nitrogen through photorespiratory pathways. Elevated levels of photorespiration are known to lead to a depletion of carbohydrates and accelerated senescence (Wingler et al. 2000). Numerous studies on diatoms and a variety of other alga taxa in the 1970s utilized in vivo isotopic labeling (O2 and CO2) and biochemical approaches to investigate metabolite fluxes and enzymatic activities for pathways related to photorespiratory and glycolate metabolism (Beardall 1989). Key findings include the detection of glycolate dehydrogenase activity in the mitochondria of the marine pennate diatoms Cylindrotheca fusiformis and Nitzscia alba (Paul et al. 1975). In vascular plants, green algae, and non-vascular cryptograms, glycolate is oxidized to glyoxylate in the peroxisome by glycolate oxidase (Somerville 2001). Other early findings concerning photorespiration in diatoms include the relative insensitivity of photosynthesis in the pennate diatom Phaeodactylum tricornutum, compared with C3 plants, to elevated O2 levels. In this aspect, P. tricornutum behaves more like the Cand Nefficient C4 plants, which have a variety of mechanisms for alleviating competition between O2 and CO2 by concentrating CO2 at the active site of RUBISCO, thereby reducing C and N losses associated with photorespiration (Beardall and Morris 1975). Also, it was shown that Thalassiosira pseudonana cells grown under different O2 concentrations had nearly constant rates of glycine and serine C labeling (Burris 1977). Therefore, although the existence of a photorespiratory pathway in diatoms was confirmed, it is evident that it differs substantially from that of C3 green lineage organisms. Recent evidence for the centric diatom Thalassiosira weissflogii does in fact support the idea that a significant fraction of the CO2 flux through RUBISCO is derived from C4 organic acids (Reinfelder et al. 2004). The existence of a CO2 concentrating mechanism (CCM) such as this one, and a variety of others involving different types of carbonic anhydrases (Lane and Morel 2000, Tanaka et al. 2005), appears to be antithetical to photorespiration and would in all likelihood greatly diminish the flux of C and N