Jodi N. Young

Rubisco is a small fraction of total protein in marine phytoplankton

Ribulose 1,5 bisphosphate carboxylase oxygenase (Rubisco) concentrations were quantified as a proportion of total protein in eight species of microalgae. This enzyme has been assumed to be a major fraction of total protein in phytoplankton, as has been demonstrated in plants, potentially constituting a large sink for cellular nitrogen. Representative microalgae were grown in batch and continuous cultures under nutrient-replete, nitrogen (N)-limited, or phosphorus (P)-limited conditions with varying CO(2). Quantitative Western blots were performed using commercially available global antibodies and protein standards. Field incubations with natural populations of organisms from the coast of California were conducted under both nutrient-replete and N-limited conditions with varying CO(2). In all experiments, Rubisco represented < 6% of total protein. In nutrient-replete exponentially growing batch cultures, concentrations ranged from 2% to 6%, while in nutrient-limited laboratory and field cultures, concentrations were < 2.5%. Rubisco generally decreased with increasing CO(2) and with decreasing growth rates. Based on a calculation of maximum Rubisco activity, these results suggest that phytoplankton contain the minimum concentration of enzyme necessary to support observed growth rates. Unlike in plants, Rubisco does not account for a major fraction of cellular N in phytoplankton.

Large variation in the Rubisco kinetics of diatoms reveals diversity among their carbon-concentrating mechanisms

Highlight Broad variations in the CO2 fixation kinetics of diatom Rubisco indicate novel mechanistic diversity and large differences in their carbon-concentrating mechanism.

Metabolic balance of coastal Antarctic waters revealed by autonomous pCO2 and ΔO2/Ar measurements

We use autonomous gas measurements to examine the metabolic balance (photosynthesis minus respiration) of coastal Antarctic waters during the spring/summer growth season. Our observations capture the development of a massive phytoplankton bloom and reveal striking variability in pCO2 and biological oxygen saturation (ΔO2/Ar) resulting from large shifts in community metabolism on time scales ranging from hours to weeks. Diel oscillations in surface gases are used to derive a high-resolution time series of net community production (NCP) that is consistent with 14C-based primary productivity estimates and with the observed seasonal evolution of phytoplankton biomass. A combination of physical mixing, grazing, and light availability appears to drive variability in coastal Antarctic NCP, leading to strong shifts between net autotrophy and heterotrophy on various time scales. Our approach provides insight into the metabolic responses of polar ocean ecosystems to environmental forcing and could be employed to autonomously detect climate-dependent changes in marine primary productivity.

Slow carboxylation of Rubisco constrains the rate of carbon fixation during Antarctic phytoplankton blooms

High-latitude oceans are areas of high primary production despite temperatures that are often well below the thermal optima of enzymes, including the key Calvin Cycle enzyme, Ribulose 1,5 bisphosphate carboxylase oxygenase (Rubisco). We measured carbon fixation rates, protein content and Rubisco abundance and catalytic rates during an intense diatom bloom in the Western Antarctic Peninsula (WAP) and in laboratory cultures of a psychrophilic diatom (Fragilariopsis cylindrus). At -1°C, the Rubisco turnover rate, kcat (c) , was 0.4 C s(-1) per site and the half saturation constant for CO2 was 15 μM (vs c. 3 C s(-1) per site and 50 μM at 20°C). To achieve high carboxylation rates, psychrophilic diatoms increased Rubisco abundance to c. 8% of biomass (vs c. 0.6% at 20°C), along with their total protein content, resulting in a low carbon : nitrogen ratio of c. 5. In psychrophilic diatoms, Rubisco must be almost fully active and near CO2 saturation to achieve carbon fixation rates observed in the WAP. Correspondingly, total protein concentrations were close to the highest ever measured in phytoplankton and likely near the maximum possible. We hypothesize that this high protein concentration, like that of Rubisco, is necessitated by slow enzyme rates, and that carbon fixation rates in the WAP are near a theoretical maximum.

Gross and net production during the spring bloom along the Western Antarctic Peninsula

This study explores some of the physiological mechanisms responsible for high productivity near the shelf in the Western Antarctic Peninsula despite a short growing season and cold temperature. We measured gross and net primary production at Palmer Station during the summer of 2012/2013 via three different techniques: incubation with H2 (18) O; incubation with (14) CO2 ; and in situ measurements of O2 /Ar and triple oxygen isotope. Additional laboratory experiments were performed with the psychrophilic diatom Fragilariopsis cylindrus. During the spring bloom, which accounted for more than half of the seasonal gross production at Palmer Station, the ratio of net-to-gross production reached a maximum greater than c. 60%, among the highest ever reported. The use of multiple techniques showed that these high ratios resulted from low heterotrophic respiration and very low daylight autotrophic respiration. Laboratory experiments revealed a similar ratio of net-to-gross O2 production in F. cylindrus and provided the first experimental evidence for an important level of cyclic electron flow (CEF) in this organism. The low ratio of community respiration to gross primary production observed during the bloom at Palmer Station may be characteristic of high latitude coastal ecosystems and partially supported by a very active CEF in psychrophilic phytoplankton.

Low temperature reduces the energetic requirement for the CO2 concentrating mechanism in diatoms

The goal of this study is to investigate the CO2 concentrating mechanism (CCM) of the dominant phytoplankton species during the growing season at Palmer station in the Western Antarctic Peninsula. Key CCM parameters (cellular half-saturation constants for CO2 fixation, carbonic anhydrase activity, CO2 /HCO3 (-) uptake, δ(13) Corg ) in natural phytoplankton assemblages were determined. Those results, together with additional measurements on CO2 membrane permeability from Fragilariopsis cylindrus laboratory cultures, were used to develop a numerical model of the CCM of cold water diatoms. The field data demonstrate that the dominant species throughout the season possess an effective CCM, which achieves near saturation of CO2 for fixation. The model provides a means to examine the role of eCA activity and HCO3 (-) /CO2 uptake in the functioning of the CCM. According to the model, the increase in δ(13) Corg during the bloom results chiefly from decreasing ambient CO2 concentration (which reduces the gross diffusive flux across the membrane) rather than a shift in inorganic carbon uptake from CO2 to HCO3 (-) . The CCM of diatoms in the Western Antarctic Peninsula functions with a relatively small expenditure of energy, resulting chiefly from the low half-saturation constant for Rubisco at cold temperatures.

The potential for co-evolution of CO2-concentrating mechanisms and Rubisco in diatoms

Diatoms are a diverse group of unicellular algae that contribute significantly to global photosynthetic carbon fixation and export in the modern ocean, and are an important source of microfossils for paleoclimate reconstructions. Because of their importance in the environment, diatoms have been a focus of study on the physiology and ecophysiology of carbon fixation, in particular their CO2-concentrating mechanisms (CCMs) and Rubisco characteristics. While carbon fixation in diatoms is not as well understood as in certain model aquatic photoautotrophs, a greater number of species have been examined in diatoms. Recent work has highlighted a large diversity in the function, physiology, and kinetics of both the CCM and Rubisco between different diatom species. This diversity was unexpected since it has generally been assumed that CCMs and Rubiscos were similar within major algal lineages as the result of selective events deep in evolutionary history, and suggests a more recent co-evolution between the CCM and Rubisco within diatoms. This review explores our current understanding of the diatom CCM and highlights the diversity of both the CCM and Rubisco kinetics. We will suggest possible environmental, physiological, and evolutionary drivers for the co-evolution of the CCM and Rubisco in diatoms.

The role of Rubisco kinetics and pyrenoid morphology in shaping the CCM of haptophyte microalgae

Variations in the pyrenoid morphology, Rubisco content, and kinetics among haptophyte algae provide insight into their carbon-concentrating mechanisms.