Abstract
Increases in atmospheric CO2 concentrations due to anthropogenic activities also cause an increase in oceanic CO2(aq), which will lead to a decline of 0.3-0.4 pH units in the surface ocean by 2100, termed ocean acidification (OA). To date, most OA studies have evaluated the effects of high CO2(aq) and low pH on calcified organisms, but little attention has been paid to determine the effects of OA on non-calcifying organisms such as fleshy macroalgae. Macroalgae depend on CO2 to support their photosynthesis, and therefore the future predicted changes in inorganic carbon (Ci) availability may directly affect their carbon metabolism, photosynthesis and consequently, growth. The giant kelp Macrocystis pyrifera (hereafter Macrocystis) is a widely distributed and highly productive macroalga of temperate reef ecosystems that plays an important ecological role in nearshore trophic dynamics. This thesis examines the effects of OA on photosynthesis, growth and carbon and nitrogen metabolism of the giant kelp Macrocystis.
Macrocystis is known to be a mixed CO2 and bicarbonate (HCO3–) user, but little is known about its Ci acquisition mechanisms. Here, an optimized method for measuring carbonic anhydrase (CA) in Macrocystis was developed. Using the optimized method, both external CA (CAext) and internal CA (CAint) activities were readily detected in Macrocystis. The CAint activity was 2× higher than CAext. The higher CAint activity was related to the Ci uptake mechanism of Macrocystis. As shown in the subsequent examination on the Ci acquisition mechanisms under different HCO3–: CO2 ratios at high (9.00) and low (7.65) pH, the main mechanism for HCO3– utilization in Macrocystis is via an anion exchange (AE) protein. Regardless of the CO2 concentration present in the medium, the second HCO3– utilization mechanism, i.e. external catalyzed HCO3– dehydration via CAext makes a lesser contribution to the photosynthetic Ci acquisition. The CAint plays an important role in maintaining internal Ci pools, cellular pH homeostasis and in dehydrating HCO3– to supply CO2 to RuBisCO.
Subsequent examination on the effects of OA on Macrocystis photosynthetic performance, growth, and CAext and CAint activities showed that increased CO2(aq) and low pH did not affect the physiology of Macrocystis. Their ability to use HCO3– as the main Ci source remained unaffected by increased CO2(aq). The photosynthesis and growth of Macrocystis are likely Ci saturated under the current Ci conditions, and therefore, their photosynthetic Ci uptake and growth will not be affected by increased CO2(aq)/low pH under a future OA scenario. Thereafter, Macrocystis nitrogen physiology relative to tissue nitrogen (N) status was examined to determine whether other environmental factors such as nutrient availability will regulate the species response to OA. However, I found that the thallus N status of Macrocystis (deplete and replete N pool) did not modify its response to OA. Consequently, OA affected neither the growth nor NO3– uptake and assimilation (i.e. NR) in Macrocystis, but some distinct responses such as enhanced NO3– uptake were observed in N-deplete Macrocystis blades grown under an OA treatment.
Kelp forests of Macrocystis are known to modify bulk water carbonate chemistry inside and outside the canopy. Seaweeds can also modify their microenvironment, i.e. at the thallus surface within the diffusion boundary layer (DBL) via physiological processes such as photosynthesis, respiration, and nutrient uptake. Knowledge of the metabolic fluxes (OH–/H+) is of great importance to elucidate how macroalgae may respond to a low pH (high [H+]) under a future OA scenario. The present study showed that metabolic fluxes related to the high photosynthetic rates of Macrocystis rather than due to inorganic nutrient uptake (i.e. NO3– and NH4+) are responsible for modifying pH within their DBL. Moreover, the pH within the DBL was greatly increased under an OA treatment compared to the ambient seawater pH condition.
Overall, this thesis reveals that OA will not affect rates of photosynthesis, growth, and carbon and nitrogen metabolisms of the giant kelp Macrocystis. The results obtained in the present study also suggest that other predicted environmental local changes such as eutrophication and low light availability may have a more significant effect on the physiology of Macrocystis than OA. This thesis elucidates how this species might respond to OA, and to the understanding of the carbon and nitrogen metabolism of the species, which will be of great importance for further studies to determine how future predicted global and local environmental changes might interactively affect Macrocystis’ physiology and ecology.