Effects of ocean acidification, temperature and copper on the development of early life stages of the native kelp Macrocystis Pyrifera and the invasive kelp Undaria Pinnatifida from Southern New Zealand

Anthropogenic activities have increased atmospheric CO2 concentrations from pre-industrial concentrations of 280 ppm to current values of 400 ppm. These atmospheric emissions of CO2 are responsible for the ongoing increase in seawater temperature and the reduction of pH in the ocean’ surface, phenomena known as ocean warming (OW) and ocean acidification (OA), respectively. Model-based projections indicate that the global ocean surface temperature will increase by 4°C whereas the pH will decrease from a current 8.10 to 7.74 by 2100. However, these two global events are not occurring in isolation because anthropogenic activities also threatens coastal environments at local levels. For instance, in coastal environments, natural concentrations of copper are low but are increasing due to human industrialization. In addition, the speciation and bioavailability of copper in seawater is highly dependent on seawater chemistry. Therefore, reductions in seawater pH due to OA will increase the toxic, free ionic form of copper in oceans by 20% by the end of the current century. These abiotic changes can have important impacts on marine biota and ecosystems. Fleshy (non-calcifying) macroalgae such as those that belong to the Order Laminariales are important components of coastal environments. Macroalgae, as sessile organisms, are exposed to constant changes in abiotic factors and the community dynamics (e.g., growth and reproduction) depend on their tolerance to stress. Despite their importance, few studies have focused on the effects of OW, OA and/or copper pollution on fleshy macroalgae and even less have focused on their early life stages. Early life stages have been reported to be the most sensitive phase of the macroalgal life cycle to stressors. Therefore, the main purpose of this work was to evaluate the separate and interactive effects of seawater temperature, pH and copper concentration on the development of microscopic stages of the native kelp M. pyrifera and the invasive kelp U. pinnatifida from southern New Zealand.The first result of this work was that, in M. pyrifera, sporogenesis occurred in basal sporophylls (specialized reproductive laminae) as well as in non-reproductive laminae such as pneumatocyst-bearing adult blade and young apical scimitars. The sorus surface area was greater on sporophylls (57%) than in adult blades and young scimitars (25%). Meiospore release was greater in apical scimitars, followed by adult blades and sporophylls. However, germination of meiospores from different laminae was not significantly different, indicating that meiospores produced in all types of fertile laminae were equally viable.

The first climate change-related experiment consisted of monitoring meiospore development of M. pyrifera and U. pinnatifida cultured under four seawater pH treatments (pH 7.20, extreme OA predicted for 2300; pH 7.65, OA predicted for 2100; pH 8.01, ambient pH; and pH 8.40, pre-industrial pH) for 15 days. Reduced seawater pH (7.20 and 7.65) had no effects on meiospore germination but had positive effects on germling growth rates and gametophyte size of both species compared to higher pH (8.01 and 8.40). Gametophyte sex ratio was biased towards females under all pH treatments. Germling growth rate under OA was significantly higher in M. pyrifera compared to U. pinnatifida but gametophyte development was equal for both kelps under all seawater pH treatments, indicating that the microscopic stages of the native M. pyrifera and the invasive U. pinnatifida will respond similarly to OA.

The second experiment climate change-related experiment consisted of monitoring meiospore development of M. pyrifera and U. pinnatifida cultured under four seawater pH treatments (pH 7.20, 7.65, 8.03, and 8.40) and two temperature treatments (12°C, ambient temperature; and 16°C, OW predicted for 2100) for 15 days. Reduced seawater pH and elevated temperature had no effects on meiospore development and positive effects on germling growth rates and gametophyte size of both species compared to higher pH (8.01 and 8.40) and lower temperature (12°C), whereas gametophyte sex ratio was not affected by the interaction between the two factors. Despite some small differences between species, results of this experiment suggest that microscopic stages of the native M. pyrifera and the invasive U. pinnatifida will respond similarly to OA and OW.

The single effects of the local stressor, copper pollution, on the development of M. pyrifera and U. pinnatifida meiospore were examined. After settlement, meiospores of both kelps were exposed to five nominal copper treatments (control, 100, 200, 300 and 400 µg L-1 Cu) for 9 days. Analyses of total dissolved copper (CuT) concentrations in the blanks showed that nominal copper concentrations were reduced to 54, 91, 131 and 171 µg L-1 CuT (i.e., > 50% of the CuT was adsorbed onto the culture vessel walls). In the media with meiospores, the CuT also decreased: to 39, 86, 97 and 148 µg L-1 CuT in M. pyrifera, and to 39, 65, 97 and 146 µg L-1 CuT in U. pinnatifida (i.e., 6 – 15% of the dissolved copper was adsorbed by the cells). Meiospore germination decreased with increasing copper concentrations but gametogenesis was arrested under all copper treatments. The effective copper concentration causing 50% of arrested germination (Cu EC50) was higher for U. pinnatifida (231 µg L-1 CuT) than for M. pyrifera (157 µg L-1 CuT), suggesting ecological success for the invasive species in copper polluted environments; however, the subsequent inhibition of gametogenesis under all copper treatments indicated no difference in copper tolerance between both kelp early life stages.

The reduction of CuT during the previous experiment occurred because copper might be adsorbed onto glass and/or plastic and this can be avoided using a proper trace metal clean procedure. Therefore, a review on the methodologies used in the literature on copper ecotoxicology of marine macro- and microalgae, specifically the use of trace metal clean procedures such as the labware used (glassware vs plasticware), methods of cleaning the labware (acid soaking and ultrapure water rinsing), stock solution preparation (copper source and acidification), and measurement and reporting of dissolved copper concentrations was performed. The main results of this review were that 50% of the articles did not specify the laboratory–ware, 25% used glassware and 25% plasticware; only ~30% of the studies specified cleaning protocols for labware to remove trace metal impurities; the copper form used to prepare the stock solutions was specified in ~80% of studies but acidification to stabilize the dissolved copper was performed in only ~20%; and the dissolved copper concentration was measured in only ~30% of studies. Based on these finding, a trace metal procedure was recommended for conducting copper ecotoxicological studies.

A four-factor experiment was performed to investigate the interactive effects of OA, OW and copper pollution on the meiospore development of M. pyrifera and U. pinnatifida. Meiospores of both species were cultured under two seawater pH treatments (7.65 and 8.16), and two temperature treatments (12 and 16°C), and to the species-specific Cu-EC50 for 18 days. In both species, meiospore germination and germling growth rates significantly decreased in the copper treatment, irrespective of pH and temperature whereas gametophyte development for both species was inhibited by copper in all pH and temperature treatments. These results suggest that a local stressor (e.g., copper) is more important to the development of microscopic stages of M. pyrifera and U. pinnatifida than global climate change factors.

In summary, results of this study indicate that meiospore development of M. pyrifera and U. pinnatifida will be able to tolerate future OA and OW. That tolerance might be related to: 1) macro- and microscopic stages of both kelps being able to use HCO3- and CO2 to support photosynthesis, therefore, the higher CO2(aq) availability due to OA (pH 7.20 to 7.65) will not affect their physiology; and 2) both M. pyrifera and U. pinnatifida have a wide temperature tolerance (4 to 30C°) which may allow them to perform well in a future warmer ocean (+ 4C°). In contrast, relatively high copper concentrations inhibited meiospore development of both kelp species. This finding indicates that local drivers (e.g., copper pollution) may be more important to physiological processes during meiospore development than global climate change factors. Furthermore, the responses of meiospores to the experimental abiotic factors (i.e., OA, OW and Cu) were similar between the study species, indicating that the invasive U. pinnatifida is unlikely to have an advantage over the native M. pyrifera in natural coastal environments.

Leal Sandoval P. P., 2016. Effects of ocean acidification, temperature and copper on the development of early life stages of the native kelp Macrocystis Pyrifera and the invasive kelp Undaria Pinnatifida from Southern New Zealand. PhD thesis, University of Otago, 280 p. Thesis.

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