No abstract

Eutrophic coastal regions are highly productive and greatly influenced by human activities. Primary production supporting the coastal ecosystems is supposed to be affected by progressive ocean acidification driven by increasing CO2 emissions. In order to investigate the effects of high pCO2 (HC) on eutrophic plankton community structure and ecological functions, we employed 9 mesocosms and carried out an experiment under ambient (∼410 ppmv) and future high (1000 ppmv) atmospheric pCO2 conditions, using in situ plankton community in Wuyuan Bay, East China Sea. Our results showed that HC along with natural seawater temperature rise significantly boosted biomass of diatoms with decreased abundance of dinoflagellates in the late stage of the experiment, demonstrating that HC repressed the succession from diatoms to dinoflagellates, a phenomenon observed during algal blooms in the East China Sea. HC did not significantly influence the primary production or biogenic silica contents of the phytoplankton assemblages. However, the HC treatments increased the abundance of viruses and heterotrophic bacteria, reflecting a refueling of nutrients for phytoplankton growth from virus-mediated cell lysis and bacterial degradation of organic matters. Conclusively, our results suggest that increasing CO2 concentrations can modulate plankton structure including the succession of phytoplankton community and the abundance of viruses and bacteria in eutrophic coastal waters, which may lead to altered biogeochemical cycles of carbon and nutrients.

Many marine organisms are exposed to decreasing O2 levels due to warming-induced expansion of hypoxic zones and ocean deoxygenation (DeO2). Nevertheless, effects of DeO2 on phytoplankton have been neglected due to technical bottlenecks on examining O2 effects on O2-producing organisms. Here we show that lowered O2 levels increased primary productivity of a coastal phytoplankton assemblage, and enhanced photosynthesis and growth in the coastal diatom Thalassiosira weissflogii. Mechanistically, reduced O2 suppressed mitochondrial respiration and photorespiration of T. weissflogii, but increased the efficiency of their CO2 concentrating mechanisms (CCMs), effective quantum yield and improved light use efficiency, which was apparent under both ambient and elevated CO2 concentrations leading to ocean acidification (OA). While the elevated CO2 treatment partially counteracted the effect of low O2 in terms of CCMs activity, reduced levels of O2 still strongly enhanced phytoplankton primary productivity. This implies that decreased availability of O2 with progressive DeO2 could boost re-oxygenation by diatom-dominated phytoplankton communities, especially in hypoxic areas, with potentially profound consequences for marine ecosystem services in coastal and pelagic oceans.

Abstract. Trichodesmium species, as a group of photosynthetic N2 fixers (diazotrophs), play an important role in the marine biogeochemical cycles of nitrogen and carbon, especially in oligotrophic waters. How ongoing ocean warming may interact with light availability to affect Trichodesmium is not yet clear. We grew Trichodesmium erythraeum IMS 101 at three temperature levels of 23, 27, and 31∘C under growth-limiting and growth-saturating light levels of 50 and 160 µmol quanta m−2 s−1, respectively, for at least 10 generations and then measured physiological performance, including the specific growth rate, N2 fixation rate, and photosynthesis. Light availability significantly modulated the growth response of Trichodesmium to temperature, with the specific growth rate peaking at ∼27∘C under the light-saturating conditions, while growth of light-limited cultures was non-responsive across the tested temperatures (23, 27, and 31∘C). Short-term thermal responses for N2 fixation indicated that both high growth temperature and light intensity increased the optimum temperature (Topt) for N2 fixation and decreased its susceptibility to supra-optimal temperatures (deactivation energy – Eh). Simultaneously, all light-limited cultures with low Topt and high Eh were unable to sustain N2 fixation during short-term exposure to high temperatures (33–34∘C) that are not lethal for the cells grown under light-saturating conditions. Our results imply that Trichodesmium spp. growing under low light levels while distributed deep in the euphotic zone or under cloudy weather conditions might be less sensitive to long-term temperature changes that occur on the timescale of multiple generations but are more susceptible to abrupt (less than one generation time span) temperature changes, such as those induced by cyclones and heat waves.

Although the marine N 2 -fixers Trichodesmium spp. are affected by increasing pCO 2 and by ultraviolet radiation (UVR) in their habitats, little is known on their potential responses to future ocean acidification in the presence of UVR. We grew Trichodesmium at two pCO 2 levels (410 and 1000 μatm) under natural sunlight, documented the filament length, growth, and chlorophyll content after its acclimation to the pCO 2 treatments, and measured its carbon and N 2 fixation rates under different solar radiation treatments with or without UVR. We showed that the elevated pCO 2 did not significantly alter the diazotroph's growth, filament length, or pigment content, and its photosynthetic rate was only affected by solar radiation treatments rather than the pCO 2 levels. The presence of UV-A and UV-B inhibited photosynthesis by 10-22% and 17-21%, respectively. Inhibition of N 2 fixation by UV-B was proportional to its intensity, whereas UV-A stimulated N 2 fixation at low, but inhibited it at high, intensities. Elevated pCO 2 only stimulated N 2 fixation under moderate levels of solar radiation. The simulated depth profile of N 2 fixation in the water column showed that UV-induced inhibition dominated the combined effects of elevated pCO 2 and UVR at 0-30 m depth and the combination of these factors enhanced N 2 fixation at 30-60 m depth, but this effect diminished in deeper water. Our results suggest that Trichodesmium could be influenced more by UVR than by pCO 2 and their combined action result in negative effects on N 2 fixation under high solar radiation, but positive effects under low to moderate solar radiation.