Effect of nutritional condition on photosymbiotic consortium of cultured Globigerinoides sacculifer (Rhizaria, Foraminifera).

Several foraminifers found in warm and low-nutrient ocean surface water have photosynthetic algae as endosymbionts (photosymbiosis). To understand the trophic interactions, we studied Globigerinoides sacculifer, a spinose planktic foraminifer that has a dinoflagellate endosymbiont. We controlled two nutritional factors, feeding and inorganic nutrients in the seawater. The growth of the host and the symbionts and the photophysiological parameters were monitored under four experimental conditions. The results demonstrated that the holobionts primarily relied on phagotrophy for growth. The foraminifers grew considerably, and the chlorophyll a content per foraminifer, which is an indicator of the symbiont population, increased in the fed groups, but not in the unfed groups. The nutrient-rich seawater used for some of the cultures made no difference in either the growth or photophysiology of the holobionts. These observations indicated that the symbionts mainly utilized metabolites from the hosts for photosynthesis rather than inorganic nutrients in the seawater. Additionally, we observed that the symbionts in the starved hosts maintained their photosynthetic capability for at least 12 days, and that the hosts maintained at least some symbionts until gametogenesis was achieved. This suggests that the hosts have to retain the symbionts as an energy source for reproduction. The symbionts may also play an indispensable role in the metabolic activities of the hosts including waste transport or essential compound synthesis. Overall, our results revealed a novel mode of photosymbiosis in planktic foraminifers which contrasts with that found in benthic photosymbiotic foraminifers and corals.

Asian dust-deposition flux to the subarctic Pacific estimated using single quartz particles.

Iron availability limits marine ecosystem activities in large areas of the ocean. However, the sources and seasonal supply of iron, critically important for controlling surface ocean biogeochemistry and carbon cycling, are poorly understood. The western subarctic Pacific is a high-nutrient and low-chlorophyll region, and despite high concentrations of macronutrients, iron limits phytoplankton production in summer. Here, we determine the seasonal deposition flux of Asian dust using scanning electron microscope-cathodoluminescence analysis of single quartz particles derived from the western subarctic Pacific during 2003-2022 to trace provenance. We found a high (up to 6.9 mg m-2 day-1) deposition flux of Asian dust in May, June, and early July, with an annual average of 1.0 ± 0.2 mg m-2 day-1. The supply of dissolved-iron flux calculated from Asian dust was 0.9 ± 0.3 µg m-2 day-1 during the high productivity season (April-July), which is approximately half that from the deeper part of the ocean, calculated from vertical profiles of dissolved iron. Our study provides a reliable approach for estimating iron supply from dust to the surface ocean that may be critical for sustaining biological productivity under future ocean stratification, which suppresses nutrient supply from the subsurface ocean.

Response of N2O production rate to ocean acidification in the western North Pacific.

Ocean acidification induced by the increase of anthropogenic CO2 emissions has a profound impact on marine organisms and biogeochemical processes.1 The response of marine microbial activities to ocean acidification might play a crucial role in the future evolution of air-sea fluxes of biogenic gases such as nitrous oxide (N2O), a strong greenhouse gas and the dominant stratospheric ozone-depleting substance.2 Here, we examine the response of N2O production from nitrification to acidification in a series of incubation experiments conducted in subtropical and subarctic western North Pacific. The experiments show that, when pH was reduced, the N2O production rate during nitrification measured at subarctic stations increased significantly whereas nitrification rates remained stable or decreased. Contrary to what was previously thought, these results suggest that the effect of ocean acidification on N2O production during nitrification and nitrification rates are likely uncoupled. Collectively these results suggest that, if seawater pH continues to decline at the same rate, ocean acidification could increase the marine N2O production during nitrification in subarctic North Pacific by 185 to 491% by the end of the century.