The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative ...importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.
Photosymbiosis has played a key role in the diversification of foraminifera and their carbonate production throughout geologic history. However, identification of photosymbiosis in extinct taxa ...remains challenging, and even among the extant species the occurrence and functional relevance of photosymbiosis remain poorly constrained. Here, we investigate photosymbiosis in living planktonic foraminifera by measuring active chlorophyll fluorescence with fast repetition rate fluorometry. This method provides unequivocal evidence for the presence of photosynthetic capacity in individual foraminifera, and it allows us to characterize multiple features of symbiont photosynthesis including chlorophyll a (Chl a) content, potential photosynthetic activity (Fv∕Fm), and light-absorption efficiency (σPSII). To obtain robust evidence for the occurrence and importance of photosymbiosis in modern planktonic foraminifera, we conducted measurements on 1266 individuals from 30 species of the families Globigerinidae, Hastigerinidae, Globorotaliidae, and Candeinidae. Among the studied species, 19 were recognized as symbiotic and 11 as non-symbiotic. Of these, six species were newly confirmed as symbiotic and five as non-symbiotic. Photosymbiotic species have been identified in all families except the Hastigerinidae. A significant positive correlation between test size and Chl a content, found in 16 species, is interpreted as symbiont abundance scaled to the growth of the host and is consistent with persistent possession of symbionts through the lifetime of the foraminifera. The remaining three symbiont-bearing species did not show such a relationship, and their Fv∕Fm values were comparatively low, indicating that their symbionts do not grow once acquired from the environment. The objectively quantified photosymbiotic characteristics have been used to design a metric of photosymbiosis, which allows the studied species to be classified along a gradient of photosynthetic activity, providing a framework for future ecological and physiological investigations of planktonic foraminifera.
Annual shell growth patterns of the three venerid bivalve species,
Gafrarium pectinatum
,
Pitar citrinus
, and
Katelysia japonica
were investigated based on the results of sclerochronological and ...stable oxygen isotope analyses of live-caught specimens from the intertidal zone of Iriomote Island, southern Ryukyu Archipelago. In the study area, these three species temporally stopped shell deposition, when sea surface temperature (SST) dropped to 23–26 °C, during the first three years. However, the shutdown temperature for shell growth increased slightly to higher than 26 °C after 6 years old for
G. pectinatum
combined with a shortening in the length of shell growing period. Seasonal changes in daily shell growth in these species were controlled mainly by SST and primary production. Shell δ
18
O-derived summer temperatures recorded in the annual increments were higher by 3–5 °C than the highest SST records of the habitat. This data mismatch might be caused by an abrupt decrease in seawater δ
18
O values during the summer and fall typhoon seasons because of the influx of fresh water into the study area from nearby rivers. This study suggests that in the study area the annual shell growth patterns and shell δ
18
O values in the three species examined were controlled by mutually related biological and environmental factors such as ontogenetic age and seasonal changes in SST, salinity and primary production.
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.
The CO
2
concentration of air has increased over the last two centuries and recently surpassed 400 ppm. Carbon cycle models project CO
2
concentrations of 720 to 1000 ppm for the IPCC intermediate ...scenario (RCP 6.0), resulting in an increase in global mean temperature of ~ 2.6 °C and a decrease in seawater pH of ~ 0.3. Together, global warming and ocean acidification are often referred to as the “evil twins” of climate change, potentially inducing severe threats in the near future. In this paper, our discussion is focused on the response of two major calcifiers, foraminifera and corals, which contribute much to the global carbonate burial rate. Photosymbiosis is regarded as an adaptive ecology for living in warm and oligotrophic oceans, especially for reef-building corals and larger reef-dwelling benthic foraminifera. As a consequence of global warming, bleaching may be a global threat to algal symbiont-bearing marine calcifying organisms under conditions of high temperature and light intensity. If CO
2
is dissolved in seawater, the partial pressure of CO
2
in seawater (
p
CO
2
) and dissolved inorganic carbon (DIC) increases while pH and the saturation state of carbonate minerals decreases without any change in total alkalinity. Generally, marine calcifying organisms show decreases in calcification rates in response to acidified seawater. However, the response often differs depending on situations, species, and life-cycle stage. Some benthic foraminifera showed a positive response to low pH conditions. The
Acropora digitifera
coral calcification of adult branches was not reduced markedly at higher
p
CO
2
conditions, although calcification tended to decrease versus
p
CO
2
in both aposymbiotic and symbiotic polyps. New analytical technologies help identify important constraints on calcification processes. Based upon Ca isotopes, the transport path of Ca
2+
and the degree of its activity would predominantly control the carbonate precipitation rate. Visualization of the extracellular pH distribution shows that proton pumping produces the high internal pH and large internal–external pH gap in association with foraminiferal calcification. From the perspective of a long-term change in the Earth’s surface environment, foraminifera seem to be more adaptive and robust than corals in coping with ocean warming and acidification but it is necessary to further understand the mechanisms underlying variations in sensitivity to heat stress and acidified seawater for future prediction. Since CO
2
is more soluble in lower temperature seawater, ocean acidification is more critical in the polar and high-latitude regions. Additionally, older deep-water has enhanced acidity owing to the addition of CO
2
from the degradation of organic matter via a synergistic effect with high pressure. With current ocean acidification, pH and the saturation state of carbonate minerals are decreasing without any change in total alkalinity. However, in the Earth’s history, it is well known that alkalinity has fluctuated significantly. Therefore, it is necessary to quantitatively reconstruct alkalinity, which is another key factor determining the saturation state of carbonate minerals. The rapid release of anthropogenic CO
2
(in the present day and at the Paleocene/Eocene boundary) induces severe ocean acidification, whereas in the Cretaceous, slow environmental change, even at high levels of
p
CO
2
, could raise alkalinity, thereby neutralizing ocean acidification.
Comprehensive reconstruction of changes in eukaryotic communities in the recent past is useful for determining the response of the local ecosystems to global changes during the Anthropocene. We used ...DNA barcoding technology to reconstruct the marine eukaryotic communities of Beppu Bay, the Seto Inland Sea, Japan, over the past 50 years based on a short sediment core. Highly vulnerable DNA fragments were preserved in the sediments, possibly due to seasonally euxinic conditions. Analysis of the 18S rRNA V9 gene region indicated the temporal variability in eukaryotic communities, which consisted mainly of dinoflagellates and diatoms, in response to changes in the nutrient regime. The dominant species in the dinoflagellate genus Alexandrium changed as the water temperature increased. In addition, enhanced contributions by terrestrial plants and mosses were detected in flood sediments. Our results suggest that DNA fragments can be used as a proxy for the paleoenvironmental and paleoecological conditions in Beppu Bay.
Plain Language Summary
Information about past environmental changes is important for understanding the present and future environments. In this study, we used the DNA preserved in sea‐bottom sediments in Beppu Bay to reconstruct phytoplankton communities over the past 50 years. The DNA in sediments contains comprehensive information about past phytoplankton communities, but it is vulnerable to degradation under normal conditions. However, the sediments from Beppu Bay yielded significant amounts of DNA, possibly because seasonally reducing conditions suppressed the degradation of DNA. We found that phytoplankton communities changed in response to changes in nutrient availability. Further, the genes of terrestrial plants and mosses were more abundant in flood sediments. Our results suggest that the DNA fragments can be used as a proxy of the paleoenvironmental and paleoecological conditions in Beppu Bay.
Key Points
DNA barcoding technology was used to reconstruct the marine eukaryotic communities in Beppu Bay, Japan
Highly vulnerable DNA fragments were preserved in the sediments, possibly due to seasonally euxinic conditions
Eukaryotic communities have shifted over recent decades in response to changes in nutrient availability
Photosymbiosis is an important ecological strategy that allows the host to thrive in oligotrophic environments, and the photosymbiosis of planktic foraminifers is no exception. Here, we present ...ontogenetic information about the photosymbiosis of planktic foraminifers that we obtained via analyses of the chlorophyll fluorescence fast repetition rate (FRR) fluorometry of symbiotic algae within the host. We cultured two symbiont-bearing planktic foraminifers (Globigerinoides sacculifer and Globigerinella siphonifera Type II) until their natural death, and conducted FRR measurements on individual host–algal consortia through the culture study. Time-series FRR analyses revealed no clear temporal trend in photophysiology but did reveale species specificity. The light-absorption efficiency of the photosynthetic system was significantly higher in Gn. siphonifera than in Gs. sacculifer indicating higher potential to acclimate to low-light environment for Gn. siphonifera. In contrast to the physiology, the chlorophyll a content of foraminifer, a metric of the quantity of symbionts, showed conspicuous ontogenetic changes; the chlorophyll a content, initially less than 30ng, reached a maximum of more than 140ng, then it was all digested or lysed at the end of the host's ontogeny. The changes of symbiont biomass and relatively invariant photophysiology indicate dynamic rise and fall of potential photosynthesis of symbiont population during the host's life processes, not just the progressive increase of photosynthesis. Because photosynthesis of symbionts can alter the geochemical composition of the foraminiferal calcifying microenvironment, our results will also contribute to better understanding of the effect of photosynthesis on the foraminiferal tests that are important for paleoecological and paleoceanographic works.
•Photosynthetic parameters of symbionts were measured using fluorometric technique.•Chlorophyll content per foraminifer increased during immature stage of ontogeny.•Chlorophyll content (i.e., symbiont number) reached zero before release of gametes.•Dynamic change of chlorophyll content did not affect photophysiologies of symbionts.•Photophysiological parameters of symbionts showed species-specific natures.
The occurrence of Oceanic Anoxic Event 2 (OAE2) 94 million years ago is considered to be one of the largest carbon cycle perturbations in the Earth's history. The marked increase in the spatial ...extent of the anoxic conditions in the world's oceans associated with OAE2 resulted in the mass accumulation of organic-rich sediments. Although extensive oceanographic studies of OAE2 have been undertaken in the Atlantic Ocean, the Tethys Sea, and the epicontinental seas of Europe and America, little is known about OAE2 in the Pacific Ocean. Here, we present high-resolution carbon-isotope and degree of pyritization (DOP) data from marine sequences that formed along the continental margins of North America and Asia below the northeastern and northwestern Pacific Ocean. The predominance of low DOP values in these areas revealed that the continental margins of the Pacific Ocean were oxic for most of the OAE2 interval.
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