ALBERT

Description: The eastern Mediterranean is a hotspot of biological invasions. Numerous species of Indo-pacific origin have colonized the Mediterranean in recent times, including tropical symbiont-bearing foraminifera. Among these is the species Pararotalia calcariformata. Unlike other invasive foraminifera, this species has been discovered only two decades ago and is restricted to the eastern Mediterranean coast. Combining ecological, genetic and physiological observations, we attempt to explain the recent invasion of this species in the Mediterranean Sea. Using morphological and genetic data, we confirm the species attribution to P. calcariformata McCulloch 1977 and identify its symbionts as a consortium of diatom species dominated by Minutocellus polymorphus. We document photosynthetic activity of its endosymbionts using Pulse Amplitude Modulated Fluorometry and test the effects of elevated temperatures on growth rates of asexual offspring. The culturing of asexual offspring for 120 days shows a 30-day period of rapid growth followed by a period of slower growth. A subsequent 48-day temperature sensitivity experiment indicates a similar developmental pathway and high growth rate at 28°C, whereas an almost complete inhibition of growth was observed at 20°C and 35°C. This indicates that the offspring of this species may have lower tolerance to cold temperatures than what would be expected for species native to the Mediterranean. We expand this hypothesis by applying a Species Distribution Model (SDM) based on modern occurrences in the Mediterranean using three environmental variables: irradiance, turbidity and yearly minimum temperature. The model reproduces the observed restricted distribution and indicates that the range of the species will drastically expand westwards under future global change scenarios. We conclude that P. calcariformata established a population in the Levant because of the recent warming in the region. In line with observations from other groups of organisms, our results indicate that continued warming of the eastern Mediterranean will facilitate the invasion of more tropical marine taxa into the Mediterranean, disturbing local biodiversity and ecosystem structure.

Description: Fossil shells of planktonic foraminifera serve as the prime source of information on past changes in surface ocean conditions. Because the population size of planktonic foraminifera species changes throughout the year, the signal preserved in fossil shells is biased towards the conditions when species production was at its maximum. The amplitude of the potential seasonal bias is a function of the magnitude of the seasonal cycle in production. Here we use a planktonic foraminifera model coupled to an ecosystem model to investigate to what degree seasonal variations in production of the species Neogloboquadrina pachyderma may affect paleoceanographic reconstructions during Heinrich Stadial 1 (~18-15 cal. ka B.P.) in the North Atlantic Ocean. The model implies that during Heinrich Stadial 1 the maximum seasonal production occurred later in the year compared to the Last Glacial Maximum (~21-19 cal. ka B.P.) and the pre-industrial era north of 30 ºN. A diagnosis of the model output indicates that this change reflects the sensitivity of the species to the seasonal cycle of sea-ice cover and food supply, which collectively lead to shifts in the modeled maximum production from the Last Glacial Maximum to Heinrich Stadial 1 by up to six months. Assuming equilibrium oxygen isotopic incorporation in the shells of N. pachyderma, the modeled changes in seasonality would result in an underestimation of the actual magnitude of the meltwater isotopic signal recorded by fossil assemblages of N. pachyderma wherever calcification is likely to take place.

Description: The use of planktonic foraminifera in paleoceanographic studies relies on the assumption that morphospecies represent biological species with ecological preferences that are stable through time and space. However, genetic surveys unveiled a considerable level of diversity in most morphospecies of planktonic foraminifera. This diversity is significant for paleoceanographic applications because cryptic species were shown to display distinct ecological preferences that could potentially help refine paleoceanographic proxies. Subtle morphological differences between cryptic species of planktonic foraminifera have been reported, but so far their applicability within paleoceanographic studies remains largely unexplored. Here we show how information on genetic diversity can be transferred to paleoceanography using Globorotalia inflata as a case study. The two cryptic species of G. inflata are separated by the Brazil-Malvinas Confluence (BMC), a major oceanographic feature in the South Atlantic. Based on this observation, we developed a morphological model of cryptic species detection in core top material. The application of the cryptic species detection model to Holocene samples implies latitudinal oscillations in the position of the confluence that are largely consistent with reconstructions obtained from stable isotope data. We show that the occurrence of cryptic species in G. inflata, can be detected in the fossil record and used to trace the migration of the BMC. Since a similar degree of morphological separation as in G. inflata has been reported from other species of planktonic foraminifera, the approach presented in this study can potentially yield a wealth of new paleoceanographical proxies.

Description: Invasive species allow an investigation of trait retention and adaptations after exposure to new habitats. Recent work on corals from the Gulf of Aqaba (GoA) shows that tolerance to high temperature persists thousands of years after invasion, without any apparent adaptive advantage. Here we test whether thermal tolerance retention also occurs in another symbiont-bearing calcifying organism. To this end, we investigate the thermal tolerance of the benthic foraminifera Amphistegina lobifera from the GoA (29° 30.14167 N 34° 55.085 E) and compare it to a recent "Lessepsian invader population" from the Eastern Mediterranean (EaM) (32° 37.386 N, 34°55.169 E). We first established that the studied populations are genetically homogenous but distinct from a population in Australia, and that they contain a similar consortium of diatom symbionts, confirming their recent common descent. Thereafter, we exposed specimens from GoA and EaM to elevated temperatures for three weeks and monitored survivorship, growth rates and photophysiology. Both populations exhibited a similar pattern of temperature tolerance. A consistent reduction of photosynthetic dark yields was observed at 34°C and reduced growth was observed at 32°C. The apparent tolerance to sustained exposure to high temperature cannot have a direct adaptive importance, as peak summer temperatures in both locations remain 〈32°C. Instead, it seems that in the studied foraminifera tolerance to high temperature is a conservative trait and the EaM population retained this trait since its recent invasion. Such pre-adaptation to higher temperatures confers A. lobifera a clear adaptive advantage in shallow and episodically high temperature environments in the Mediterranean under further warming.

Description: The transition from benthos to plankton requires multiple adaptations, yet so far it remains unclear how these are acquired in the course of the transition. To investigate this process, we analyzed the genetic diversity and distribution patterns of a group of foraminifera of the genus Bolivina with a tychopelagic mode of life (same species occurring both in benthos and plankton). We assembled a global sequence dataset for this group from single-cell DNA extractions and occurrences in metabarcodes from pelagic environmental samples. The pelagic sequences all cluster within a single monophyletic clade within Bolivina. This clade harbors three distinct genetic lineages, which are associated with incipient morphological differentiation. All lineages occur in plankton and benthos, but only one lineage shows no limit to offshore dispersal and has been shown to grow in the plankton. These observations indicate that the emergence of buoyancy regulation within the clade preceded the evolution of pelagic feeding and that the evolution of both traits was not channeled into a full transition into the plankton. We infer that in foraminifera, colonization of the planktonic niche may occur by sequential cooptation of independently acquired traits, with holoplanktonic species being recruited from tychopelagic ancestors.

Description: We present a new coccolithophore productivity reconstruction spanning the last 300 ka in core GeoB12613-1 retrieved from the western tropical Indian Ocean (IO), an area that mainly derives its warm and oligotrophic surface waters from the eastern IO. Application of a calibrated assemblage-based productivity index indicates a reduction in estimated primary productivity (EPP) from 300 ka to the present, with reconstructed EPP values ranging from 91 to 246 g C/m2/yr. Coccolithophore assemblages and coccolith fraction Sr/Ca indicate three main phases of productivity change, with major changes at 160 and 46 ka. The productivity and water-column stratification records show both dominant precession and obliquity periodicities, which appear to control the paleoproductivity in the study area over the last two glacial-interglacial cycles. Shallowing of the thermocline due to strengthening of the trade winds in response to insolation maxima resulted to peaks in EPP. Comparison with the eastern IO productivity and stratification coccolithophore data reveals good correspondence with our records, indicating a strong tropical Pacific influence in our study area. Both of these records show high productivity from 300 ka to 160 ka, interpreted to be due to stronger Walker Circulation while the declining productivity from 160 ka to the present day is a consequence of its weakening intensity.

Description: The chronology of deglacial meltwater pulses from the Laurentide Ice Sheet is well documented. However, the deglacial history of the North American-Arctic (north-eastern Laurentide and Innuitian) and western Greenland ice sheets draining into the Labrador Sea via Baffin Bay is less well constrained. Here we present new high-resolution, radiocarbon-dated records from the central Baffin Bay spanning ~17 to 10 kyr BP and documenting the full deglacial history of Baffin Bay. Sedimentological and geochemical data confirm the presence of two periods of enhanced detrital carbonate delivery, termed Baffin Bay Detrital Carbonate Events (BBDCs). These events are dated to ~14.2-13.7 kyr BP and ~12.7-11 kyr BP. They are synchronous across Baffin Bay and their mineralogical signature indicates a common source of detrital carbonate from northern Baffin Bay. The first event, BBDC 1, postdates Heinrich Event 1 and the second event, BBDC 0, predates the recently revised timing of Heinrich Event 0. The onset of the BBDC events appears not to be systematically linked to Greenland temperature change as they occur during both interstadial and stadial periods. This indicates that deglaciation of North American-Arctic and western Greenland ice sheets with the associated iceberg and meltwater discharge were decoupled from the dominant North Atlantic climate mode, where iceberg discharge events from the Laurentide Ice Sheet occurred during stadial periods.

Description: Species of planktonic foraminifera exhibit specific seasonal production patterns and different preferred vertical habitats. The seasonality and vertical habitats are not constant throughout the range of the species and changes therein must be considered when interpreting paleoceanographic reconstructions based on fossil foraminifera. Accounting for the effect of vertical and seasonal habitat tracking on foraminifera proxies at times of climate change is difficult because it requires independent fossil evidence. An alternative that could reduce the bias in paleoceanographic reconstructions is to predict species-specific habitat shifts under climate change using an ecosystem modeling approach. To this end, we present a new version of a planktonic foraminifera model, PLAFOM2.0, embedded into the ocean component of the Community Earth System Model, version 1.2.2. This model predicts monthly global concentrations of the planktonic foraminiferal species: Neogloboquadrina pachyderma, N. incompta, Globigerina bulloides, Globigerinoides ruber (white), and Trilobatus sacculifer throughout the world ocean, resolved in 24 vertical layers to 250m depth. The resolution along the vertical dimension has been implemented by applying the previously used spatial parameterization of biomass as a function of temperature, light, nutrition, and competition on depth-resolved parameter fields. This approach alone results in the emergence of species-specific vertical habitats, which are spatially and temporally variable. Although an explicit parameterization of the vertical dimension has not been carried out, the seasonal and vertical distribution patterns predicted by the model are in good agreement with sediment trap data and plankton tow observations. In the simulation, the colder-water species N. pachyderma, N. incompta, and G. bulloides show a pronounced seasonal cycle in their depth habitat in the polar and subpolar regions, which appears to be controlled by food availability. During the warm season, these species preferably occur in the subsurface, while towards the cold season they ascend through the water column and are found closer to the sea surface. The warm-water species G. ruber (white) and T. sacculifer exhibit a less variable shallow depth habitat with highest biomass concentrations within the top 40m of the water column. Nevertheless, even these species show vertical habitat variability and their seasonal occurrence outside the tropics is limited to the warm surface layer that develops at the end of the warm season. The emergence in PLAFOM2.0 of species-specific vertical habitats that are consistent with observations indicates that the population dynamics of planktonic foraminifera species may be driven by the same factors in time, space, and with depth, in which case the model can provide a reliable and robust tool to aid the interpretation of proxy records.