Growth and Mortality in the Marine Calcifier Globigerina Bulloides

Introduction
The marine calcifier Globigerina bulloides is a significant contributor to the global carbon cycle and an important tool for studying past ocean-climate interactions through e.g., changes in the geochemistry of its fossil shells. Despite the widespread applications of the species, however, there are extensive knowledge gaps surrounding its biological behaviours and life cycle, potentially creating significant uncertainties in scientific applications. One of these uncertainties is the presence of a potential coupling between growth rate, mortality, and diet, which could have a significant impact on shell cycling through the water column and geochemistry.

To this end, we designed a set of live culturing experiments to be carried out in summer 2023, to closely monitor how diet, growth and mortality relate to one another, and subsequently investigate how these parameters impact the geochemistry of their shells. These new growth results will improve understanding of calcification and the life cycle of G. bulloides, and thus their contribution to the global carbon cycle, while also offering necessary insight into how biological factors can impact the geochemistry of fossil shells used for past ocean reconstructions.

Method
Culturing was carried out onboard RV Celtic Explorer in summer 2023, as part of the Seawater for culturing was taken at 55 5.120 N, 11 7.230 W. The water was filtered and manipulated to match the water conditions used in previous experiments. These were designed to mirror expected salinity, pH, and temperature at the collection site. Water was spiked with 135Ba so that any chamber growth in culture could be tracked through geochemical analysis. 143 live individuals of G. bulloides were caught at 55 38.880N, 14 0.530 W. Each specimen was placed into an individual culturing well and observed and photographed regularly.

The specimens were split by dietary treatment. Food sources included a N. Atlantic phytoplankton blend, copepods, Artemia sp. nauplii and the diatom Attheya longicornis. One group was fed nothing to act as a control. The PyroScience temperature and pH sensors were used to monitor temperature and pH stability of the experiment throughout its continuation. O2 levels were measured in the initial and post growth waters.

Preliminary Data
Intermittent monitoring using the PyroScience FireSting sensor system shows that temperature and pH remained stable throughout the experiment, meaning variation in individual responses can be attributed to diet as the controlled variable.

Initial data suggests that mortality rate may have been impacted by diet in the case of treatment 1, however, the link is less clear for growth.
Final growth rates for each treatment will be obtained by measuring the individuals using microscopy imagery. The consequence on geochemistry will be further studied through laser ablation mass spectrometry of the shells.

G. bulloides were collected from temperate waters in the North Atlantic and kept in culture for 4 weeks under different dietary regimes. The impact on mortality and growth was observed. The results of this study will improve the understanding of a marine calcifier and thus its application to palaeoceanographic reconstructions.

Fig. 3 Initial mortality data for the diffferent treatments

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Fig. 1: Calibrating the pH sensor in preparation for manipulating water chemistry

Fig. 2 Collecting specimens for culture

Fig. 4 Healthy specimen of G. bulloides after one week in culture. All chambers are filled with cytoplasm, spines are present, and rhizopods are extended