Biological Studies of Modern Larger Benthic Foraminifers

概要

Studying calcification organisms are essential for understanding environmental changes in the tropical to subtropical seas. Although symbiont-bearing larger benthic foraminifers (LBF) which is one of important calcification organism with complex calcium carbonate shells living in coral reefs provide accurate and long-term information of climate changes, their biological and ecological knowledges are still not enough for environmental predictions. In this study, I investigated the symbiont-bearing LBF by means of MicroCT for improving our understanding about biological states of them under natural conditions and examined response of LBF to global warming by culturing experiment in laboratory.

In Chapter 1 and 2, I investigated the symbiont-bearing benthic foraminifer Palaeonummulites venosus to examine the chamber building rate (CBR), test diameter increases rate (DIR), reproduction time, longevity and growth oscillation using the ‘natural laboratory approach’. This is based on the decomposition of monthly obtained frequency distributions of chamber number and test diameter into normal-distributed components. The shift of the component parameters ‘mean’ and ‘standard deviation’ during the 15-month investigation period was used to calculate Michaelis-Menten functions applied to estimate the averaged CBR and DIR under natural conditions. The individual dates of birth were estimated using the inverse averaged CBR and the inverse DIR fitted by the individual chamber number or the individual test diameter at the sampling date. Distributions of frequencies and densities (i.e. frequency divided by sediment weight) based on CBR and DIR both revealed a continuous reproduction throughout the year with two peaks, a stronger one in June determined as the onset of the summer generation (generation 1) and a weaker one in November determined as the onset of the winter generation (generation 2). This reproduction scheme explains the presence of small and large specimens in the same sample. Oscillations in cell growth of P. venosus are mainly caused by differing light intensities during chamber construction that can be correlated with tidal cycles. For nummulitids living in the mid and deep euphotic zone, weak differences in water depth caused by variable tidal heights enriches the photosynthetic rate of the endosymbiotic diatoms during neap tides. A continuous reproduction rate of P. venosus throughout the year is increased in subtropical calms by higher summer temperature and the input of inorganic nutrients during raining seasons. The expected lifetime of both megalospheric generations ranges from 8 (very rare) to 21 months with an optimum at 18 months.

In Chapter 3, I cultured asexually-produced individuals of Sorites orbiculus under six different temperature conditions to examine the effects of SST on LBF growth. Experimental results indicate that growth rate, measured by shell weight, shell volume and the number of chambers added, generally increase with highering temperature from 19 °C to 27 °C after 12 weeks of culture. On the other hand, calculated shell density is in constant whole range of experiment temperature, it suggested that shell weight was most closely dependent upon their shell volume. Their growth rates of shell weight and shell volume were reversed approximately at 27 °C, in spite of their growth rate of number of chambers added was still in constant from 27 °C to 29 °C. Therefore, the optimum temperatures for their chamber growth rate and calcification rate might be showed some deviation. This deviation would be caused by deviation of optimum temperatures of LBF and their photosynthesis symbionts. Finally, in future predicted global warming will be disturbed LBF calcification.