Photosynthetic carbon assimilation and electron transport rate in two symbiont-bearing planktonic foraminifera

Photosymbiosis is one of the important features in planktonic foraminifera. The number of symbiont cells within one host is reported to be well over a few thousand, which means that photosynthesis by photosymbiosis might be a “hot spot” of primary production, especially in oligotrophic oceans. Information of photosynthetic activity of symbionts is also essential when interpreting the geochemical proxies recorded in foraminiferal tests because the microenvironmental condition in the vicinity of foraminifera is greatly affected by rapid biological activities such as photosynthesis and respiration. Recently, active chlorophyll fluorometry is increasingly being used as a useful and instant tool to estimate photosynthesis. However, the carbon assimilation rate is the only direct measure of photosynthetic carbon flow. Therefore, confirming the relationship between the active fluorometry-based photosynthetic rate (electron transport rate, ETR) and carbon assimilation rate (CAR) is required before utilizing ETR to understand the dynamics of carbon in the foraminifera-symbiont system.

Here, we compared CAR and ETR for two species, Trilobatus sacculifer (dinoflagellate-bearing) and Globigerinella siphonifera Type II (pelagophyte-bearing). CAR was estimated using ¹⁴C‐tracer experiment and ETR was estimated using active fluorometric measurement by fast repetition rate fluorometry.

The results showed that the CAR and ETR were correlated positively (p << 0.01) for both species. However, the regression slopes of the two species were largely different. The slope, representing the apparent electron requirement for carbon assimilation (e⁻/C), was estimated to 28.5 for T. sacculifer and 101.1 for G. siphonifera. These values were strikingly high. Theoretically, under optimal growth conditions, phototrophs’ e⁻/C should be 4 based on the minimum number of electrons derived from 2 water molecules to generate 1 oxygen molecule. So, we hypothesized that the observed high e⁻/C in the foraminifera-algal consortia is partly attributable to the utilization of unlabeled respiratory carbon (resulting in underestimation of CAR). Considering the theoretical and empirically realistic e⁻/C, we estimated the proportion of the carbon source for photosynthesis. The results showed that a considerable amount of carbon should be derived from the host’s respired CO₂. The higher contribution of the respired CO₂ was suggested in G. siphonifera than in T. sacculifer.

From the viewpoint of utilizing test geochemistry such as δ¹³C as paleoceanographic proxies, one should beware that the potential magnitude of the photosynthetic effect can differ between species. This study suggests that in G. siphonifera, photosynthetic carbon incorporation from seawater is smaller, and utilization of the host-derived carbon by symbionts is more efficient, indicating that G. siphonifera would be less susceptible to the alteration of geochemical composition by photosynthesis and respiration. This attempt to couple the ETR and CAR could comprehensively disclose an interesting perspective of these intimate interactions in the photosymbiotic system.