Abstract
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.
Highlights
Algal photosymbiosis is a form of mixotrophy
There was a clear difference between the growth profiles of the fed and unfed groups (Figs. 2 and 3)
Some specimens in the unfed groups shed their original or newly precipitated chamber(s) (Fig. 2c, d), which was not observed in the fed groups
Summary
It allows for greater flexibility regarding resource acquisition as dissolved inorganic nutrients are obtained by the algal symbionts, and organic particulate foods are made available through feeding by the host (Fig. 1). This pattern is commonly observed in various marine organisms such as hermatypic corals, sea anemones, foraminifers, and radiolarians, especially those inhabiting oligotrophic environments Despite this, photosymbiosis in plankton are poorly understood at the organismal level, compared with that in benthic organisms. This is partly because of difficulties in culturing such minute free-floating plankton
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