CHANGES IN THE ULTRASTRUCTURE OF SYMBIOTIC ZOOXANTHELLAE (SYMBIODINIUM SP., DINOPHYCEAE) IN FED AND STARVED SEA ANEMONES MAINTAINED UNDER HIGH AND LOW LIGHT¹

Abstract

The ultrastructure of symbiotic dinoflagellates (Symbiodinium sp., zooxanthellae) in the sea anemone Aiptasia pallida Verrill was examined in well‐fed or starved (up to 120 days) anemones maintained under two light levels (5 and 50 μmol · m−2· s−1). Cell size of zooxanthellae was not affected by feeding history; however, both light and feeding history affected the relative cell volume of chloroplasts, lipids, and vacuoles. Stereological analysis of transmission electron micrographs showed that algae in low‐light starved anemones had 10 times as much lipid (17.4% of cell volume) as those in well‐fed anemones under the same light conditions (1.8%). The lipid content of algae from anemones in high light increased from 15.4% in well‐fed anemones to 30.1% in starved anemones. The starch content of zooxanthellae in low‐light anemones was law (4.1%) and not affected by feeding history, while the starch content of zooxanthellae in high‐light anemones was greater (10.7%), with some differences among groups. Algal photoacclimation to low light included an increase in chloroplast relative volume from 17% (in well‐fed high‐light anemones) to 33% in well‐fed low‐light anemones. Starvation of the host resulted in a significant decrease in chloroplast volume in zooxanthellae in anemones at both light levels. Morphometry provides quantitative confirmation of biochemical and physiological data on zooxanthellae, because the changes in zooxanthellae with starvation of the host are consistent with other indicators of nutrient limitation of zooxanthellae of A. pallida held without food for long periods of time.

Cell Biology of Cnidarian-Dinoflagellate Symbiosis

“…One hypothesis is that the host could limit symbiont growth, either by restricting the supply of nutrients or by actively "blocking" symbiont mitosis (see the previous section), thus generating a surplus of photosynthetic carbon for release (105,267). It is important to note, however, that nutrient limitation may cause carbon storage rather than release in symbiotic dinoflagellates (78,251), as it also does in free-living microalgae (375).…”

Early ontogenetic expression of specificity in a cnidarian-algal symbiosis

“…Potential sources include zooxanthellae released by cnidarians (Steele 1977, Hoegh-Guldberg et al 1987, HoeghGuldberg & Smith 1989, Stimson & Kinzie 1991, Davy et al 1997, Maruyama & Heslinga 1997, those excreted by fish (Muller-Parker et al 1996, Augustine & Muller-Parker 1998, and possibly populations in the benthos (Carlos et al 1999). Given that infection rate is correlated to zooxanthella concentrations (Montgomery & Kremer 1995, Kinzie et al 2001), a comparison of the rates of zooxanthella acquisition under known zooxanthella concentrations with that observed in situ provides a rough estimate of zooxanthella abundance in the field.…”

Zooxanthellae that open calcium channels: implications for reef corals

“…Algal toxins may speed coral calcification during nutrient shortages. Nutrient shortages freeze zooxanthellae in the G1 phase of the cell cycle (Muscatine et al 1989, Muller-Parker et al 1996, Smith & Muscatine 1999, Wang et al 2008; Fig. 6a), where dinoflagellates are often most toxic (Pan et al 1999, Taroncher-Oldenburg et al 1999, Guisande et al …”

“…6a), they can take coral nutrients as they leave, and they will likely die if they remain inside a bleaching coral (Dunn et al 2004). The coral may also evict the algae, if they become too toxic, and mainly produce lipids (Muller-Parker et al 1996).…”