In many of these partnerships, the symbiotic microalgae provide their hosts with up to 100% of their energy requirements in the form of glycerol, peptides, and carbohydrates. In exchange, the microalgae obtain inorganic nutrients from hosts and an ideal habitat.

Background:

Previously, we showed how giant clams were in a unique symbiotic partnerships with the dinoflagellate microalgae. The symbiotic microalgae provide their host with up to 100% of their energy budgets in the form of glycerol, peptides, and carbohydrates. In return, the microalgae obtain inorganic nutrients from the hosts and an ideal habitat.  This partnership provides advantages to the host through acquiring higher energy budgets, and to symbionts through the availability of nutrients, and this is especially efficient in nutrient-poor environments such as the tropical coral reefs.

Unlike the corals that hold symbiotic microalgae in the endodermal cells lining the gastrovascular cavity, symbionts in giant clams are found intercellularly within the siphonal mantle, specifically in the tertiary tubes of the zooxanthellal tubular system (See the following figure). These tubules carry high densities of endosymbionts and are located underneath the iridocytes, which is a layer of light-scattering cells on the surface of mantle tissue that give rise to the wide variety of mantle colours.

Armstrong et al. 2018
Proposed model of zooxanthellae tubule system relative to endosymbiotic arrangement in corals (Extracted from Armstrong et al. 2018).

How it works – Tiny catalysts running the engines!

The mechanisms of inorganic nutrient transport between host clams and symbionts were not well-known, other than the location and arrangement of symbionts in the giant clams. Now there is more information on how tiny catalysts are promoting Symbiodiniaceae photosynthesis in the giant clam siphonal mantle tubule system!

There has been an exceptional effort by researchers in the past 5 years discovering these pathways through the use of molecular approach. They have identified a number of enzymes (e.g. anhydrases, ATPases, H+ transporters, etc…) that are involved in facilitating the movement of inorganic carbon and nitrogen across the tissue membranes between host clams and symbionts.

Here’s a quick overview of what it could look like inside the giant clam! To help simplify, there are these enzymes that catalyses chemical reactions – dissociating and associating chemical bonds, and in the process, move these chemicals across boundaries and then turning into useful compounds that can be used by the host or symbionts. 😉

Ip et al. 2018
Proposed schematic on the involvement of the host Vacuolar-type H+-ATPase (WHA) in the uptake of exogenous inorganic carbon in the ctenidium and the transport of absorbed inorganic carbon to the symbiotic microalgae in the outer mantle of Tridacna squamosa. (a) Between ambient seawater and hemolymph of host clam, (b) Between hemolymph of host clam and its zooxanthellae tubules. (Extracted from Ip et al. 2018).
Chew et al. 2019
A proposed schematic on how Carbonic Anhydrase (CA) 4-like (CA4-like), CA20like, and possibly other transporters in the inner mantle of Tridacna squamosa to absorb inorganic carbon from ambient seawater. Several other unknowns (such as PT?, BT?) remain unidentified for now. (Extracted from Chew et al. 2019).

When these results are taken altogether, they help to give us some inkling on how these nutrients move efficiently between host and symbionts, and how they may be used to support the formation of shell (Ip et al. 2015; Hiong et al. 2017; Chew et al. 2019), assimilation and excretion of urea (Boo et al. 2018; Chan et al. 2018), and possibly the prevention of oxidative stresses (Hiong et al. 2018).

I have to admit that you have to read these papers collectively and draw the conclusions together to get a better idea of the overall system! 😀 Good luck with that! So, if you are interested in any of the papers, drop me an email to request for them! Thank you! 🙂

Further readings:

  • Armstrong EJ, JN Roa, JH Stillman & M Tresguerres (2018) Symbiont photosynthesis in giant clams is promoted by V-type H+-ATPase from host cells. Journal of Experimental Biology 221: jeb177220.
  • Boo MV, KC Hiong, EJK Goh, CYL Choo, WP Wong, SF Chew & YK Ip (2018) The ctenidium of the giant clam, Tridacna squamosa, expresses an ammonium transporter 1 that displays light-suppressed gene and protein expression and may be involved in ammonia excretion. Journal of Comparative Physiology B 188: 765–777.
  • Boo MV, KC Hiong, CYL Choo, AH Cao-Pham, WP Wong, SF Chew & YK Ip (2017) The inner mantle of the giant clam, Tridacna squamosa, expresses a basolateral Na+/K+-ATPase alpha-subunit, which displays light-dependent gene and protein expression along the shell-facing epithelium. PLoS ONE 12(10): e0186865.
  • Cao-Pham AH, KC Hiong, MV Boo, CYL Choo, CZ Pang, WP Wong, ML Neo, SF Chew & YK Ip (2019) Molecular characterization, cellular localization, and light-enhanced expression of Beta-Na+ / H+ Exchanger-like in the whitish inner mantle of the giant clam, Tridacna squamosa, denote its role in light-enhanced shell formation. Gene 695: 101–112.
  • Chan CYL, KC Hiong, MV Boo, CYL Choo, WP Wong, SF Chew & YK Ip (2018) Light exposure enhances urea absorption in the fluted giant clam, Tridacna squamosa, and up-regulates the protein abundance of a light-dependent urea active transporter, DUR3-like, in its ctenidium. Journal of Experimental Biology 221: jeb176313.
  • Chew SF, CZY Koh, KC Hiong, CYL Choo, WP Wong, ML Neo & YK Ip (2019) Light-enhanced expression of Carbonic Anhydrase 4-like supports shell formation in the fluted giant clam Tridacna squamosa. Gene 683: 101–112.
  • Hiong KC, CZY Koh, MV Boo, CYL Choo, WP Wong, SF Chew & YK Ip (2018) The colorful mantle of the giant clam, Tridacna squamosa, expresses a light-dependent manganese superoxide dismutase to ameliorate oxidative stresses due to its symbiotic association with zooxanthellae. Coral Reefs 37: 1039–1051.
  • Hiong KC, CYL Choo, MV Boo, B Ching, WP Wong, SF Chew & YK Ip (2017) A light-dependent ammonia-assimilating mechanism in the ctenidia of a giant clam. Coral Reefs 36: 311–323.
  • Hiong KC, AH Cao-Pham, CYL Choo, MV Boo, WP Wong, SF Chew & YK Ip (2017) Light-dependent expression of a Na+/H+ exchanger 3-like transporter in the ctenidium of the giant clam, Tridacna squamosa, can be related to increased H+ excretion during light-enhanced calcification. Physiological Reports 5(8): e13209.
  • Ip YK, KC Hiong, LJY Lim, CYL Choo, MV Boo,WP Wong, ML Neo & SF Chew (2018) Molecular characterization, light-dependent expression, and cellular localization of a host vacuolar-type H+-ATPase (VHA) subunit A in the giant clam, Tridacna squamosa, indicate the involvement of the host VHA in the uptake of inorganic carbon and its supply to the symbiotic zooxanthellae. Gene 659: 137–148.
  • Ip YK, CZY Koh, KC Hiong, CYL Choo, MV Boo, WP Wong, ML Neo & SF Chew (2017) Carbonic anhydrase 2-like in the giant clam, Tridacna squamosa: characterization, localization, response to light, and possible role in the transport of inorganic carbon from the host to its symbionts. Physiological Reports 5(23): e13494.
  • Ip YK, KC Hiong, EJK Goh, MV Boo, CYL Choo, B Ching, WP Wong & SF Chew (2017) The whitish inner mantle of the giant clam, Tridacna squamosa, expresses an apical plasma membrane Ca2+-ATPase (PMCA) which displays light-dependent gene and protein expressions. Frontiers in Physiology 8: 781.
  • Ip YK, B Ching, KC Hiong, CYL Choo, MV Boo, WP Wong & SF Chew (2015) Light induces changes in activities of Na+/K+-ATPase, H+/K+-ATPase and glutamine synthetase in tissues involved directly or indirectly in light-enhanced calcification in the giant clam, Tridacna squamosaFrontiers in Physiology 6: 68.
  • Koh CZ, KC Hiong, CY Choo, MV Boo, WP Wong, SF Chew, ML Neo & YK Ip (2018) Molecular characterization of a Dual Domain Carbonic Anhydrase from the ctenidium of the giant clam, Tridacna squamosa, and its expression levels after light exposure, cellular localization, and possible role in the uptake of exogenous inorganic carbon. Frontiers in Physiology 9: 281.
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