Not just who, but how many: the importance of partner abundance in reef coral symbioses

Abstract

The performance and function of reef corals depends on the genetic identity of their symbiotic algal partners, with some symbionts providing greater benefits (e.g., photosynthate, thermotolerance) than others. However, these interaction outcomes may also depend on partner abundance, with differences in the total number of symbionts changing the net benefit to the coral host, depending on the particular environmental conditions. We suggest that symbiont abundance is a fundamental aspect of the dynamic interface between reef corals and the abiotic environment that ultimately determines the benefits, costs, and functional responses of these symbioses. This density-dependent framework suggests that corals may regulate the size of their symbiont pool to match microhabitat-specific optima, which may contribute to the high spatiotemporal variability in symbiont abundance observed within and among colonies and reefs. Differences in symbiont standing stock may subsequently explain variation in energetics, growth, reproduction, and stress susceptibility, and may mediate the impacts of environmental change on these outcomes. However, the importance of symbiont abundance has received relatively little recognition, possibly because commonly-used metrics based on surface area (e.g., symbiont cells cm-2) may be only weakly linked to biological phenomena and are difficult to compare across studies. We suggest that normalizing symbionts to biological host parameters, such as units of protein or numbers of host cells, will more clearly elucidate the functional role of symbiont abundance in reef coral symbioses. In this article, we generate testable hypotheses regarding the importance of symbiont abundance by first discussing different metrics and their potential links to symbiosis performance and breakdown, and then describing how natural variability and dynamics of symbiont communities may help explain ecological patterns on coral reefs and predict responses to environmental change.

Cell Cycle Dynamics of Cultured Coral Endosymbiotic Microalgae (Symbiodinium) Across Different Types (Species) Under Alternate Light and Temperature Conditions

“…Growth status of Symbiodinium is important for maintaining a stable symbiosis with the host (e.g. Cunning and Baker ). Consequently, application of cell cycle analysis to Symbiodinium in hospite, not only under steady‐state conditions but also under periodic stress, is further needed to better understand mechanisms of maintenance and hence of breakdown of symbiosis at the level of individual cells.…”

Mutualism or parasitism: Partner abundance affects host fitness in a fish reproductive interaction

“…Partner abundance is a key source of context dependency in symbioses (Chomicki & Renner, 2017;Cunning & Baker, 2014;Kiers, Palmer, Ives, Bruno, & Bronstein, 2010). Maximum host fitness occurs at intermediate partner density in many symbioses (Brown et al, 2012;Izzo & Vasconcelos, 2002;Morales, 2000;Palmer & Brody, 2013).…”

Heritability of symbiont density reveals distinct regulatory mechanisms in a tripartite symbiosis

“…Even in the case of vertically transmitted symbionts, strict regulation of symbiont density within the host is essential for the efficient functioning of the partnership (Falkowski et al 1993;Rio et al 2006;Wilkinson et al 2007;Cunning and Baker 2014;Laughton et al 2014). Too few symbiont cells will cause a deficiency of gene products for the host and inefficient vertical transmission for the symbiont, while too many cells will incur some cost to the host without a proportionate benefit.…”