Effect of Competition on Growth and Fitness of Symbiodinium Populations in Culture

Abstract

According to the resource-ratio hypothesis, autotrophs with a lower minimum nutrient requirement will tend to dominate, as they further reduce nutrient availability to competitors, thus leading to dominance via competitive exclusion. With the use of bioreactors, minimum nitrogen and light requirements for Symbiodinium can be quantified and used to predict competitive outcomes among clades that co-occur in host corals. However, competitive outcomes are likely influenced by physical factors such as temperature and light, which can impact metabolic rates. Thus, a species may only be a superior competitor for limiting nutrients within its thermal or irradiance optima. We show that temperature drives different competitive outcomes in mixed cultures of Symbiodinium clades D and A. Clade D was always displaced by clade A, but more so at 32˚C (D:A cell ratio = 0.00), than at 26˚C (0.02) after 33 days of competition. In co-dominant populations, a slightly reduced photosynthetic efficiency (YII=0.376) was observed in comparison to pure cultures (YII=0.399). We hypothesis that reduced YII is a consequence of resources being allocated to competition and fewer to beneficial metabolic traits. The cost of hosting competing Symbiodinium populations might explain the rarity of mixed Symbiodinium populations in situ.