Abstract
Corals display circadian physiological cycles, changing from autotrophy during the day to heterotrophy during the night. Such physiological transition offers distinct environments to the microbial community associated with corals: an oxygen-rich environment during daylight hours and an oxygen-depleted environment during the night. Most studies of coral reef microbes have been performed on samples taken during the day, representing a bias in the understanding of the composition and function of these communities. We hypothesized that coral circadian physiology alters the composition and function of microbial communities in reef boundary layers. Here, we analyzed microbial communities associated with the momentum boundary layer (MBL) of the Brazilian endemic reef coral Mussismilia braziliensis during a diurnal cycle, and compared them to the water column. We determined microbial abundance and nutrient concentration in samples taken within a few centimeters of the coral's surface every 6 h for 48 h, and sequenced microbial metagenomes from a subset of the samples. We found that dominant taxa and functions in the coral MBL community were stable over the time scale of our sampling, with no significant shifts between night and day samples. Interestingly, the two water column metagenomes sampled 1 m above the corals were also very similar to the MBL metagenomes. When all samples were analyzed together, nutrient concentration significantly explained 40% of the taxonomic dissimilarity among dominant genera in the community. Functional profiles were highly homogenous and not significantly predicted by any environmental variables measured. Our data indicated that water flow may overrule the effects of coral physiology in the MBL bacterial community, at the scale of centimeters, and suggested that sampling resolution at the scale of millimeters may be necessary to address diurnal variation in community composition.
Highlights
The rules governing associations between bacteria and corals represent a current theme of debate (Thompson et al, 2015; Douglas and Werren, 2016; Theis et al, 2016)
We investigated the microbial community associated with the momentum boundary layer (MBL) over the Brazilian endemic reef coral Mussismilia braziliensis in response to light availability
Most studies investigating temporal variability in coral microbiomes are focused on relatively long time scales, weeks to months, with little information available on temporal variability at the diel scale (Glasl et al, 2016; Zaneveld et al, 2016)
Summary
The rules governing associations between bacteria and corals represent a current theme of debate (Thompson et al, 2015; Douglas and Werren, 2016; Theis et al, 2016). Temporal dynamics of bacterial community composition and functional roles in coral holobionts add important information to this debate (Garren and Azam, 2012; Bourne et al, 2016). The majority of studies on the changes in coral bacterial community composition over time have been focused on stress and disease conditions (Ritchie, 2006; Garren et al, 2009; Sweet et al, 2014; Zaneveld et al, 2016). Understanding the dynamics of coral-microbe associations over time is necessary in order to predict coral’s response to disturbance and design manipulation strategies to improve coral resilience (Ainsworth and Gates, 2016; Sweet and Brown, 2016)
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