© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published inWeber, L., & Apprill, A. Diel, daily, and spatial variation of coral reef seawater microbial communities. PLoS One, 15(3), (2020): e0229442, doi: 10.1371/journal.pone.0229442.
Reef organisms influence microorganisms within the surrounding seawater, yet the spatial and temporal dynamics of seawater microbial communities located in proximity to corals are rarely investigated. To better understand reef seawater microbial community dynamics over time and space, we collected small-volume seawater samples during the day and night over a 72 hour period from three locations that differed in spatial distance from 5 Porites astreoides coral colonies on a shallow reef in St. John, U.S. Virgin Islands: near-coral (sampled 5 cm horizontally from each colony), reef-depth (sampled 2 m above each colony) and surface seawater (sampled 1 m from the seawater surface). At all time points and locations, we quantified abundances of microbial cells, sequenced small subunit rRNA genes of bacterial and archaeal communities, and measured inorganic nutrient concentrations. Prochlorococcus and Synechococcus cells were consistently elevated at night compared to day and these abundances changed over time, corresponding with temperature, nitrite, and silicate concentrations. During the day, bacterial and archaeal alpha diversity was significantly higher in reef-depth and near-coral seawater compared to the surface seawater, signifying that the reef influences the diversity of the seawater microorganisms. At night, alpha diversity decreased across all samples, suggesting that photosynthesis may favor a more taxonomically diverse community. While Prochlorococcus exhibited consistent temporal rhythmicity, additional taxa were enriched in reef seawater at night compared to day or in reef-depth compared to surface seawater based on their normalized sequence counts. There were some significant differences in nutrient concentrations and cell abundances between reef-depth and near-coral seawater but no clear trends. This study demonstrates that temporal variation supersedes small-scale spatial variation in proximity to corals in reef seawater microbial communities. As coral reefs continue to change in benthic composition worldwide, monitoring microbial composition in response to temporal changes and environmental fluctuations will help discern normal variability from longer lasting changes attributed to anthropogenic stressors and global climate change.
This work was supported by a National Science Foundation (NSF; https://www.nsf.gov/) Graduate Research Fellowship award to L.Weber. This research was also supported by NSF award OCE-1536782 to A. Mooney, J. Llopiz, and A. Apprill and NSF award OCE-1736288 to A. Apprill. Additionally, this work was supported by the NOAA Cooperative Institutes award NA19OAR4320074 to A.A. and E. Kujawinski and the Andrew W. Mellon Foundation Endowed Fund for Innovative Research to A.A.
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