A mass coral spawning event on the Heron Island reef flat in 2005 provided a unique opportunity to examine the response of a coral reef ecosystem to a large episodic nutrient addition. A post-major spawning phytoplankton bloom resulted in only a small drawdown of dissolved inorganic phosphorus (DIP minimum 5 0.37 mmol L 21 ), compared with almost complete removal of dissolved inorganic nitrogen (DIN) (minimum NO, suggesting that pelagic primary production is potentially N limited on the timescale of this study. DIN, DIP, dissolved organic nitrogen (DON), and dissolved organic phosphorus were used in the production of biomass, and mass balance calculations highlighted the importance of organic forms of N and P for benthic and pelagic production in tropical coral reef environments characterized by low inorganic N and P. The input of N and P via the deposition of coral spawn and associated phytodetritus resulted in large changes to N cycling in the sediments, but only small changes to P cycling, because of the buffering capacity provided by the large pool of bioavailable P. It is most likely that this large pool of bioavailable P in the sediments drives potential N limitation of benthic coral reef communities. For example, there was sufficient bioavailable P stored in the top 10 cm of the sediment column to sustain the prespawning rates of benthic production for over 200 d. Most of the change in benthic N cycling occurred via DON and N 2 pathways, driven by changes in the quantity and quality of organic matter deposited and decomposed post-major spawning. The heterotrophic and autotrophic microbial communities within the coral reef sands were able to rapidly (6 to 7 d) process the large episodic load of N and P provided by coral mass spawning.

Coral mass-spawning represents a spectacular annual, short-term, fertilization event of many oligotrophic reef communities. The spawning event in 2005 at Heron Island, Great Barrier Reef, was followed by an intense bloom of benthic dinoflagellates. Within a day from the first observed spawning, the primary production of the water column and the benthic compartment increased by factors of 4 and 2.5, respectively. However, the phototrophic communities were intensively grazed by macrozoans, and after 4-5 d the net photosynthesis (P) returned to the pre-spawning background level. The heterotrophic activity (R) mirrored the phototrophic response: a short term of elevated activity was followed by a rapid decline. However, the net autotrophic microbial communities exhibited a marked increase in the P : R ratio just after coral mass-spawning, indicating a preferential phototrophic recycling of nutrients rather than a microbial exploitation of the release of labile organic carbon. The heterotrophic and phototrophic activity of the benthic community exceeded the pelagic activity by ,2-and ,5-fold, respectively, underlining the importance of benthic activity for coral reef ecosystem function. Mass balance calculations indicated an efficient recycling of spawn-derived nitrogen (N) and carbon (C) within the benthic reef community. This was presumably facilitated by advective solute transport within the coarse, permeable, carbonate sand.

Transmission electron microscopy was employed to determine the morphological diversity of virus-like particles (VLPs) associated with the coral surface microlayer (CSM) of Acropora muricata and Porites spp. from the Great Barrier Reef, Australia. VLPs were assigned to one of 17 sub-groups within 5 major morphological groups including tailed bacteriophages, polyhedral, filamentous and lemon-shaped VLPs. Polyhedral VLPs in the 30 to 60 nm size class dominated the CSM of A. muricata and Porites spp., comprising 29.4 and 26.9% of total VLPs, respectively. Tailed bacteriophages comprised < 6% of total VLPs within the CSM of both A. muricata and Porites spp. Filamentous VLPs (FVLPs) of varying lengths and widths accounted for up to 19.9% of total CSM VLPs, with no significant difference between the CSM samples and overlying water. Unique VLPs, which could not be classified into any known viral morphological group, accounted for 1.2 to 11.7% of total VLPs within the CSM and were absent from overlying water. While some exchange of VLPs likely occurred between the CSM and overlying water, our results suggest that the majority of CSM morphotypes were specific to the CSM micro-niche. The similarity of many of these VLPs to previously described viruses suggests that a range of potential hosts exist in the CSM, including bacteria, archaea, cyanobacteria, fungi, algae (possibly including zooxanthellae) and the coral animal. Research on coral-microbial interactions and their role in coral health and functioning is in its infancy and the present study provides important information on the largely unstudied viral component of the coral microbiota.

Variations in the abundance and community characteristics of virus-like particles (VLP) and heterotrophic bacteria within a shallow, near-shore coral reef were determined using flow cytometric analysis. Mean concentrations of 6.5 × 10 5 and 1.3 × 10 5 ml -1 were observed for VLP and bacterioplankton, respectively, although concentrations of both populations varied significantly (p < 0.05) between 4 distinct reef water types. Significant (p < 0.05) variability in the percentage of high DNA (HDNA) bacteria, applied here as an estimate of the proportion of active bacterial cells, and the virus:bacteria ratio (VBR) was also observed between different reef water types. Microscale profiles were taken in the 12 cm layer of water directly above the surface of coral colonies to determine the small-scale spatial relationships between coral colonies and planktonic microbial communities. Across these profiles, mean changes of 2-and 3.5-fold were observed for bacterioplankton and VLP communities, respectively, with VLP abundance positively correlated to bacteria in 75% of profiles. Bacterial and VLP abundance, percentage of HDNA bacteria, and VBR all generally exhibited increasing trends with proximity to the coral surface. VLP abundance was significantly higher (p < 0.05) in the 4 cm closest to the coral surface, and the VBR was higher at the coral surface than in any other zone. The patterns observed here indicate that VLP represent an abundant and dynamic community within coral reefs, are apparently coupled to the spatial dynamics of the bacterioplankton community, and may consequently significantly influence nutrient cycling rates and food-web structure within coral reef ecosystems. KEY WORDS: Coral reefs · Virus-like particles · BacterioplanktonResale or republication not permitted without written consent of the publisher

Bacterial and virus-like particle (VLP) abundances and physical and chemical parameters were measured in reef water and sediments over a ten day period, coinciding with mass coral spawning at Heron Island, Great Barrier Reef (GBR). Bacterial abundances in reef water increased 2-fold after spawning and remained elevated for 3 days, before declining to below prespawning values. Reef water VLP abundances were also elevated 2 days after spawning, however, VLP abundances exhibited a general declined over the study. Dissolved oxygen (DO) and total nitrogen (TN) concentrations appeared to be dominant factors driving reef water bacterial and VLP dynamics. Sediment bacterial and VLP abundances exceeded those in the water column by up to three orders of magnitude. In contrast to no relationship occurring between reef water bacteria and VLPs, bacteria and VLPs in sediments exhibited strong positive correlations for all investigated depths. While short-lived peaks in bacterial and VLP abundances within sediments lagged behind water column trends by 2 days, reef water total phosphorous (TP) concentrations were strongly correlated with sediment bacterial and VLP abundances. Shifts in bacterial and VLP abundances in reef water and sediments during the study corresponded with two distinct periods; one prior to, and one after the first night of intense spawning. Scavenging by sedimenting coral spawn material is proposed as a direct mechanism contributing to these shifts, by removing bacteria and VLPs from the water column. The input of organic matter and associated nutrients from mass coral spawning, and the immediate and strongly correlated responses of bacteria and VLPs, indicate viruses are important players in nutrient cycling processes in coral reefs.