BackgroundScleractinian corals and their algal endosymbionts (genus Symbiodinium) exhibit distinct bathymetric distributions on coral reefs. Yet, few studies have assessed the evolutionary context of these ecological distributions by exploring the genetic diversity of closely related coral species and their associated Symbiodinium over large depth ranges. Here we assess the distribution and genetic diversity of five agariciid coral species (Agaricia humilis, A. agaricites, A. lamarcki, A. grahamae, and Helioseris cucullata) and their algal endosymbionts (Symbiodinium) across a large depth gradient (2-60 m) covering shallow to mesophotic depths on a Caribbean reef.ResultsThe five agariciid species exhibited distinct depth distributions, and dominant Symbiodinium associations were found to be species-specific, with each of the agariciid species harbouring a distinct ITS2-DGGE profile (except for a shared profile between A. lamarcki and A. grahamae). Only A. lamarcki harboured different Symbiodinium types across its depth distribution (i.e. exhibited symbiont zonation). Phylogenetic analysis (atp6) of the coral hosts demonstrated a division of the Agaricia genus into two major lineages that correspond to their bathymetric distribution (“shallow”: A. humilis / A. agaricites and “deep”: A. lamarcki / A. grahamae), highlighting the role of depth-related factors in the diversification of these congeneric agariciid species. The divergence between “shallow” and “deep” host species was reflected in the relatedness of the associated Symbiodinium (with A. lamarcki and A. grahamae sharing an identical Symbiodinium profile, and A. humilis and A. agaricites harbouring a related ITS2 sequence in their Symbiodinium profiles), corroborating the notion that brooding corals and their Symbiodinium are engaged in coevolutionary processes.ConclusionsOur findings support the hypothesis that the depth-related environmental gradient on reefs has played an important role in the diversification of the genus Agaricia and their associated Symbiodinium, resulting in a genetic segregation between coral host-symbiont communities at shallow and mesophotic depths.

Microorganisms play an important role in coral ecology; however, little is known about cold-water coral (CWC) reefs. Microbial abundance in bottom water of Lophelia pertusadominated CWC reefs in the Skagerrak, North Sea, was elevated by ca. 40% for prokaryotes and 70 to 90% for viruses compared to bottom water where no corals were present. This indicates that CWCs influence the microbial food web in bottom water. Furthermore, results suggest that microorganisms contributed significantly to the stock of organic nitrogen (13%) and organic phosphorus (32%) in bottom water. Prokaryotic abundances in the coelenteron of freshly collected corals were (2.0 ± 0.2) × 10 7 cells ml −l and viral abundances were (4.0 ± 0.6) × 10 7 particles ml −l . Prokaryotic abundances in coral mucus were (7.5 ± 0.7) × 10 7 cells ml −l and viral abundances were (1.7 ± 0.3) × 10 8 particles ml −l . Onboard incubations with elevated prokaryotic and viral abundances in the incubation water indicated that (1) corals accumulate microorganisms in the coelenteron as a potential food source and (2) control mechanisms exist for the abundance of prokaryotic cells and viral particles in the mucus. Our data suggest that L. pertusa controls microbial biomass in the CWC reef environment and coral microhabitats. KEY WORDS: Cold-water coral · Microbial abundance · Lophelia pertusa · Viruses · Prokaryotes · Bacteria · ReefsResale or republication not permitted without written consent of the publisher Aquat Biol 16: 209-216, 2012 210 barrier against incoming fowling or pathogenic prokaryotes (Reshef et al. 2006) and as a buffer against environmental change (Rosenberg et al. 2007).Recent research has shown that viral abundance decreases with distance from the coral (Seymour et al. 2005, Patten et al. 2006 and that the mucus contains an astounding morphological diversity of viruses (Davy & Patten 2007). To our knowledge, no data have been published on the abundance of viruses in the mucus or coelenteron of corals, and the viral ecology of the coral holobiont remains largely unstudied.This study had 3 objectives: (1) to quantify prokaryotic and viral abundances (and inorganic and organic nutrients) in a CWC reef environment in the Skagerrak, North Sea, (2) to quantify prokaryotic and viral abundance in 2 major microhabitats of Lophelia pertusa, viz. the coelenteron and the mucus, and (3) to test whether an increase in the abundance of prokaryotes and viruses in ambient water by ca. 1 order of magnitude can change abundances in the coelenteron and mucus. MATERIALS AND METHODSSamples were obtained during the BIOSYS cruise (www. nioz. nl/public/dmg/rpt/crs/64pe263.pdf) on the RV 'Pelagia' in March 2007 to the Skagerrak of Norway at Fjellknausene (59.07°N, 10.74°E) and Soester (59.08°N, 10.76°E), ca. 100 km south of Oslo (Norway). Colonies of Lophelia pertusa and water samples were collected at 78 to 128 m depth from a reef site that was dominated by L. pertusa. The water column was characterized by a conductivity-temperaturedensity (CTD) rosette ...

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