Increasing algal cover on tropical reefs worldwide may be maintained through feedbacks whereby algae outcompete coral by altering microbial activity. We hypothesized that algae and coral release compositionally distinct exudates that differentially alter bacterioplankton growth and community structure. We collected exudates from the dominant hermatypic coral holobiont Porites spp. and three dominant macroalgae (one each Ochrophyta, Rhodophyta and Chlorophyta) from reefs of Mo'orea, French Polynesia. We characterized exudates by measuring dissolved organic carbon (DOC) and fractional dissolved combined neutral sugars (DCNSs) and subsequently tracked bacterioplankton responses to each exudate over 48 h, assessing cellular growth, DOC/DCNS utilization and changes in taxonomic composition (via 16S rRNA amplicon pyrosequencing). Fleshy macroalgal exudates were enriched in the DCNS components fucose (Ochrophyta) and galactose (Rhodophyta); coral and calcareous algal exudates were enriched in total DCNS but in the same component proportions as ambient seawater. Rates of bacterioplankton growth and DOC utilization were significantly higher in algal exudate treatments than in coral exudate and control incubations with each community selectively removing different DCNS components. Coral exudates engendered the smallest shift in overall bacterioplankton community structure, maintained high diversity and enriched taxa from Alphaproteobacteria lineages containing cultured representatives with relatively few virulence factors (VFs) (Hyphomonadaceae and Erythrobacteraceae). In contrast, macroalgal exudates selected for less diverse communities heavily enriched in copiotrophic Gammaproteobacteria lineages containing cultured pathogens with increased VFs (Vibrionaceae and Pseudoalteromonadaceae). Our results demonstrate that algal exudates are enriched in DCNS components, foster rapid growth of bacterioplankton and select for bacterial populations with more potential VFs than coral exudates.

At the Bermuda Atlantic Time-Series Study (BATS) site, the field observations of dissolved organic carbon (DOC) dynamics indicate that seasonally produced ''semilabile'' DOC is resistant to rapid microbial degradation in the surface waters but available for microbial remineralization once it is delivered into the mesopelagic zone after convective overturn. In this study, we employed an experimental simulation of convective overturn events to determine whether the remineralization of semilabile DOC would occur in a controlled laboratory setting. Seawater culture experiments were conducted in which surface (Յ10 m) and mesopelagic (250 m) 0.2-m filtrates were inoculated with unfiltered water from Յ10 and 250 m in an assortment of combinations to simulate various mixtures of nutrients, DOC quantity and quality, and microbial assemblages. Results indicate that (1) microbial inocula from the upper euphotic zone were incapable of remineralizing the seasonally accumulated semilabile DOC (mol C L Ϫ1 resolution) on the timescales of the incubations; (2) the utilization of semilabile DOC was greatest when the inoculum source was from 250 m and the filtrate source was from the upper 10 m; and (3) the decrease in bacterioplankton diversity, estimated with the Shannon-Wiener diversity index, was greater in treatments in which inoculum from 250 m was mixed with filtrate from 10 m than in treatments in which the surface inoculum was mixed with the surface filtrate. Our findings are that a portion of the surface semilabile DOC can be metabolized by microorganisms in a laboratory setting and that mesopelagic nutrients alone are insufficient to stimulate DOC drawdown Ͼ1.3 mol L Ϫ1. Transformations of microbial community structure were associated with the drawdown of surface DOC in simulated mixing events and suggest that microbial community structure is a factor in surface-layer DOC dynamics.The production of oceanic dissolved organic carbon (DOC), regardless of mechanism (see Carlson 2002), is ul-

Bacterial abundance, DOC concentrations, and bacterioplankton community structure (using PCR-based techniques) were measured in 5 seawater culture experiments conducted near the Bermuda Atlantic Time-series Study (BATS) site in the northwestern Sargasso Sea. Cultures were amended with inorganic and organic nutrients, alone or in combination, to test the existence of the 'malfunctioning microbial loop' during late spring and summer at BATS. Objectives of the study were to determine whether (1) alleviating grazing pressure and inorganic nutrient limitation stimulated DOC remineralization by bacterioplankton; (2) a combination of organic and inorganic nutrients affect bacterial production and utilization of seasonally accumulated DOC; and (3) shifts in bacterioplankton community structure are associated with nutrient amendment and DOC utilization. In unamended cultures natural assemblages of surface bacterioplankton did not utilize detectable amounts of naturally occurring 'semi-labile' DOC over time-scales of days to weeks. Neither bacterial production nor utilization of DOC was enhanced with the addition of inorganic N or P (alone or in combination). Labile DOC amendments stimulated bacterial production and DOC utilization, even in the absence of measurable inorganic nutrients, indicating that the bacterioplankton assemblage was initially energy limited, but did not stimulate utilization of seasonally accumulated DOC. The combination of inorganic N and P with labile DOC enhanced both bacterial production and utilization of 'semi-labile' DOC. Changes in bacterioplankton community rDNA gene profiles were minor in the control and inorganic treatments; however, utilization of 'semi-labile' DOC in the organic plus inorganic nutrient treatments coincided with significant changes in bacterioplankton community structure. These data suggest that bacterioplankton community structure, as well as nutrient regime, may be important factors governing the utilization of recalcitrant DOC substrates in the northwestern Sargasso Sea. KEY WORDS: Dissolved organic carbon · Nutrient limitation · 16S rRNA · BATSResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 30: 19-36, 2002 Hodson 1977, Azam et al. 1983). Net DOC production within the euphotic zone has been documented for a variety of oceanic sites (Duursma 1963, Parsons et al. 1970, Eberlein et al. 1985, Copin-Montégut & Avril 1993, Carlson et al. 1994, Børsheim & Myklestad 1997, Hansell & Carlson, 1998b, Hansell & Carlson 2001. The subsequent vertical mixing of a portion of the accumulated DOC out of the surface waters can be a significant carbon export term in certain oceanic systems (Copin-Montégut & Avril 1993, Carlson et al. 1994, Hansell & Carlson 1998a, Hansell & Carlson 2001. However, for DOC to play a role in carbon export, the temporal scale of the DOC turnover has to be greater than the timescale of physical mixing processes (Thingstad 1993, Carlson et al. 1998, Hansell & Carlson 2002. Thus, factors that prevent ...

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I.M., Fluorescent dissolved organic matter as a multivariate biogeochemical tracer of submarine groundwater discharge in coral reef ecosystems, Marine Chemistry (2015), AbstractIn Hawaiʻi and other Pacific high islands submarine groundwater discharge (SGD) can be a significant and continuous source of solutes to nearshore reefs and may play a key role in the structure and function of benthic coral and algal communities.Identifying SGD sources and linking them to reef biogeochemistry is technically challenging. Here we analyzed spectra of fluorescent dissolved organic matter (fDOM) in coral reefs in the context of a suite of biogeochemical parameters along gradients of SGD to characterize fDOM composition and evaluate the utility of fDOM signatures in tracking groundwater dispersal and transformation. We spatially mapped water column chemistry in Maunalua Bay, O'ahu, Hawaiʻi by collecting 24 water samples in grids at each of two ~0.15 km 2 regions during both high and low tides over a two-day period. We observed clear horizontal gradients in the majority of 15 measured parameters, including inorganic and organic solutes and organic particles that tracked concentrations of conservative SGD tracers (radon, salinity and silicate). Multivariate scanning excitation-emission fluorometry successfully differentiated two distinct groundwater sources and delineated regions of SGD dispersion in each reef from the surrounding water column samples without detectable groundwater. Groundwater was consistently depleted in DOC and enriched in nutrients; although the two SGD sources varied widely in fDOM quantity and fluorophore proportions, indices of humification were consistently elevated in SGD at both sites. Our results provide a robust spectral characterization of fDOM in SGD-influenced coral reefs and indicate the potential for this rapid and cost-effective measurement technique to be useful in tracking SGD dispersal in nearshore ecosystems.