Abstract
Benthic primary producers in marine ecosystems may significantly alter biogeochemical cycling and microbial processes in their surrounding environment. To examine these interactions, we studied dissolved organic matter release by dominant benthic taxa and subsequent microbial remineralization in the lagoonal reefs of Moorea, French Polynesia. Rates of photosynthesis, respiration, and dissolved organic carbon (DOC) release were assessed for several common benthic reef organisms from the backreef habitat. We assessed microbial community response to dissolved exudates of each benthic producer by measuring bacterioplankton growth, respiration, and DOC drawdown in two-day dark dilution culture incubations. Experiments were conducted for six benthic producers: three species of macroalgae (each representing a different algal phylum: Turbinaria ornata – Ochrophyta; Amansia rhodantha – Rhodophyta; Halimeda opuntia – Chlorophyta), a mixed assemblage of turf algae, a species of crustose coralline algae (Hydrolithon reinboldii) and a dominant hermatypic coral (Porites lobata). Our results show that all five types of algae, but not the coral, exuded significant amounts of labile DOC into their surrounding environment. In general, primary producers with the highest rates of photosynthesis released the most DOC and yielded the greatest bacterioplankton growth; turf algae produced nearly twice as much DOC per unit surface area than the other benthic producers (14.0±2.8 µmol h−1 dm−2), stimulating rapid bacterioplankton growth (0.044±0.002 log10 cells h−1) and concomitant oxygen drawdown (0.16±0.05 µmol L−1 h−1 dm−2). Our results demonstrate that benthic reef algae can release a significant fraction of their photosynthetically-fixed carbon as DOC, these release rates vary by species, and this DOC is available to and consumed by reef associated microbes. These data provide compelling evidence that benthic primary producers differentially influence reef microbial dynamics and biogeochemical parameters (i.e., DOC and oxygen availability, bacterial abundance and metabolism) in coral reef communities.
Highlights
Tropical coral reef ecosystems often support enormous biological diversity [1], including structurally and functionally complex benthic communities [2]
Changes in dissolved organic carbon (DOC) and Dissolved oxygen (DO) in benthic producer incubations Concentrations of DOC and DO in all light treatment primary producer incubations exhibited changes significantly different from zero over the course of the 8 hour incubations (p,0.01) and rates of release in all treatments were significantly greater than the controls (Dunnett’s p,0.01; Figure S2)
Starting concentrations of DOC in controls, turf, Porites, and calcareous (Halimeda) and crustose (CCA) did not differ from ambient reef concentrations (68–73 mmol L21, Dunnett’s p.0.8) while starting DOC concentrations were elevated in all three macroalgal treatments as expected due to carryover contamination from the organisms and/or immediate DOC release associated with stress
Summary
Tropical coral reef ecosystems often support enormous biological diversity [1], including structurally and functionally complex benthic communities [2]. Ambient oceanographic conditions play a fundamental role in the structure and function of a given reef community, benthic primary producers can significantly alter key biogeochemical parameters (e.g. oxygen and organic carbon concentrations) in their surroundings via basic physiological processes [6,7,8]. This primary producer-derived DOC is known to affect activity and growth of microbial communities, which in turn play an important role in the remineralization of organic to inorganic constituents [23] or transfer of energy to higher trophic levels [5,8,24,25,26,27]. The source, quality, and quantity of this bioavailable organic material may fundamentally regulate variation in overall ecosystem functioning [31,32]
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