Role of sponges in nitrogen cycling and total respiration in coral reef ecosystems
Coastal ecosystems are regions of remarkable biological productivity and diversity, yet they are among our most disturbed natural environments. Although many human activities cause change in the coastal zone, they occur against a background of natural change. Effective coastal-zone management requires that we identify and understand these separate causes of ecosystem change. This requires an understanding of the biogeochemical processes that sustain the natural resources of tropical and subtropical ecosystems. In this study we focus on marine sponges because they strongly effect nitrogen (N) cycling in coastal environments through pumping and filtering tremendous volumes of water while their hosted microorganisms affect a wide range of ecologically important N transformations. Our new in situ measurements of whole sponge respiration, water pumping rates, and chemical fluxes indicate that previous studies based upon enclosure or laboratory experiments can dramatically underestimate net fluxes and cannot provide data needed to establish actual net fluxes of dissolved inorganic nitrogen (DIN) and other N species.
The overall goal of our research is to quantify sponge impacts on coastal N cycles. Previous work has identified high rates of N transformations within sponge communities. These include nitrification (i.e., the chemolithoautotrophic conversion of ammonia to nitrogen oxides), dissolved/particulate organic N uptake and degradation, and potentially N2 fixation and denitrification (i.e., the bacterial-mediated respiration of organic matter using nitrogen oxides as electron acceptors in lieu of oxygen). Thus, marine sponges may be adding or subtracting bioavailable N from coastal systems. Our initial investigations of N cycling in sponges demonstrated that they play a major role in the N budget of coral reefs and other shallow water tropical ecosystems (see AQUARIUS Mission 4 August 15-24, 2005 and Mission 6 17-25 September 2007). This mission will examine the role of sponges in the coastal N cycle by (i) using in situ methodologies to construct accurate N budgets for sponges that will identify them as nutrient sources or sinks, and (ii) to examine environmental controls over sponge mediated N transformations. The net impact of sponges on biogeochemical N cycling in coastal environments is largely unknown. Our research program is the first to use in situ methods to quantify the nitrogen cycling in sponges and the resultant flux of DIN and dissolved organic nitrogen from sponges.
Of paramount importance in calculation of chemical fluxes in the oceans is linking the magnitude of the water flow transporting the chemicals with real time measurements of the substrates and products of biogeochemical reactions. During our mission, we will measure sponge excurrent pumping rates using acoustic Doppler velocimeters (ADVs) on a variety of sponge species. Our in situ data collected over the past three years have documented sponge pumping rates as great as 100,000 L seawater per day per L of sponge tissue, and sponge excurrent velocities > 25 cm/s. We will use newly developed underwater in situ membrane inlet mass spectrometry instrumentation that makes possible extremely sensitive and rapid measurements of N2, Ar, CH4, O2, CO2 and other trace gases. These data will be combined with discrete samples on which we will measure inorganic nutrient, total dissolved inorganic carbon, and nitrous oxide concentrations as well as the stable isotopic compositions of nitrate, nitrous oxide and total dissolved inorganic carbon. A major goal of our mission will be to compare pumping rate data of dissolved oxygen, total dissolved inorganic carbon, and various N species in the sponge excurrent plume vs. the concentration of these species in the ambient water surrounding the sponge. With these data we can measure the net chemical flux for several sponge species to determine the impacts sponges have on coastal N cycles.
We now recognize that sponges play a crucial role in the nutrient balance of coastal environments because they are abundant in tropical, temperate and polar habitats, they process tremendous amounts of water, and they can host abundant and active microbial populations. The impact of sponges on biogeochemical cycling in these environments however, is largely unknown. The information we collect on the sponge species living in proximity to AQUARIUS is likely to have broad implications for nitrogen biogeochemistry in coastal ecosystems with abundant sponges. This research is supported by the National Science Foundation (OCE-0624406 to Christopher S. Martens and Niels L. Lindquist and OCE 0624703 to Brian N. Popp) and NOAA/NURC through the Coral Reef Alliance.