For clinical utility, cardiac grafts should be thick and compact, and contain physiologic density of metabolically active, differentiated cells. This involves the need to control the levels of nutrients, and most critically oxygen, throughout the construct volume. Most culture systems involve diffusional transport within the constructs, a situation associated with gradients of oxygen concentration, cell density, cell viability, and function. The goal of our study was to measure diffusional gradients of oxygen in statically cultured cardiac constructs, and to correlate oxygen gradients to the spatial distributions of cell number and cell viability. Using microelectrodes, we measured oxygen distribution in a disc-shaped constructs (3.6 mm diameter, 1.8 mm thickness) based on neonatal rat cardiomyocytes cultured on collagen scaffolds for 16 days in static dishes. To rationalize experimental data, a mathematical model of oxygen distribution was derived as a function of cell density, viability, and spatial position within the construct. Oxygen concentration and cell viability decreased linearly and the live cell density decreased exponentially with the distance from the construct surface. Physiological density of live cells was present only within the first 128 microm of the construct thickness. Medium flow significantly increased oxygen concentration within the construct, correlating with the improved tissue properties observed for constructs cultured in convectively mixed bioreactors.

We studied the removal of dissolved organic carbon (DOC) and bacterioplankton by the encrusting sponges Halisarca caerulea, Mycale microsigmatosa and Merlia normani in coral reefs along Curaçao, Netherlands Antilles. Sponge specimens were collected from coral reef cavities and incubations were done on the fore-reef slope at 12 m depth. The concentrations of DOC and bacterioplankton carbon (BC) were monitored in situ, using incubation chambers with sponges and without sponges (incubations with coral rock or ambient reef water only). Average (± SD) DOC removal rates (in μmol C cm -3 sponge h -1 ) amounted to 13.1 ± 2.5, 15.2 ± 0.9 and 13.6 ± 2.4 for H. caerula, M. microsigmatosa and M. normani, respectively. The DOC removal rates by the 3 sponges were on average 2 orders of magnitude higher than BC removal rates and accounted for more than 90% of the total organic carbon removal. Total organic carbon removal rates presented here were the highest ever reported for sponges. In an additional experiment with H. caerulea, the fate of organic carbon was reconstructed by measuring dissolved oxygen (O 2 ) removal and dissolved inorganic carbon (DIC) release in a laminar flow chamber. H. caerulea respired 39 to 45% of the organic carbon removed. The remaining 55 to 61% of carbon is expected to be assimilated. We argue that H. caerulea may have a rapid turnover of matter. All 3 sponge species contained associated bacteria, but it is unclear to what extent the associated bacteria are involved in the nutrition of the sponge. We conclude that the 3 sponge-microbe associations are (related to the availability of dissolved and particulate carbon sources in the ambient water) 'dissolved organic matter (DOM)-feeders' and encrusting sponges are of quantitative importance in the removal of DOC in coral reef cavities.KEY WORDS: DOC · Sponges · Nutrition · Carbon budget · Nutrient cycling · Coral reef · Reef framework cavities · Caribbean · Curaçao Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 357: [139][140][141][142][143][144][145][146][147][148][149][150][151] 2008 organic carbon by sponges (Reiswig 1971) and the vast volumes of water sponges can process over time (Reiswig 1974b). He found a discrepancy between the supply and demand of carbon in benthic suspension feeders, and DOM was proposed to be the missing link (Reiswig 1981). It is generally assumed that only sponges with sponge-associated bacteria, sometimes comprising up to half of the total biomass of the sponge, are capable of utilizing DOM (Frost 1987, Ribes et al. 1999. Sponges have been demonstrated to take up the amino acid glycine (Stephens & Schinske 1961), 0.1 μm beads (Leys & Eerkes-Medrano 2006), as well as virus particles (Hadas et al. 2006) from ambient water. Both viral particles and 0.1 μm beads easily pass a 0.2 μm filter and are therefore operationally defined as 'dissolved '. Yahel et al. (2003) were the first to show extensive removal of bulk dissolved organic carbon (DOC) by the sponge Theone...

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ABSTRACT. Two microelectrode s t u d~e s demonstrate the effect of light intensity and photosynthesis on areal oxygen respiration in a hypersaline mat at Guerrero Negro, Mexico, and in an intertidal sediment at Texel, The Netherlands. The hypersaline mat was studied in the laboratory at light intensities of 0, 10, 25, 50, 100, 200, and 500 pE m-2 S -' using the light-dark shift technique to measure gross photosynthesis rates. Areal gross photosynthesis increased from 0 to 31.3 nmol O2 cni-' m i n ' and areal net photosynthesis increased from -3.9 to 16.7 nmol O2 cm-' min-' with increasing light intensities. Areal respiration, calculated from the difference between areal gross and areal net photosynthesis, increased from 3 9 to 14.4 nmol O? cin-Z min-l with increasing surface irradiance. This light-enhanced areal respiration was related to an increase in oxygen penetration depth from 0.2 to 2.0 mm, thus expanding the volume of sediment involved in oxygen respiration beneath the mat surface. The mean rate of oxygen respiration per volume of mat remained constant at a rate of -100 nmol O2 min-l. Oxygen profiles for the intertidal sediment were recorded in s~t u during the day at prevailing light intensities. A l-dimensional diffusion-reaction model was used to estimate gross photosynthesis and oxygen resplration per volume of sediment, as well as the euphotic depth and the s e d i m e n t -~d t e r interface concentration of oxygen. Areal gross photosynthesis ranged from 9.7 to 26.3 nmol 0, cm-' min-' and areal net photosynthesis ranged from 1.1 to 8.1 nmol O2 cm-' mln-l, both showing maximum values in the early afternoon. Areal respiration ranged from 8.1 to 18.2 nn~ol 0, cm-l min.' and showed a good positive linear correlation w~t h areal gross photosynthes~s ( R = 0.97, n = 9 ) . The enhanced areal respiration In the intertidal sediment was due to an lncrease in mean resp~ratlon per volume of sedlment from 18.5 to 52 nmol O L c m 7 m i n ' . The oxygen penetration depth only varied between 3.5 and 4.3 mm. Both studies thus indicate that areal respiration is enhanced significantly with increasing light intensities due to increased areal gross photosynthesis By assumlng that areal respiration in darkness is equal to areal respiration in the l~g h t , bell jar incubations would have underestimated areal resplration i.n the light 3.7-fold and areal gross photosynthesis 1.5-fold for the hypersaline mat and 10.7-and 2.6-fold for the intertidal sediment.