Growth, nitrate uptake kinetics, and biofiltration potential of eucheumatoids with different thallus morphologies.

The declining production of commercially important eucheumatoids related to serious problems, like increasing susceptibility to ice-ice disease and epiphytism, may be ameliorated by nutrition. This ushered an increasing interest in incorporating seaweeds into an integrated multi-trophic aquaculture (IMTA) setup to take up excess inorganic nutrients produced by fish farms for their nourishment. In this regard, it is important to understand the nutrient uptake capacity of candidate seaweeds for incorporation into an IMTA system. Here, we examined the growth, nitrate ( NO3- ) uptake kinetics, and biofiltration potential of Eucheuma denticulatum and three strains of Kappaphycus alvarezii (G-O2, TR-C16, and SW-13) with distinct thallus morphologies. The NO3- uptake rates of the samples were determined under a range of NO3- concentrations (1-48 µM) and uptake rates were fitted to the Michaelis-Menten saturation equation. Among the examined eucheumatoids, only SW-13 had a linear response to NO3- concentration while other strains had uptake rates that followed the Michaelis-Menten saturation equation. Eucheuma denticulatum had the lowest Km (9.78 ± 1.48 µM) while G-O2 had the highest Vmax (307 ± 79.3 µmol · g-1 · min-1 ). The efficiency in NO3- uptake (highest Vmax /Km and α) was translated into the highest growth rate (3.41 ± 0.58% · d-1 ) measured in E. denticulatum. Our study provided evidence that eucheumatoids could potentially take up large amount of NO3- and fix CO2 when cultivated proximate to a fish farm as one component of an IMTA system. During a 45 -d cultivation period of eucheumatoids, as much as 370 g NO3- can be sequestered by every 1 kg initial biomass of E. denticulatum growing at 3% · d-1 . Furthermore, based on our unpublished photosynthetic measurements, the congeneric K. striatus can fix 27.5 g C · kg-1 DW during a 12 h daylight period.

Inorganic carbon utilization of tropical calcifying macroalgae and the impacts of intensive mariculture-derived coastal acidification on the physiological performance of the rhodolith Sporolithon sp.

Fish farming in coastal areas has become an important source of food to support the world's increasing population. However, intensive and unregulated mariculture activities have contributed to changing seawater carbonate chemistry through the production of high levels of respiratory CO2. This additional CO2, i.e. in addition to atmospheric inputs, intensifies the effects of global ocean acidification resulting in localized extreme low pH levels. Marine calcifying macroalgae are susceptible to such changes due to their CaCO3 skeleton. Their physiological response to CO2-driven acidification is dependent on their carbon physiology. In this study, we used the pH drift experiment to determine the capability of 9 calcifying macroalgae to use one or more inorganic carbon (Ci) species. From the 9 species, we selected the rhodolith Sporolithon sp. as a model organism to investigate the long-term effects of extreme low pH on the physiology and biochemistry of calcifying macroalgae. Samples were incubated under two pH treatments (pH 7.9 = ambient and pH 7.5 = extreme acidification) in a temperature-controlled (26 ± 0.02 °C) room provided with saturating light intensity (98.3 ± 2.50 µmol photons m-2 s-1). After the experimental treatment period (40 d), growth rate, calcification rate, nutrient uptake rate, organic content, skeletal CO3-2, pigments, and tissue C, N and P of Sporolithon samples were compared. The pH drift experiment revealed species-specific Ci use mechanisms, even between congenerics, among tropical calcifying macroalgae. Furthermore, long-term extreme low pH significantly reduced the growth rate, calcification rate and skeletal CO3-2 content by 79%, 66% and 18%, respectively. On the other hand, nutrient uptake rates, organic matter, pigments and tissue C, N and P were not affected by the low pH treatments. Our results suggest that the rhodolith Sporolithon sp. is susceptible to the negative effects of extreme low pH resulting from intensive mariculture-driven coastal acidification.