High rates of alkaline phosphatase activity in an equatorial coral reef ecosystem unrelated to seasonal nutrient dynamics

Abstract

Coral reef productivity depends on fast nutrient cycling, mediated largely by enzymatic breakdown of organic matter. Alkaline phosphatases hydrolyse phosphomonoesters and are one of the key enzymes involved in marine phosphorus cycling. They are expressed by a plethora of marine organisms including both planktonic microbes and metazoans such as corals, often in response to phosphate limitation, and are potentially important for coral P nutrition and reef biogeochemical cycling. However, most alkaline phosphatase activity (APA) data are from open-ocean environments, and the rates and drivers of APA in coastal waters are not well understood. Here, we measured APA both in the water column and associated with three coral species at reefs in Singapore, where the monsoonal ocean current reversal creates strong seasonal changes in dissolved nutrient availability. Water column APA was consistently high, averaging 9 ± 10 nmol l-1 h-1, but was not correlated with dissolved phosphate or other biogeochemical parameters. Experimental phosphate addition did not reduce seawater APA but addition of labile organic carbon did increase seawater APA, indicating that the increase in APA was driven by heterotrophic activity rather than phosphate stress. Coral APA ranged from 12–163 µmol m-2 h-1 depending on species and was equivalent to the APA in several meters of overlying water. While most coral APA was associated with the coral holobiont rather than the coral mucus, corals released 12 – 55 µmol h-1 per m2 of mucus-associated APA into the water column, which is potentially significant for water column DOP cycling.

The effects of dissolved organic matter supplements on the metabolism of corals under heat stress

“…We used DOM derived from lysed zooplankton cells, which are likely highly labile. This is to mimic the capacity of corals to use DOM when it is freshly produced by plankton, or by “sloppy feeding” and extracellular enzymatic activities during coral grazing on plankton (Wijgerde et al 2011), which can be very high in reef waters (Nichols et al 2023). Our study addresses several questions.…”

Pathways and consequences of dissolved organic matter transformation in tropical coastal waters

“…. Tissue density of each coral fragment collected concurrently to this study (Moynihan, 2021). Statistical differences between species are shown (Tukey HSD, p < 0.05).…”

“…All coral tissue from coral fragments used in incubations was preserved for Symbiodiniaceae counts, and the coral skeleton was kept for coral surface area measurements. Tissue weight measurements and mucus production rates came from concurrently running experiments to measure coral-associated nitrogen fixation for which coral fragments from the same sites and colonies were collected and preserved (Moynihan, 2021).…”

“…Tissue dry weight measurements and mucus production rates came from concurrently running incubation experiments for which coral fragments from the same sites and colonies were collected and preserved (Moynihan, 2021). Briefly, fragments from the same colonies used in APA incubations were incubated in situ for 24 hours in filter-sterilized seawater (0.22 µm, Sterivex).…”

“…columna. Concurrent incubations of separate coral fragments from the same colonies over 24 hours (Moynihan, 2021) found that mucus release rates averaged 4 ± 2 g m -2 day -1 (n=32) for P. sinensis, 3 ± 1 g m -2 day -1 (n=16) for G. columna and 2 ± 2 g m -2 day -1 (n=16) for P. acuta. Based on the average mucus APA rates, we therefore estimate that the APA of mucus released by corals to the water column in a day is 56 ± 26 µmol m -2 h -1 for P. acuta, 37 ± 21 µmol m -2 h -1 for P. sinensis and 13 ± 3 µmol m -2 h -1 for G. columna.…”