What was done
The authors investigated whether diel variations in seawater pH caused by the photosynthetic activity of seagrass meadows within Chwaka Bay (Zanzibar, Tanzania) could affect the calcification and photosynthesis rates of calcareous red algae (Hydrolithon sp. and Mesophyllum sp.) and green algae (Halimeda renschii) growing within the marine meadows. This feat was accomplished by measuring rates of calcification and relative photosynthetic electron transport (rETR) of the algae in situ in open-bottom incubation cylinders either in the natural presence of the rooted seagrasses or after their leaves had been removed.

What was learned
Semesi et al. determined that "seagrass photosynthesis increased the seawater pH within the cylinders from 8.3-8.4 to 8.6-8.9 after 2.5 hours (largely in conformity with that of the surrounding seawater), which, in turn, enhanced the rates of calcification 5.8-fold for Hydrolithon sp. and 1.6-fold for the other 2 species," while "the rETRs of all algae largely followed the irradiance throughout the day and were (in Mesophyllum sp.) significantly higher in the presence of seagrassses."

What it means
The three researchers concluded that "algal calcification within seagrass meadows such as those of Chwaka Bay is considerably enhanced by the photosynthetic activity of the seagrasses, which in turn increases the seawater pH." In fact, they state that the high pH values that were created by seagrass photosynthesis are what actually caused the elevated calcification rates of the macroalgae; and this observation has some further implications. It suggests, for example, that the lowering of seawater pH caused by higher atmospheric CO2 concentrations - which results in a lowering of the calcium carbonate saturation state of seawater, and which has been predicted to lead to reductions in coral calcification rates - may well be counteracted by CO2-induced increases in the photosynthetic activity of the symbiotic zooxanthellae of earth's corals, which phenomenon may boost the pH of seawater in intimate contact with the coral host, allowing the host to actually increase its calcification rate in CO2-enriched seawater (or to at least not suffer a major reduction in calcification), which end result is what has generally been observed in the real world over the course of the increase in the air's CO2 concentration that has been experienced since the inception of the Industrial Revolution.