Acid lysis of macroalgae by marine herbivorous fishes: myth or digestive mechanism?

The lysis of algal cells by gastric acidity is widely cited as a digestive mechanism used by herbivorous fishes to gain access to intracellular nutrients, despite limited supporting evidence. This study examined acid lysis by (1) determining the gastric pH of eight species of marine herbivorous fishes from north-eastern New Zealand, and (2) experimentally assessing the susceptibility of ten species of marine macroalgae to the effects of low pH. The gastric pH in the fishes ranged from 1.9 to 3.5. The algae were first treated with acidified seawater, then incubated with α-amylase and amyloglucosidase which specifically hydrolyse starch; this starch is found only inside the algal cells. Quantification of the glucose released was used as a measure of cell lysis. At the pH found in the fish stomachs, seven of the ten algae were lysed to an extent that allowed the α-amylase and amyloglucosidase to enter the cells. The results from the remaining three algal species were equivocal, not eliminating the possibility of acid lysis in these algae. The upper limit of pH at which algae were susceptible to acid lysis was lower than found in previous studies. Transmission electron microscopy of the acid-treated algae showed that the major visible structural effects were on cell membranes and not on cell walls. Acid treatment primarily increased the permeability of the cells by lysing the plasma membrane, with little or no major structural modification of the cell wall.