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 nutrie nts, despite limited supporting evidence. This study examined acid lysis by (1) determining the gastric pH of eight species of marine herbivorous fish es from north-eastern New Zealand, and (2) experimentally assessing the sus ceptibility of ten species of marine macroalgae to the effects of low pH. T he gastric pH in the fishes ranged from 1.9 to 3.5. The algae were first tr eated with acidified seawater, then incubated with a-amylase and amylogluco sidase which specifically hydrolyse starch; this starch is found only insid e the algal cells. Quantification of the glucose released was used as a mea sure of cell lysis. At the pH found in the fish stomachs, seven of the ten algae were lysed to an extent that allowed the alpha-amylase and amylogluco sidase to enter the cells. The results from the remaining three algal speci es were equivocal, not eliminating the possibility of acid lysis in these a lgae. The upper limit of pH at which algae were susceptible to acid lysis w as lower than found in previous studies. Transmission electron microscopy o f the acid-treated algae showed that the major visible structural effects w ere on cell membranes and not on cell walls. Acid treatment primarily incre ased the permeability of the cells by lysing the plasma membrane, with litt le or no major structural modification of the cell wall. (C) 1999 Elsevier Science B.V. All rights reserved.