Carboxylic ester hydrolase that may be involved in ulvan degradation (Probable). Ulvan is the main polysaccharide component of the Ulvales (green seaweed) cell wall. It is composed of disaccharide building blocks comprising 3-sulfated rhamnose (Rha3S) linked to D-glucuronic acid (GlcA), L-iduronic acid (IduA), or D-xylose (Xyl) (Probable). Catalyzes the hydrolysis of 6-phosphogluconolactone to 6-phosphogluconate (By similarity). 6-phospho-D-glucono-1,5-lactone + H2O = 6-phospho-D-gluconate + H(+) Carbohydrate degradation; pentose phosphate pathway; D-ribulose 5-phosphate from D-glucose 6-phosphate (oxidative stage): step 2/3. By ulvan and rhamnose. Belongs to the cycloisomerase 2 family.