Holdfasts and alginic acid are characteristic of which group of protists?

Answer is: brown algae.
Holdfast is a root like structure that anchors aquatic organisms (in this case brown algae) to the substrate.
Alginic acid is an anionic polysaccharide distributed in the cell walls of brown algae. Alginic acid with water viscous gum. 
Brown algae (Phaeophyceae) are large group of marime algae, located in colder Northern Hemisphere waters.

It allows water to resist freezing and thawing cycle.