Guide to Bacteriastrum

Bacteriastrum is strictly planktonic and widely distributed in warm, temperate marine waters. Living cells contain numerous, small discoid chloroplasts. Cells are joined in unbranched chains, in most species. Resting spores, which are formed endogenously, are known in a single species.

Valves are circular, formed from a central annulus. Frustules are cylindrical. The valve face is flat, or slightly depressed with a shallow mantle. At the valve mantle transition, the distinctive, hollow setae emerge in a radiate pattern. In all but one of the chain-forming species, the frustules are held together by fusion of the setae from adjacent valves. The fusion of the setae involves only the outer walls of the setae as they are hollow along their entire length.

There are two types of valves in a filament – intercalary and separation. The former hold the chain of cells together and the latter occur only at the ends of a filament. The setae of two adjacent intercalary valves are fused for a certain distance and then bifurcate at an acute angle. This bifurcation typically occurs in a plane parallel to the valvar plane but in some species is parallel to the pervalvar axis of the cells. The setae of the separation valves are unbranched and usually curved. The setae of both valves may be smooth or possess spines that are spinulose, T-shaped, or Y-shaped depending on the species. Pores of variable size may be found on the setae.

Rimoportulae are found only in the separation valves; these are centrally located on the valve face and externally are either circular or a flattened tube and internally as a narrow slit. In some species, the valve is unperforated while in others small pores occur all over the valve surface or are restricted to a region around the hollow base of the setae. In addition to the ornamentation of spines, the periphery of the intercalary valves may possess either small, shoehorn-like or T-shaped outgrowths.

The girdle consists of a connecting band which is lobed in a sinuous pattern along its abvalvar margin and numerous, small imbricate intercalary bands which are advalvar to the former. Both are porous. During their formation the setae emerge through gaps in the overlapping lobes of the connecting bands from two adjacent valves.

According to Bosak et al. (2015) there are approximately 15 validly described species. Two are worth mentioning here. Bacteriastrum parallelum is the only species that does not form chains, and B. jadranum forms chains that are held together by a cross-linked polysaccharide network, rather than fused setae.

Species of Bacteriastrum could be confused with those of Chaetoceros which also forms unbranched filaments via fusion of the setae. However, valves of the former are circular, whereas those of most Chaetoceros species are elliptical. The setae of Bacteriastrum are numerous and emerge from the valve in a radially symmetric pattern. Chaetoceros has only a single seta projecting from each apex (two per valve, or four per frustule). Recently Bosak et al. (2015), using nuclear-encoded large-subunit (LSU) rDNA sequences, showed that Bacteriastrum was monophyletic, but that the two sections of the genus based on the form and direction of the terminal setae were not.