Crocosphaera watsonii

Crocosphaera watsonii (strain WH8501) is a unicellular marine cyanobacterium capable of nitrogen fixation (diazotrophy). Isolated by John Waterbury at the Woods Hole Oceanographic Institution, C. watsonii is widespread in the oceans and plays an important role in the global nitrogen cycle through atmospheric N₂ fixation.

Role in Our Research

C. watsonii served as the primary organism in the earliest studies of hopanoid function in bacteria:

Detergent-resistant membranes. We showed that BHT-CE, the principal hopanoid in C. watsonii, is enriched 10-fold in detergent-resistant membrane fractions — the first evidence connecting hopanoids to putative lipid raft-like domains in bacteria (Sáenz, 2010, Organic Geochemistry).

Hopanoid distribution survey. In a survey of 16 marine cyanobacterial strains, C. watsonii was one of only 6 that produced detectable Bacteriohopanepolyols. Its high membrane hopanoid content (~5–10% of TLE by mass) is consistent with hopanoids having an important effect on bulk membrane physical properties (Sáenz et al., 2012, Geobiology).

Hopanoid Profile

C. watsonii produces:

• BHT-CE (bacteriohopanetetrol cyclitol ether): ~99% of total BHP content
• BHT (bacteriohopanetetrol): trace amounts (~1%)

BHP abundance does not vary significantly under nitrogen-limiting versus nitrogen-replete conditions, nor between light and dark periods, arguing against a direct role in nitrogen fixation physiology. We hypothesized that high hopanoid content may reduce membrane permeability to lipophilic antimicrobial compounds in the environment — an untested proposal motivated by the analogy to hopanoid-mediated ethanol tolerance in Zymomonas mobilis.

Culture Conditions

Typically grown in SN media at 28 °C with a 14:10 h light:dark cycle. Cells were also grown on nitrogen-free (SO) and ammonium-amended (SNH4) media for nitrogen regulation experiments.