Atlantic microbial lipid makeup tracks phosphate scarcity and bulk particle chemistry

Why it matters

Ocean models assume fixed element ratios inside lipids. This work shows those ratios change a lot, and that they move together with the bulk particle ratios. Lipid biomarkers can also show nutrient stress where nutrient concentrations are too low to measure, which may help track expanding low-nutrient ocean regions.

What they did

The authors reused public datasets of lipids, nutrients and particulate organic matter from cruises in the western Atlantic, from about 40°S to about 55°N. They added 77 lipids to the existing lipid dataset, including a sulfur-based lipid class they named DGCS. They computed carbon, nitrogen, phosphorus and sulfur ratios in the lipid pool and compared them with phosphate, nitrate plus nitrite, and bulk particle ratios. They also tested existing and new lipid ratios as signs of nutrient stress, using Spearman rank correlations.

Key findings

• Lipid C:P, N:P and S:P in the mixed layer were all negatively correlated with phosphate, and all peaked in the Sargasso Sea. Lipid C:P ranged from about 83:1 to about 215:1. None of the three ratios correlated significantly with nitrate plus nitrite.
• In the North Atlantic, lipid C:P and S:P rose with bulk particulate C:P. The response levelled off above a bulk ratio of about 200:1, which suggests a minimum phosphorus quota in lipids.
• Lipid nitrogen averaged about 1% of total particulate nitrogen and reached up to about 4%, more than often assumed.
• The two classic stress ratios, SQDG:PG and BL:PC, disagreed in places, and only BL:PC correlated significantly with phosphate. The ornithine lipid ratio OL:PE and the glycolipid ratio GADG:PE tracked phosphate scarcity in heterotrophic bacteria.
• The ratio of DGCS to PDPT in coccolithophores was highest in the phosphorus-poor Sargasso Sea and lowest north of the Gulf Stream, where nitrogen was depleted. This points to a shift from phosphorus stress to nitrogen stress.

Limitations

• Paired lipid and bulk particle data came from one North Atlantic cruise. It is unclear whether the patterns hold in other ocean basins.
• Added lipid classes had no commercial standards, so their amounts rely on best-matched standards. The cause of the rise in bulk C:P is unclear, because storage-lipid carbon did not correlate with it.
• The coccolithophore culture responses to phosphorus limitation were not significant for DGCS or PDPT alone. The results are correlations from a snapshot survey.

Glossary

• Redfield ratio: The classic average ratio of carbon, nitrogen and phosphorus atoms in ocean particles, 106:16:1.
• Headgroup substitution: Swapping the phosphorus-containing part of a membrane lipid for one built from nitrogen, sulfur or sugar, to save phosphorus.
• POM: Particulate organic matter: small suspended particles of organic material in seawater.
• Lipid biomarker: A lipid molecule or lipid ratio whose abundance signals a specific group of organisms or a physiological state.