Sulfolipid substitution ratios of Microcystis aeruginosa and planktonic communities as an indicator of phosphorus limitation in Lake Erie

Phosphorus (P) availability frequently limits primary production in lakes, influences the physiology of phytoplankton, shapes community structure, and can stimulate or constrain the formation of cyanobacterial blooms. Given the importance of P, numerous methods are available to assess P stress in phytoplankton communities. Marine phytoplankton are known to substitute sulfolipids for phospholipids in response to P limitation. We asked whether sulfolipid substitution might serve as an additional indicator of P stress in freshwater phytoplankton communities. The question was addressed using cultures of Microcystis aeruginosa, Lake Erie microcosms, and surveys of lipid profiles in Lake Erie during a Microcystis spp. bloom. Peak area response ratios of the intact polar lipids sulfoquinovosyldiacylglycerol (SQDG) to phosphatidylglycerol (PG) were used as the metric of lipid substitution. In cultures of M. aeruginosa NIES-843, the SQDG : PG ratio increased from similar to 0.9 to similar to 3.3 with decreasing P concentration. In P-limited communities, the SQDG : PG ratio increased from similar to 6 to similar to 11 after 48 h in microcosm controls, while P amendments reduced the ratio to similar to 3. In Lake Erie surveys, the SQDG : PG ratio ranged from similar to 0.4 to similar to 7.4 and was negatively correlated (Pearson r = similar to 0.62) with total dissolved P. The SQDG : PG ratio was not correlated with concentrations of chlorophyll a, soluble reactive P, or N : P molar ratios. These results demonstrated that M. aeruginosa and Microcystis-dominated communities remodel lipid profiles in response to P scarcity, providing a potential short-term, time-integrated biomarker of nutrient history and P stress in fresh waters.