A novel niche for Prochlorococcus sp. in low-light suboxic environments in the Arabian Sea and the Eastern Tropical North Pacific

Prochlorococcus marinus is present in all tropical and subtropical oceans, where it is often found throughout the euphotic zone, contributing significantly to phytoplankton biomass and primary production and growing at rates comparable to those of other picoplankters. Clearly, Prochlorococcus and eucaryotic picoplankton share significant niche dimensions in the open ocean. Here we report the discovery of populations of Prochlorococcus in layers below the oxyclines of the oxygen minimum zones of the Arabian Sea and the Eastern Tropical North Pacific off Mexico. The unusual aspects of these populations are that these were at times virtual monoalgal cultures found at a depth of 80 to 140 m, often below the euphotic zone, where irradiance ranged from less than 0.1 to 2% of the surface irradiance (I0). The pigment complement of these deep populations was characterized in detail. The previously unidentified Chl-c-like pigment of Prochlorococcus is Mg-3,8-divinylpheoporphyrin a5 monomethylester. The carotenoid complement of populations in these deep layers was similar to that of cultured Prochlorococcus strains, except for high concentrations of a 7′,8′-dihydro-derivative of zeaxanthin, quite likely parasiloxanthin. Even though cellular concentrations of pigments were very high in these populations, suggesting acclimation to low irradiance, ambient light experienced by these populations in the Arabian Sea, <0.1% I0, may not have been sufficient to support normal photoautotrophic growth. Off Mexico these deep Prochlorococcus populations were located at 0.2 and 2% I0 isolumes, an irradiance likely sufficient for slow growth. Environmental conditions in these layers, except for concentrations of oxygen, are similar to those found at and below the subsurface chlorophyll maxima of the subtropical central gyres. As only Prochlorococcus thrives in these layers but Prochlorococcus and eucaryotic picoplankton coexist in and below subsurface chlorophyll maxima, we conclude that the low oxygen concentrations at the deep Prochlorococcus maxima are the determining factor, but we are not able to identify any specific physiological functions that are affected by low oxygen concentrations in eucaryotes but not Prochlorococcus.