Impact of Biogeochemical Cycling of Phytoplankton-Produced Lipid Compounds in Ocean Systems on Paleoceanographic Records Buried in Sediments

Abstract

Deciphering historic records buried in ancient marine sediments is critically important for understanding the past and current changes in environmental and climate conditions of the earth. Many molecular approaches, including phytoplankton-produced lipid compounds and their stable isotopic compositions, have been developed to conduct this mission [1][2][3][4]. Applications of these approaches are based on a presumption that once these chemical and isotopic signals are generated by phytoplankton in surface water, they remain intact until ultimate burial in sediments. However, most (>95%) phytoplankton-produced organic compounds are recycled by biological and biogeochemical processes in water and surface sediments [5][6][7][8]. Therefore, a logical question has been raised on whether or how these molecular signals are altered during transport/recycling processes.The first example is the record of paleo-surface water temperature using alkenone-based index Uk-37' [9,10]. Although the variability in the Uk-37' index is largely dependent on the temperature in surface water at which the algae grow, other factors have been recognized to affect the index. For example, cell growth rate or growth stage may play a role along with temperature in controlling the unsaturation degree of alkenones [11]. Alkenones are disproportionally synthesized by Prymnesiophyceae cells either as membrane compounds or as metabolic energy storage compounds during different growth stages [12,13], leading to potential deviations (up to 3°C) in the estimated temperature [14][15][16][17]. Grazing activities of zooplankton and benthic fauna seem to have little impact on alkenone degradation [18][19][20], but microbial processes play a dominating role in alkenone degradation [21,22]. Contrasting effects of microbial degradation of alkenones on the Uk-37' index have been observed in different systems or under different environmental conditions, which are likely due to involvement of different microorganisms [23], different degrading reactions [24], and variable redox conditions [25].The second example is to determine paleo-CO 2 level using alkenone δ 13 C signals, based on a presumption that isotopic composition of one single alkenone compound preserved in sediments is linked to carbon isotopic fractionation (ε p ) of phytoplanktonic organic matter while a simple linear relationship exists between CO 2 concentration and the (ε p ) [26,27]. This simple linear relationship was challenged by a finding that cell growth rate also affects isotopic fractionation of phytoplankton organic matter and the (ε p ) is strongly related to the ratio of growth rate to CO 2 concentration (μ/C e ) rather than [CO 2 ] alone [28,29]. More studies [30][31][32][33] have documented that nutrientlimitation (either nitrate or phosphate) and Fe-limitation control the cell growth rate and thus affect isotopic fractionation. On the other hand, many field observations and laboratory experiments [34][35][36] have repeatedly shown strong evidence on isotopic depletion (negative shif...