Pervasive growth of microencrusters (Lithocodium/Bacinella) characterizes short-lived stratigraphic intervals in Lower Aptian shallow-water carbonate platform settings of the central and southern Tethys. Although a causal relationship between this peculiar shallow-water facies and the early Aptian oceanic anoxic event (OAE) 1a seems likely, the temporal and spatial distributions of Lithocodium and Bacinella mass occurrences in the rock record point to a rather complex pattern. Possible controls include various global and regional drivers expressed in environmental perturbation of neritic ecosystems. Microencruster blooms have hitherto been reported to occur at the onset and in the aftermath of OAE 1a, slightly pre- or postdating the deposition of dark organic-rich black shales in pelagic basins. This study presents evidence for a punctuated and well-expressed early Aptian bloom of the ulvophycean green algae Lithocodium aggregatum observed at the proto–North Atlantic margin, Lusitanian Basin, Portugal. Similar to southern and central Tethyan sections, a Lithocodium-dominated interval temporally replaces the rudist-dominated platform ecosystem. This Lithocodium-rich horizon is composed of oncoidal floatstones and boundstones, including a well-exposed biohermal level, and reaches a thickness of up to 6 m. High-resolution carbon-isotope stratigraphy shown here clearly argues for Lithocodium to represent a biotic harbinger of environmental change in the run-up of OAE 1a. Chemostratigraphy provides evidence that the subsequent carbonate platform breakdown in the proto–North Atlantic was coeval with an early Aptian carbonate platform drowning episode observed in the northern Tethys prior to OAE 1a. A regionally enhanced nutrient level and a relative sea-level rise are suggested to have triggered the transient mass occurrence of Lithocodium microencrusters, which also may have been influenced by elevated atmospheric CO2 concentrations related to the emplacement of the Ontong Java large igneous province. Upwelling water masses and enhanced terrigenous runoff due to increased weathering may have served as possible sources of the required nutrients, which tentatively diminished and finally interrupted the metazoan carbonate platform factory.