General Botany Study Guide: Plant Cytology and Cellular Biology

Introduction to Cytology and Cell Theory

• Etymological Foundations and Definitions:

  • The cell (Latin cellula, diminutive of cella meaning "cell") is the fundamental structural and functional unit of all living organisms.
  • The scientific discipline dedicated to the study of the cell is Cytology (Greek Kytos meaning "Cell").
  • Organisms are categorized by cellularity into unicellular organisms (composed of a single cell) and pluricellular or multicellular organisms (composed of multiple cells).

• Cellular Dimensions and Size Ranges:

  • Cell size varies dramatically across living systems, ranging from a ten-thousandth of a millimeter ($\frac{1}{10000}\,\text{mm}$ or $0.1\,\mu\text{m}$) to several centimeters.
  • The vast majority of plant and animal cells measure between $5\text{--}100\,\mu\text{m}$ in diameter.

• Organismic Theory:

  • Postulated by Kaplan (1992) and expanded by Arana (2014), Organismic Theory asserts that rather than viewing individual cells as the primary building blocks of plants, the entire organism must be understood as a single, continuous structure divided into sub-compartments.
  • These internal sub-compartments are created by septa or partitions (tabiques) that are inherently discontinuous due to microscopic pores termed plasmodesmata, which facilitate continuous substance exchange.
  • Under this paradigm, the plant organism is fundamentally symplastic.
  • Multicellularity in plants arises from incomplete septation during compartment formation rather than through the physical aggregation of independent cells.
  • Cells are functionally defined as subdivisions of the organism. Rather than isolated entities, they are constituent parts of the SYMPLAST (SIMPLASTO)—a continuous cytoplasmic continuum bounded by a single plasma membrane, of which the cell wall forms a fundamental, integrated, and metabolically active component.

• Coenocytic Organization:

  • Certain algal lineages display a coenocytic organization, where the basic functional unit is a coenocyte (cenocito).
  • A coenocyte is a large, multinucleate "super-cell" formed when multiple rounds of nuclear division (karyokinesis) occur without accompanying cytoplasmic division (cytokinesis).

• Taxonomic Domains and Major Cell Types:

  • Living organisms are divided into two main categories based on cellular nuclear organization:
  • Prokaryotes (Greek Pro meaning "Before", Karion meaning "Nucleus"): Organisms whose cells lack a membrane-bound nucleus.
  • Eukaryotes (Greek Eu meaning "True", Karion meaning "Nucleus"): Organisms possessing a defined, membrane-bound nucleus.
  • Among eukaryotes, cellular structure is primarily differentiated into Plant Cells and Animal Cells.
  • Plant cells are uniquely characterized by the following diagnostic structural features:
  • Cell wall (pared celular).
  • Phragmoplast (fragmoplasto).
  • Plasmodesmata (plasmodesmos).
  • Chloroplasts (cloroplastos) and plastids in general.
  • Large central vacuole, which forces the cell nucleus into an eccentric position.
  • Microtubule Organizing Center (MTOC / COMT) or centrosome that lacks centrioles.

Cell Wall Structure, Composition, and Biogenesis

• Functional Architecture:

  • The cell wall serves as the structural exoskeleton of the plant cell, establishing cell shape and mechanical consistency.
  • It counteracts osmotic turgor pressure, keeping vacuolated cells in mechanical-osmotic equilibrium.

• Chemical and Physical Matrix Composition:

  • Composed of cellulose microfibrils (or actin microfibrils in select algae and fungi) embedded within a basic, gelatinous, amorphous matrix.
  • The cell wall is a dynamic secretion product of the living cell protoplasm.

• Biogenesis and Step-by-Step Wall Synthesis:

  • Cell Plate Formation: Cell wall assembly initiates during cell division with the internal secretion and confluence of Golgi-derived vesicles to produce the Cell Plate (Placa Celular). The cell plate contains no skeletal microfibrils and consists entirely of matrix material—predominantly highly hydrated pectins and a low proportion of protein.
  • Primary Cell Wall Formation: Following cell division, each daughter cell secretes parietal lamellae containing structural skeletal microfibrils, establishing the Primary Wall (Pared Primaria).
  • The primary wall is initially plastic and flexible.
  • Its main chemical constituent is Protopectin (a polymer of galacturonic acid).
  • During cell development, the primary wall thickens via successive deposition of parietal lamellae, accompanied by a progressive increase in dry matter content.
  • Wall plasticity increases alongside matrix substance accumulation.
  • Dry microfibrillar mass increases until it comprises $\frac{1}{4}$ ($25\%$) of total wall dry weight.
  • In green algae and all land plants (from mosses to angiosperms), these microfibrils consist of flexible, high-tensile-strength cellulose.
  • Progressive accumulation of hemicelluloses and structural proteins increases wall rigidity, halting wall expansion once the final cell volume is reached.
  • Upon reaching growth cessation, the rigid cell wall envelope is frequently designated as the Sacoderm (Sacodermo; Greek Sákkos meaning "Dress/Sack", Dérma meaning "Skin").
  • Secondary Cell Wall Formation:
  • In multicellular plants, cellular differentiation produces chemical modifications of the sacoderm or the deposition of additional internal layers termed the Secondary Wall (Pared Secundaria), typically synthesized after cell elongation has ceased.
  • Microstructure and composition of secondary layers are directly tailored to specialized cellular functions.
  • Tertiary Cell Wall and Matrix Modifications:
  • The Tertiary Wall (Pared celular terciaria) consists of specialized substances deposited onto preexisting wall strata, such as lignin or suberin (súber).
  • Lignified "mechanical" walls provide structural support in skeletal tissues, whereas suberized/cutinized walls establish impermeable barriers in protective insulating tissues.

• The Plant Cuticle:

  • The cuticle (cutícula) is synthesized endogenously by epidermal cells and deposited externally onto the outer face of epidermal cell walls.
  • Structure (Tafolla-Arellano et al., 2013): Composed of an outermost epicuticular wax layer, underlaid by a cuticular layer consisting of cutin and embedded waxes intermingled with cell wall polysaccharides (pectins, cellulose, and other carbohydrates).

Plasmodesmata and Intercellular Communication

• Structure and Composition:

  • Plasmodesmata (plasmodesmos) are microscopic cytoplasmic channels traversing cell walls to link contiguous plant cells.
  • Wall structure of each channel is formed by the interpenetration of plasma membranes belonging to adjacent cells.
  • The central core of each plasmodesma is traversed by a desmotubule (desmotúbulo), which represents a direct cylindrical extension of the Endoplasmic Reticulum (ER).
  • The channel lumen is wrapped externally by a collar sheath of Callose (Calosa).

• Physiological Roles:

  • Enable continuous cell-to-cell transport of organic metabolites, inorganic ions, and macromolecular signals.
  • In specialized tissues, plasmodesmata serve as conduits for the conduction of bioelectric and chemical stimuli.

• Tissue Distribution and Dynamic Density:

  • Highest plasmodesmata density occurs between cells engaged in intense physiological cooperation, such as phloem sieve tube elements and their adjacent companion cells (células anexas).
  • Lowest density occurs in physiologically isolated cells, such as stomatal guard cells (células oclusivas).
  • Spatial distribution is non-uniform, typically aggregating into distinct clusters termed primary pit fields (campos primarios de punteaduras).

• Cellular Isolation and Wall Adaptations:

  • Pathology and Cell Death: If a cell within a tissue dies, surrounding plasmodesmata compress and seal closed via rapid thickening of their callose sheaths, shielding neighboring cells from disruption and allowing them to survive unaffected.
  • Asymmetric Intercellular Boundaries: When a cell wall borders a space or dead structure with no living cell on the opposite side, no plasmodesmata develop within that wall section.
  • Transfer Cells: Specialized transport cells engaged in high-rate solute secretion or absorption develop irregular internal wall projections termed Wall Protuberances or Parietal Outgrowths (Protuberancias Parietales). These outgrowths increase effective plasma membrane surface area by more than ten-fold ($>10\times$) relative to flat cell walls.

Plasma Membrane and Transmembrane Transport

• Structural Organization:

  • The Plasma Membrane (also termed Cell Membrane or Plasmalemma) forms an uninterrupted, fluid boundary defining the cell periphery.
  • Structurally organized as a fluid lipid bilayer with embedded proteins held together by non-covalent hydrophobic interactions, imparting exceptional structural flexibility.

• Membrane Protein Topology and Function:

  • Integral / Transmembrane Proteins: Span the entire lipid bilayer, functioning as selective molecular channels or membrane transporters.
  • Peripheral / Surface Proteins: Bound to either the external membrane face (facing the cell wall) or the internal face (facing the cytoplasm).

• Selective Permeability and Transport:

  • Exhibits selective permeability, permitting free passage of water and controlled flux of dissolved solutes (molecules and ions) via specific transport proteins.
  • Maintains precise concentration gradients across the cell boundary, ensuring distinct intracellular versus extracellular environments.

• Signal Transduction:

  • Membrane-bound receptor proteins exhibit high binding specificity for extracellular signaling ligands, such as phytohormones, initiating intracellular signal cascades.

Cytoplasm and Cytoskeletal Architecture

• Cytoplasmic Matrix Properties:

  • The Cytoplasm (also called Fundamental or Basic Plasma) is a viscous, transparent, gelatinous matrix containing ultra-fine granulations.
  • Its physical and chemical composition varies according to cell age, tissue location, and plant species.
  • Occupies the entire cell volume except for spaces inhabited by organelles and the nucleus.
  • Houses ribosomes, cytoskeletal networks, and metabolic storage inclusions.
  • Protoplasm: Comprehensive term referring to the combined assembly of cytoplasm, nucleus, cellular organelles, and inclusion bodies.

• Biochemical Composition and Metabolism:

  • Primary solvent is water, containing dissolved organic molecules composed of carbon ($C$), nitrogen ($N$), oxygen ($O$), and hydrogen ($H$).
  • Structural and enzymatic proteins constitute $10\%\text{--}30\%$ of cytoplasmic solute concentration.
  • Major metabolic pathways occurring in the cytoplasm include:
  • Glycolysis.
  • Reserve lipid formation.
  • Synthesis of amino acids, nucleotides, and sucrose.
  • Amino acid activation and tRNA charging.
  • Protein translation.

• Rheology, Aging, and Plasmolysis:

  • Reversibly alternates between a fluid Sol state (driving cytoplasmic streaming) and a semi-solid Gel state (predominant in animal cells).
  • During cell maturation and aging, the cytoplasm becomes increasingly vacuolated. In mature, specialized dead cells (e.g., structural support or xylem/phloem sap transport elements), the cytoplasm disappears entirely.
  • A continuous peripheral cytoplasmic layer (parietal layer) remains attached to the inner plasma membrane face.
  • Immersion of plant cells in hypertonic sugar or salt solutions causes osmotic water loss, inducing cytoplasmic contraction and visible detachment of the parietal cytoplasm from the cell wall (plasmolysis).

• Ionic Environment and Internal pH:

  • Active ion pumps located in the plasma membrane and endomembranes maintain a strictly regulated cytoplasmic ionic composition:
  • Relatively high intracellular potassium ($K^+$) concentration.
  • Low intracellular sodium ($Na^+$) and calcium ($Ca^{2+}$) concentrations relative to extracellular fluids.
  • Cytoplasmic $pH$ is maintained above $7.0$ ($pH > 7$), providing the optimal basic environment for cytoplasmic enzyme activity.

• Cytoskeletal System:

  • A dynamic, three-dimensional filament network extending throughout the cytoplasm to maintain organelle positioning, cell shape, and internal spatial organization.
  • Formed by simple protein subunits undergoing regulated polymerization and depolymerization in response to cellular requirements (driving motility and contractility).
  • Reorganizes dramatically during mitosis, cytokinesis, and morphological transitions.
  • Microtubules:
  • Tubular protein cylinders measuring approximately $25\,\text{nm}$ in diameter with variable length.
  • Assembled from heterodimers of $\alpha\text{-tubulin}$ and $\beta\text{-tubulin}$.
  • Undergo dynamic instability via subunit addition at one end and dissociation at the other.
  • Concentrated beneath the plasma membrane in the cortical gel and localized to motility structures (mitotic spindle, flagella).
  • Associated with ATP-dependent motor proteins: Kinesin (Kinesina) and Dynein (Dineína), which carry specific organelles along microtubule tracks at velocities up to $1\,\mu\text{m/s}$.
  • Microfilaments (Actin Filaments):
  • Protein microfilaments measuring approximately $6\,\text{nm}$ in diameter.
  • Assembled from globular actin monomers utilizing $ATP$ energy into long non-covalent helical polymers.
  • Associated crosslinking proteins Filamin (Filamina) and Fodrin (Fodrina) stabilize the network and elevate cytoplasmic viscosity.
  • Membrane-bound microfilaments run parallel to the plasma membrane, providing mechanical rigidity to the cell surface.
  • Myosin motor proteins hydrolyze $ATP$ to generate mechanical force along actin tracks. The actin-myosin complex drives Cytoplasmic Streaming (Corriente Plasmática), enabling rapid intracellular transport in large cells (e.g., the giant green alga Nitella) where simple diffusion is insufficient.
  • Intermediate Filaments:
  • Protein fibers with intermediate diameters ranging from $8\text{--}10\,\text{nm}$.
  • Constructed from diverse monomeric protein subunits.
  • Provide mechanical structural support and anchor cellular organelles in fixed positions.

Endomembrane System and Organelles

• Biomembranes and Compartmentalization:

  • Biomembranes (endomembranes) are closed, three-dimensional, borderless lipid-protein bilayers that establish distinct intracellular compartments.
  • Compartmentalization isolates specialized metabolic environments from the surrounding cytoplasm.
  • Endomembrane components include the Endoplasmic Reticulum, nuclear envelope, Golgi dictyosomes, vacuolar tonoplast, transport vesicles, and specialized inner organellar membranes.

• Endoplasmic Reticulum (ER):

  • An interconnected three-dimensional network of flattened membranous sacs (cisternae) and branched tubules enclosing an internal compartment termed the Lumen.
  • The central hub connects directly with the nuclear envelope (perinuclear cisterna).
  • Rough Endoplasmic Reticulum (RER / Granular ER):
  • Cisternae studded with bound ribosomes.
  • Synthesizes membrane-bound proteins, lysosomal/vacuolar luminal proteins, and proteins destined for extracellular secretion.
  • Highly abundant in cells specialized for protein secretion.
  • Smooth Endoplasmic Reticulum (REL / Agranular ER):
  • Tubular network lacking attached ribosomes.
  • Primary site for lipid biosynthesis and the metabolic processing of xenobiotics and toxic compounds.
  • Serves as a rapid storage and release repository for calcium ions ($Ca^{2+}$), which trigger intracellular signaling and motor responses.

• Dictyosomes (Golgi Apparatus):

  • Composed of discrete stacks of smooth, flattened membranous cisternae, representing the dispersed functional units of the Golgi apparatus in plant cells.
  • Synthesizes non-cellulosic cell wall matrix polysaccharides.
  • Performs protein glycosylation (attaching carbohydrate chains to proteins to form glycoproteins), sorting and packaging secretable proteins into secretory vesicles for targeted delivery.

• Ribosomes:

  • Spherical ribonucleoprotein complexes measuring $20\text{--}30\,\text{nm}$ in diameter, composed of ribosomal RNAs and structural proteins.
  • Frequently associate along single strands of messenger RNA ($\text{mRNA}$) to form functional aggregates termed Polyribosomes or Polysomes.
  • Catalyze Translation (protein biosynthesis, translating genetic nucleic acid sequences into amino acid chains).
  • Free cytoplasmic ribosomes synthesize proteins that function within the cytoplasm.
  • Abundant in active meristematic tissues and protein-secreting glands.

• Vacuoles and the Vacuome:

  • The Vacuome (Vacuoma) refers to the collective vacuolar network within a plant cell.
  • Small and numerous in young meristematic cells, vacuoles coalesce during cell differentiation into a dominant Central Vacuole.
  • Vacuolar expansion increases volume by over $400\times$, occupying up to $>95\%$ of total cell volume in mature tissue cells.
  • Because plant cells lack classical animal lysosomes, vacuoles function as the primary intracellular digestive organelle, degrading proteins, polysaccharides, nucleic acids, and lipids.
  • Cell Sap (Jugo Celular):
  • Extraplasmic fluid housed within the vacuole, possessing a lower $pH$ than the cytoplasm ($pH_{\text{sap}} < pH_{\text{cytoplasm}}$).
  • Contains dissolved inorganic ions, micromolecules, macromolecules, reserve nutrients, water-soluble pigments (anthocyanins), metabolic secretions, and chemical defenses (alkaloids and specific glycosides).
  • Turgor Pressure (Presión de Turgencia):
  • High solute concentration within cell sap drives osmotic water influx, generating Turgor or Tone ($Lt.\ Turgidus$ meaning "Swollen"; $Gr.\ Tonos$ meaning "Tension").
  • Turgor pressure pushes against the plasma membrane and cell wall, bestowing mechanical rigidity to non-lignified plant tissues.
  • Tonoplast (Tonoplasto): The single boundary membrane enclosing the vacuole, derived from the ER or Golgi apparatus.
  • Pigment storage in the vacuole generates tissue coloration, as seen in petals or the foliage of Tradescantia spathacea Sw ("oreja de gato").

• Peroxisomes:

  • Small, membrane-bound spherical vesicles approximately $1\,\mu\text{m}$ in diameter, containing a dense protein core.
  • Historically classified as "Microbodies" alongside lysosomes and glyoxysomes.
  • Function to isolate reactions that produce highly reactive free radicals during amino acid and fatty acid catabolism.
  • Abundant Catalase enzyme within peroxisomes neutralizes toxic hydrogen peroxide ($H_2O_2$) via the reaction:     $2\,H_2O_2 \rightarrow 2\,H_2O + O_2$

• Glyoxysomes:

  • Specialized peroxisomes present in select plant cells (particularly germinating oilseeds).
  • Enriched with enzymes of the Glyoxylate Cycle—a metabolic pathway unique to plants that converts stored seed lipids into carbohydrates to nourish germinating seedlings.

Nucleus, Genome Architecture, and Gene Expression

• Nuclear Structure and Protoplasmic Ratio:

  • The nucleus is a membrane-bound organelle storing and replicating the cellular genome (deoxyribonucleic acid, DNA).
  • Present in all living cells at some stage of development; represents the largest organelle in eukaryotic cells, occupying roughly $10\%$ of total plasma volume.
  • The nucleus-to-protoplast volume ratio remains constant at $1:10$ across cellular volume shifts.
  • Large nuclei are characteristic of cytoplasm-dense cells and are frequently polyploid (containing multiple copies of the chromosome set beyond the baseline diploid state).

• Nuclear Envelope and Transport:

  • Bounded by a double membrane system perforated by nuclear pores approximately $90\,\text{nm}$ in diameter.
  • Pore complexes regulate nucleocytoplasmic exchange, transporting messenger RNA ($\text{mRNA}$), transfer RNA ($\text{tRNA}$), preribosomal subunits, and structural/regulatory proteins between the nucleoplasm and cytoplasm.

• Nuclear Matrix and Chromatin Organization:

  • The inner nuclear membrane face is lined by a fibrous protein network termed the Nuclear Lamina (Lámina Nuclear), which forms an internal structural scaffold (Nuclear Matrix / Matriz Nuclear) imparting a gel-like consistency to the nucleoplasm.
  • Suspended within this nuclear gel is Chromatin (DNA complexed with basic histone proteins).
  • Chromatin exists in a maximally decondensed state during DNA replication and transcription, but packages tightly around histone proteins during cell division.

• Nucleolus:

  • Dense, non-membrane-bound nuclear sub-regions containing multiple tandem copies of ribosomal RNA ($\text{rRNA}$) genes.
  • Sites of intense transcription and ribosome assembly required to meet high cellular demand for protein synthesis machinery.

Mitochondria and Cellular Energetics

• Morphology and Structural Membrane Organization:

  • Rod-shaped (bacillar) organelles measuring approximately $0.5\,\mu\text{m}$ in inner diameter and up to $10\,\mu\text{m}$ in length.
  • Enclosed by a double membrane lacking nuclear-type pores.
  • Smooth outer membrane faces the cytoplasm.
  • Inner membrane surrounds a dense gelatinous Matrix and forms deep inward folds termed Cristae ($Lat.\ Crista$ meaning "Tuft" or "Crest").

• Respiration and Oxidative Phosphorylation:

  • Mitochondria are the cellular powerhouses responsible for Cellular Respiration.
  • Capture energy released during organic compound catabolism to synthesize Adenosine Triphosphate (ATP) via Oxidative Phosphorylation.
  • ATP synthesis and hydrolysis proceed according to the following thermodynamic coupling:     $ADP + P_i \rightarrow ATP$$ATP \rightarrow ADP + P_i + \text{Energy}$
  • Respiratory enzymes and electron transport chains are localized within the mitochondrial matrix and embedded across the inner mitochondrial membrane.

Plastid Diversity, Photosynthesis, and Endosymbiosis

• General Features:

  • Plastids are pigment-containing, double-membrane organelles characteristic of plants and algae (absent in animal cells).
  • Possess autonomous DNA and distinct intra-organellar ribosomes.

• Chloroplast Structure and Photosynthesis:

  • Chloroplasts ($Gr.\ Chloros$ meaning "Yellowish-green") contain green chlorophyll pigments and are localized in photosynthetic stem and leaf tissues.
  • Function: Capture atmospheric or aquatic carbon dioxide ($CO_2$ / carbonic acid) for carbon assimilation, utilizing light energy to drive Photosynthesis and releasing oxygen ($O_2$) as a waste product.
  • Internal Organization:
  • Light Phase (Light-Dependent Reactions): Occur within internal double-membrane sacs containing chlorophylls and carotenoids, termed Thylakoid membranes ($Gr.\ Thylakos$ meaning "Sack").
  • Grana: Disc-like stacks of tightly grouped thylakoids.
  • Stroma (Estroma): Fluid matrix surrounding the grana, containing isolated thylakoid lamellae, intra-chloroplast ribosomes (smaller than cytoplasmic ribosomes), and metabolic inclusions (plastoglobules/lipid droplets and starch grains).
  • Average chloroplast diameter is approximately $5\,\mu\text{m}$, displaying variable morphology.
  • Obligate parasitic plants lacking chlorophyll lose functional chloroplasts as they absorb preformed organic nutrients from host plants.

• Plastid Interconversion and Specialization:

  • Proplastids: Small, unpigmented precursor plastids in embryonic meristems that rapidly multiply via division.
  • Leucoplasts ($Gr.\ Leukos$ meaning "White"): Colorless, unpigmented plastids found in unilluminated tissues (roots, stem interiors, floral leaves/anthophylls); specialized for storing nutrient reserves.
  • Gerontoplasts: Senescent chloroplasts formed in autumn leaves, stained yellow due to structural unmasking of carotenoids; represent an irreversible terminal stage.
  • Chromoplasts: Carotenoid-rich plastids imparting yellow, orange, or red coloration to floral anthophylls, carpels, and non-green tissues; photosynthetically active/pigmentary.
  • Plasticity: Except for gerontoplasts, all plastid types retain the capacity to divide and interconvert into alternative plastid forms depending on light and developmental cues.

• Endosymbiotic Theory:

  • Mitochondria and plastids share striking structural and genetic homologies with prokaryotic bacteria:
  • Double membrane envelope that remains intact during organelle division.
  • Multiplication exclusively via binary fission (membrane strangulation/constriction).
  • Inner membranes with distinct lipid/protein compositions that do not fuse with endomembranes.
  • Autonomous circular genome (organellar DNA).
  • Distinct prokaryotic-like ribosomes.
  • The Endosymbiotic Theory (Teoría Endosimbióntica) postulates that mitochondria and plastids evolved phylogenetically from ancient prokaryotic intracellular endosymbionts engulfed by ancestral eukaryotic host cells.

Oleosomes and Inclusion Bodies

• Oleosomes:

  • Formerly known as Spherosomes (Esferosomas), oleosomes are spherical lipid droplets dispersed throughout the cytoplasm.
  • Contain fluid accumulations of reserve lipids (fatty oils).

• Subcellular Storage Inclusions:

  • Unenclosed cytoplasmic storage zones aggregating diverse metabolic products, including starch grains, essential oils, resins, and essences.

Oxygenic Prokaryotes: Cyanobacteria and Stromatolites

• Cyanobacterial Physiology and Pigmentation:

  • Oxygenic photosynthesis is not exclusive to Kingdom Plantae.
  • Cyanobacteria (Cyanophyceae or Blue-Green Algae) are prokaryotic, unicellular or filamentous organisms.
  • Contain a diverse suite of photosynthetic pigments (Green, Yellow, Blue, and Red) that capture light energy and confer varied colors.
  • Inhabit diverse ecological niches, including brackish water, fresh water, and terrestrial soils.

• Fossil Record and Stromatolites:

  • Cyanobacteria produce mineralized calcium carbonate structures known as Stromatolites.
  • Stromatolites represent the oldest fossil evidence of autotrophic life on Earth.